Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. UNITED STATES DEPARTMENT OF AGRICULTURE BULLETIN No. 741 ¥ Contribution from the Forest Service Bs OO HENRY S. GRAVES, Forester So" Tal. Washington, D. C. PROFESSIONAL PAPER February 7, 1919 EFFECT OF GRAZING UPON ASPEN REPRODUCTION. By Artuur W. Sampson, Plant Ecologist. CONTENTS. Page. Page., URDOS Ee eet ce ot ones ao omaran ace nee eae «8 1 | Injury to aspen reproduction—Continued. NIST OMEL BEM eens e ney eS Doe E NE OE Rare Cel 2 Effect of cattle browsing................- 15 Injury to aspen reproduction by live stock in Comparative injury to aspen sprouts by sheep StanGuneutimpeneansne case ees nae see 3 BNC Cattle Foca ncuessce ees sec cee ee eceeeeee 16 Effect of sheep browsing............-..-- 3 | Height at which reproduction is exempt from Relation between intensity of grazing STATIN GNI UTLESH- eos sees Tee eee eee 18 andyinjuny LOSProutse.2).eoase. 508 5 Height to which sheep browse..........- 18 Relation between intensity of grazing Height to which cattle browse........-.- 21 and injury to sprouts of different Rate of growth of aspen reproduction........ 21 heishiticlassesssseee eee ee eee 7 | Silvicultural management of aspen.......... 23 SUMAIMA ype en a SLU eh ee Ce ae 8 Methodsioiicuttine seas) eens. see aeons 23 Effect of cattle browsing............-...- 9 Methods of brush disposal............... 24 Injury to aspen reproduction by grazing on SUMIMARYecee eo ances ees aeeee eee 25 Clear-cutplandsy ean se G sec ar mciiges eas 10} PRecommiendationss-ao-ecre. deere eeaeee 27 Effect of sheep browsing............-.... 10 PURPOSE. It is desirable that on lands bearing a stand of aspen (Populus tremuloides)’ a proper balance be maintained between timber production and grazing. In many of the National Forests, particu- larly those in northern New Mexico and Arizona, in Colorado and Utah, and in portions of Idaho and Nevada, aspen is a tree of con- siderable commercial value. It is used chiefly for fuel, posts, corral poles, lumber, boxwood, excelsior, and mine props, and the demand for it is steadily increasing. As a protective cover for watersheds, aspen, by reason of its extensive lateral root system, 1s probably more valuable than any other tree species with which it commonly is found. 1Some taxonomists claim that the proper title of the western aspen is Populus aurea Tidestrom being distinguished from the more eastern Populus tremuloides Michaux by certain technical differences in the flowers and fruit, as well as by the fact that after frost the leaves of the former take on a golden- orange rather than a light-yellow hue. Without entering into the merits of the question, the present . usage of the Forest Service is here adhered to. 76783°—19—Bull. 741——1 2 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. : Beneath the aspen, which ordinarily grows in rather open stands, is usually a luxuriant understory of grasses, weeds, and browse that is grazed with relish by all kinds of live stock. As range land, the aspen type is much more valuable than either the spruce-fir type Just above it or the oak-brush type just below it. Unfortunately, however, the stock, especially sheep, do not confine themselves to the forage, no matter how abundant and palatable it may be; they also eat the foliage and tender twigs of the young aspen. This, of course, Means that the aspen reproduction is often badly injured or even killed, which makes it very difficult in many cases to secure a second stand of timber. A study, the results of which are presented in this bulletin, has been made to determine the extent of injury to aspen reproduction by sheep and cattle, the effects of such damage on the development of the young trees, and the best method of protecting the reproduction from injury without unnecessarily restricting the grazing of live stock. Though the results are meant to apply especially to conditions in the National Forests, they may also be of value, possibly with slight modifications, in the case of farm woodlands in the East where the forage under aspen is converted into meat or butter fat. METHOD. The study, which covered a period of five years (1902 to 1916 inclu- sive), was conducted on the Manti National Forest, in central Utah, where the timber and forage are typical of the commercial aspen type in National Forests. A general study was made on 122.-sample plots in virgin and lightly thinned aspen, at elevations between 7,500 and 10,000 feet, in both sheep and cattle ranges and covering all variations in timber and forage. A detailed study of the extent and growth of aspen reproduction and the extent of its injury by stock and other agencies was made on selected clear-cut areas, four of which were located on sheep range and two on cattle range. Two of the plots on the sheep range and one on the cattle range were securely fenced against stock (Plate ID); the others were left open to grazing. The reason for selecting clear- cut areas for the intensive study of reproduction is that aspen repro- duces mainly by sprouts from the roots of older trees and the best method of harvesting mature aspen is to cut the timber clear. On each clear-cut area transect and quadrat sample plots were established so that the sprouts originally observed could be definitely located in future examinations. In 1913 and 1914 all plots were examined twice—once in June and again in September. In subsequent seasons only the transects and quadrats on the clear-cut areas were examined twice during a EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 3 season, the plots in the standing timber being examined only in the autumn. The reproduction injured or killed was classified according to height, sprouts of from about 6 inches to 14 feet constituting the youngest class, those from 14 to 24 feet the second class, and so on up to sprouts 44 feet high. Record was also made of the intensity of grazing; that is, whether it was light, medium, or heavy. Plots were classed as “lightly grazed’’ where 50 per cent or less of the palatable vegetation had been cropped, ‘‘moderately grazed’’ where from 50 to 70 per cent of the forage had been consumed, and “‘heav- ily grazed’”’ where more than 75 per cent of the palatable vegetation had been eaten. INJURY TO ASPEN REPRODUCTION BY LIVE STOCK IN STANDING TIMBER. EFFECT OF SHEEP BROWSING. Injurious browsing of, aspen reproduction means the removal by stock of terminal or lateral shoots, or both, to a sufficient extent to interfere more or less seriously with the subsequent growth and development of the sprouts. The removal of a single lateral twig or the mere nipping of the terminal bud are disregarded, since the study has shown that to interfere seriously with the food manu- facturing power or with the form development of the young trees at least one-fourth of the total number of branches must be destroyed. If browsing is confined to the upper half of the sprout, including the terminal shoot, the damage is more serious, especially so far as concerns the ultimate form of the tree. Table1 summarizes the effects of sheep browsing on plots in standing timber, according to seasons and to the intensity of grazing. There is also given the number of sprouts injured by other things than livestock. Of a total of 16,631 sprouts observed during the five years of study, 17.1 per cent were killed and 27.3 per cent were more or less injured by browsing, while 37.5 per cent were uninjured. It is noteworthy that 11.7 per cent of the total number of sprouts were killed and 6.3 per cent injured by causes other than grazing. Un- favorable climatic conditions and the activities of bark-eating rodents were chiefly responsible for these results. It is evident that very few vigorous sprouts remain to perpetuate the stand where sheep grazing is continued. — Considerable variation in extent of injury to the reproduction occurs in different seasons. In the case of the lightly grazed plots, for example, only 3.7 per cent of the aspen reproduction was injured in 1912, but the percentage was 30.8 in 1913. Similar variations occur in the case of moderately grazed and heavily grazed plots. «a 4 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. They are probably due more or less to chance. No appreciable difference was observed in the extent and degree of injury by browsing during different portions of the season so long as the foliage remained Intact. When the wood of the stems hardens the stems are browsed practically not at all by cattle and only to a limited extent by sheep. The effect of the intensity of grazing on the Vigor and growth of the reproduction is better shown by figures covering a number of years.. These are given in Table 2, which summarizes the data on intensity of grazing presented in Table 1. The lighter the grazing the greater is the percentage of uninjured sprouts. Thus, on the lightly grazed plots more than half (58.2 per cent) of the total number observed were uninjured; on the moderately grazed plots a little less than half (43.8 per cent); and on the heavily grazed areas only about a fifth of the stand (22.2 per cent) escaped injury. TABLE 1.—Jnjury to aspen sprouts on plots in standing timber. Total | Sprouts | Sprouts in- Grazing intensity and year of | number] Uninjured | Killed by | Injured by Sled by. sues oy examination. of sprouts. grazing. grazing. z pees er aes other sprouts. maT ; eae oS: . a Per |Num-| Per |Num-| Per |Num-| Per Laghiy grazed: ber. | cent. ber! | cent.| ber. | cent.| ber. | cent.| ber. | cent. oa a aes oe eee 108.315 90451-96232) en ee Uy lee ey pees ee EE 1913 Bh lees Da eee O54 T 633 166.3} > 20 | "2211294 ws0rs | eee | BEIM Fegan pase TL ee 2 See SARS Se 1,169 |. 652 1.55.7]... 89\ |. Z Gt 219, | 81S 7h el eee NOP eee ee Saree 595 | 233 | 39.1 95/159) ar) 6.9 ii eee eee Tee - SPS eee 267.|. 276.1.165.9 |. 36 1.13.5 |) 40,) 1550.) i ee Moderately grazed: rae Seng ae ae ee Tet 7 | Ol Oe. |e 5) 3:9 [3.2 es eee 1913 Os ee a ee See SRM GUT | eOs tule sya e yee seed 252 | 28.6). <24 5) Soles bere TTYL ec ek ah eae 2,697 |1, 011 | 37.5°)° 303°] 11/27) 532 [99.7722 ee eee Tee oe ee I ia 2 1, 864.) 1571.| 30-6) 327 | 1725 |. 227-P 1200 2 Tee eee aes BOT G ee eae BE WANE ST 1,127 | 611-542) 146'/'12°9 [7-310] 27.6 Pee eS Heavily grazed C1 De Seay Ae ee tee eae 154 | 138] 89.6 9] 5.8 3.1. 1.9 Le AOIS A Set 7 ie 5a Dla] aS ae acl pete 993 ASST Ie SS ee ee THESE ie Ore Sed et he ae 2,471 | (624 | 25.21. 432.) 16.7 (121929) 48.3.1. 2 | ee TREY Gs Se a eae 2,632 |° 370°} 14.0 5. 683°] 26. 1 | “964°| "36.6 | 22. <1 eee BON Gee Rote Lees 1,127.|.- 151 | 13.4 1; 12,| 68.2 |» 39 | 21.2 |S ee eS ee Potabe es ark kis es. 16,631 |6, 242 | Saba 2, 838 |_...-- ee eS 1440 | 1,057 |_..-.- Average. 6.0) Ate oF SSR SSeS aa jesveoNeeecms 17.1 | PALL 7.3 | ee Hee 6.3 | TABLE 2.—Injured, killed, and uninjured aspen sprouts on plots in standing timber subject to sheep grazing during a five-year period. Period of examinations and of Uninjured Sprouts killed | Sproutsinjured grazing intensity. sprouts sprouts. by grazing. by grazing. a a Num- Per Num- Per Num- Per 1912-1916: ber. cent. | ber. cent. ber. cent. hichtiy. crazed). eee. SE eee 3,093 | 1,798 58. 2 241 7.8 598 19.4 Moderately: srazed =< i. 22 so. ae essences 6,696 | 2,927 43.8 | 776 11.6) 1,352 20. 2 Heavily grazed aioe! Levy yd Ss ee es eee 6,842] 1,517] 22.2) 1,821] 26.6) 2,621 38.4 EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 5 RELATION BETWEEN INTENSITY OF GRAZING AND INJURY TO SPROUTS. The character of injury to sprouts will, of course, largely determine (1) the subsequent density of the aspen stand, (2) the ultimate form of the trees, (3) the subsequent rate of growth, and (4), to a con- siderable extent at least, the size of the trees and the commercial value of the timber. The removal of the leader or terminal shoots of aspen reproduction, for instance, may’ destroy the symmetry of the tree, especially if the leader is removed more than once. The injury caused by the destruction of lateral branches is chiefly physio- logical, the nutriment on which the specimen may build being roughly proportionate to the number of laterals; 1. e., to the leaf area. The extent of browsing of leaders and laterals, or both, the mor- tality due to excessive browsing, and the number of sprouts unin- jured on the plots in standing timber during 1915 and 1916 are sum- marized in Table 3. The greatest number of specimens were under observation during these two years. TABLE 3.—Character and extent of injury to sprouts according to intensity of grazing, seasons 1915 and 1916. Plots in standing timber. Total aa ne P Ungrazed| Leaders Season and grazing intensity. ee sprouts. | browsed.| laterals browsed. Leaders and SS ee Oe ey 1915: Dated onl yee (eae A216 KM Aero Ae Sy gaa A Baar) NEL a gr a 5 2.3 Moderately enazede sey eb cia ta wel sen se Seve 17.5 30.6 4.3 had ia reavilyjonazed. Weitenss See Pence (seek. SENN Se 26.1 14.0 21.5 15.1 916: ichbhysonazed sae Si yyy ees Ges fate eee a oh oe 13.5 65. 9 7.8 bal Moderately serazedesesns jen. eae Se fe Me are a ee ae 12.9 54. 2 14.6 12.8 ieeawilygerazedi were t aes. Ue hese see ees et os eae 63. 2 12.9 4.4 16.7 Both in 1915 and 1916 the mortality was practically the same on hehtly grazed and on moderately grazed plots, the average percentage being 14.9 (fig.1). Onthe heavily grazed plots, however, the mortality of sprouts was appreciably greater, being 26.1 per cent of the total in 1915 and 63.2 per cent in 1916. The percentage of ungrazed sprouts in 1916 was more than five times as much on the lightly grazed areas and four times as much on the moderately grazed areas as on the heavily grazed plots. The general tendency is the same in the 1916 figures. The averages for 1915 and 1916 on lightly grazed plots was 52.5 per cent, on the moderately grazed areas 42.4 per cent, and on the heavily grazed plots only 13.4 per cent. | The number of terminal shoots or ‘leaders’? removed was prac-_ tically the same (averaging 4.4 per cent) on lightly and on moderately grazed plots in 1915; but on the heavily grazed areas it was consid- erably larger, amounting to 21.5 per cent. In 1916 the terminal shoots of 7.8 per cent of the specimens were consumed on lightly 6 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. grazed plots, of 14.6 per cent on moderately grazed areas, and of 4.4 per cent on heavily grazed plots. The small percentage on the heavily grazed plots is due to the high mortality of specimens on these plots in previous seasons, which left for observation relatively few specimens having terminal shoots. Table 5 shows that for the whole period from 1912 to 1916, inclusive, the greatest number of grazed sprouts, especially those up to a foot or so in height, were in general found on heavily cropped plots, and that the least injuries occurred on the lightly grazed plots. When the total number of sprouts occurring on the plots and the number injured are considered col- lectively for the five years, regard- less of grazing intensity, a little less than half (40 per cent) is found to be injured by browsing. This is considerably lower than on the heavily grazed plots. On most plots the stature of the specimens was appreciably more reduced on the heavily grazed lands than on those where grazing was light or moderate, the portion of the speci- mens remaining often bemg mere snags, as is shown in figure 2 of Plate I and figures 1 and 2 of Plate Il. In the most extreme eases all the leaves, much of the tender woody growth, some of the buds, and portions of the bark were ight ly Grazed Moderately Grazed Heavily Grazea Leader & Leterals Browsed eaten. sttti==t Leader Browsed The proportion of sp ecimens with === Ungrazed both leaders and laterals browsed LS Forel Morbanty is least serious on the lightly grazed Fic. 1.—Relation of injury ofsprouts tointensity plots. On moderately grazed areas Ces See EL aye the percentage is considerably more, and on heavily grazed areas it is the highest of all. This holds true both for 1915 and 1916. For the five-year period 1912-1916, Table 4 shows that, regardless of height classes, the average percentage of injured sprouts is least on the lightly grazed plots, mtermediate on the moderately grazed areas, and largest on the heavily grazed plots, the figures being 14.1, 24.9, and 53.1, respectively. The results are similar EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 7 in the case of the number of killed sprouts, the percentages being 12.3, 14.5, and 16.9 on the corresponding plots. On the lightly grazed areas the average per cent of uninjured sprouts was 46.2, while on the moderately and heavily grazed plots it was 27.1 and 9.1, respectively. RELATION BETWEEN INTENSITY OF GRAZING AND INJURY TO SPROUTS OF DIFFERENT HEIGHT CLASSES. The extent of injury to reproduction of different height classes varies considerably. There appears to be no constant relation, how- ever, between the different height classes of reproduction and the percentage of injured or killed sprouts. In Table 4 the sprouts under observation during the 5-year period have been grouped into height classes and assembled according to intensity of grazing. i: Tape 4.—LHxtent of injury to sprouts in standing timber, according to height classes and grazing intensities. Height Total g prouts Sprouts Grazing intensity, tS oe Ur t| Uninjured. | killed by | injured by tion. _|sprouts. grazing. ——— ey ye) ey Feet. ber. ne ber. | cent. | ber. | cent. ASUS (OOP GR) 5 a cosuseaseac bookeag||bdekoqocoasd|ldoocsecd |Bedsac PAs Whek Ae Te) ese 24.9 RNY Foavily erazed sc... oo5-2cccceedececdenceseee .5t01.5| 1,395 | 246| 17.6] 318 | 22.8| 665| 47.7 A A Ghee oop aka keep ee 1.5t02.5| 628] 26] 4.1| 131 | 20.81] 373] 59.4 TD Coyisila' aaa Bie Oa MEM BON Mie Ulpeerr San A ee 250, LOWOaO 241 23 9.5 21 S97 159 66.0 BIE) CR NOISE est A SN oa ea tlhe 3.5 to 4.5 109 Gil oso 17 | 15.6 43} 39.5 INVONBZS (GO CHT) Gee SSeS ees oSe She olbascsogecceHellosacousslsecose Hei eeSese 1639) | seer 53.1 The number of specimens remaining on the plots decreases with the increase in height class. Thus in the 34 to 44 foot class is found the sparsest stand of reproduction examined, all intensities of grazing considered. This is accounted for by the number of sprouts killed by adverse climatic conditions and by rodents, as well as by the number killed by browsing. The shoots of the first year’s sprouts (6-inch to 14-foot class) are more succulent than those of sprouts of greater age, but the new twigs and branches produced each season by the older specimens are quite as palatable as are the shoots of younger sprouts. Hence, while the terminal shoot is less liable to injury in the case of the taller and older sprouts than sprouts 14 feet or less in height, the taller specimens are nevertheless subject to injury of varying serious- 8 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. ness so long as the branches are within reach of the sheep. Further- more, the sprouts listed in Table 5 which have the least height growth, viz, those ‘‘less than one season old”’ and those ‘‘below 6 inches, one season old,’ are undoubtedly protected from grazing by the rather luxuriant cover of herbaceous vegetation which characteristically occurs in the aspen type after about August 1 and hides much of the reproduction from view. ‘The sprouts in these classes show a con- siderably smaller percentage of grazing than those of greater height. However, the loss chargeable to grazing is considerable, and owing to the succulence of the entire aerial parts during the first season of growth there were numerous individual sample plots representing large areas where on account both of the climatic conditions and af browsing the greatest injury to sprouts was inflicted during the initial season of growth. In view of the mortality of 1-year-old sprouts due to browsing and causes other than browsing, 1t may be concluded that even light cropping of the lands is sure to have a determining effect on the ulti- mate timber stand. The extent to which the land should be grazed when the sprouts are just beginning to appear, therefore, should be determined by the timber stand ultimately desired. TABLE 5.—Injuries to sprouts 14 feet or less in height according to_varying ree of grazing; data grouped by seasons and eu classes and according to intensity of grazing. Plots in standing timber. ' | Lightly grazed. | Moderately grazed. Heavily grazed. Total Tine [ | ber pe Season ace height | Total] Num-| Per | Total] Num-]| Per | Total! Num-| Per |& ae all SS num-| ber | cent |num-| ber | cent |num-| ber | cent |, °— | inten- ber. |grazed.|grazed.| ber. |grazed.|grazed.| ber. |grazed.|grazed.| “4,0 | Sities. ea ae sal baad reed Ie 122 10. 38 21 5. 61 IR1S - 2 nee cee cece sacs 899 287 | 31.92 871 262 48. 90 773 | 34.69 IRI be ee ee Ee 783 299 | 38.18 | 1,826 737 68.00 | 2,498 | 52.48 1S os ee sc chee eeee | 386 54 | 13.98 | 1,405 261 62.18 | 1,678 | 42.00 LOT (Ps cas 2 es a ee | 216 30 | 13.90 | 924 248 10.30 366 | 18.40 DVT EE eo eee cones £2: 382-1 610) eee 5,148 } 1,513 |_..-.--] 5,802 | 3,153 |.--...- 5,336 |.-.---- SA verare = 2-2-2 [eee ee | Bp 3i Ly eee Saas by 4 oe E 54-34 |i 2a oS: 40. 02 Height class: | Sprouts less than 1 season old...-..- 551| 107/ 19.4 | 1,711 | 365 52.8 | 1,455 | 35.2 1-year-old sprouts ito. £5 teet...- 315 66 | 20.9 | 1,108 202 51.5 | 1,088} 26.0 Sprouts below 6in- inches (1 season olds te FEL 625| 146] 23.4 | 991] 328 37.8 797 | 32.1 6 inches to 1.5 feet | (1 season old)..--| 891 355 | 39.9 | 1,343 436 73.7 .| 1,867 | 51.4 SUMMARY. The effect of sheep browsing in standing aspen timber may be summarized as follows: (1) Of the total number of sprouts examined during 1912 and 1913, inclusive, the percentage of seriously injured and killed sprouts was Bul. 741, U. S. Dept. of Agriculture. PLATE lI. Fia. 1.—CLEAR-CUT PROTECTED ASPEN AREA ON SHEEP RANGE AS IT APPEARED AFTER FENCING IN THE SPRING, 1913. se ee Fia. 2.—TyPiCAL RESULTS FROM BROWSING BY SHEEP OF LEADERS OF ASPEN SPROUTS OF THE FIRST SEASON’S GROWTH ON HEAVILY GRAZED PLOT Following such browsing the specimens usually die, as was the case in this instance. PLATE Il. Bul. 741, U. S. Dept. of Agriculture. ‘poonpoid to0q, OUT} Ye, Ye PvY [eurUr104 ou puv [Mors ory AIOA OPVUL OTST pue GI6. UL IYsTL oY, OF UoUTTOOds oY OTT “poTp 4JoT OY) OF uourfoods OY}. GT61 UL “PI6T Ut poydeasojoyd 010M SuoUTIOOds ot], "HDIH SSHON| OF LNOYdS Nadsy JO S1IVYSLV] GNV Ysqva7 4O ONISMONG snolYyaS ‘LHOIY SYSGVAY SHL NO ‘dasMOYG ATLNANDASAaNS SLOOHS VYSLV7 Ad Y3I0Va SHL 3O LNIWFOVId -3Y JHL ONIMOHS ‘(14357 SMAGVAY SHL NO) LOTd G43ZVYD ATIAWAH NO SLNOUdS GIO-YvAA-J3aYUH 1 —'S “DI-} *poonpoid O1OAL SJOOYS OL0UL OW YOTYM JoJJe “WOsvos oY} JO puo oY} qe porp synoids 10}} ef oy, “posMo.aq ATsnorsos A] }UENbosqns 010M, SPN SHOMTMUApe wor, poonpoid ‘syooys Jo 4os puodes ou, "HLMOU) JO NOSVaS 1sul4 SHL ONIUNG SIVNINYS] SHL JO ONISMONG 3HL Ad GATTI 3Y3M HOIHM 4O SLOOHS 3HL ‘LOId OaZVYH AMAVAH NO SLNOYdS G1IO-YvVSA-OML—"| "DI EFFECT OF GRAZING UPON ASPEN REPRODUCTION. G considerably larger than the percentage of the remaining uninjured sprouts. In addition to the mortality and injury due to browsing, - 11.7 per cent of the sprouts were killed and 6.3 per cent injured in varying degrees of seriousness by causes other than browsing. (2) The extent of.injury to and mortality of sprouts in standing timber was in direct proportion to the intensity of grazing; also the character of injury, such as the removal of the leader, for example, was notably greater on heavily grazed plots than on moderately and lightly grazed areas. In 1915, for example, an average of 4.4 per cent of the leaders were removed on lightly and moderately grazed plots, while on the heavily grazed areas 21.5 per cent of the leaders were removed. (3) Considerable variation in the extent of injury and mortality to the reproduction occurred in different seasons, the variation in injured sprouts being over 25 per cent and in killed sprouts over 10 per cent during the five years of study. Within a single season, however, no appreciable variation in degree and number of injured sprouts was observed so long as the leafage remained intact. EFFECT OF CATTLE BROWSING. Plots in standing aspen timber grazed exclusively by cattle were selected early in the spring of 1915. In addition to the detailed data on character and extent of injury to aspen reproduction by cattle in 1915 and 1916, data on the extent of injury by browsing prior to the establishment of the sample plots were afforded by the older speci- mens under observation. Accordingly, injuries covering a great many years were recorded. The general grazing injuries to the reproduction are summarized in Table 6. The data are grouped according to intensity of grazing, both as to total injury regardless of the height classes of the sprouts and as to injury to the different height classes. Table 6 shows that the extent of injury in 1915 and 1916 varies directly with the close- ness to which the range was grazed, being least, indeed practically nominal, on lightly and moderately cropped plots and relatively heavy (17.5 per cent in 1915 and 36.1 per cent in 1916) on heavily grazed areas. While the number of browsed sprouts was fairly large on the closely grazed plots, the injuries were in no instance especially serious, nor was the extent of damage anywhere near as great as that caused by sheep. The damage done by cattle to repro- duction of a given height was in no case severe. The average per cent of injury to sprouts above 14 feet in height was approximately the same as that of sprouts of the 6-inch to 14-foot class. Thrifty aspen reproduction of varying age and size is found throughout the aspen type in localities where cattle only have been permitted to graze moderately for a number of years. This would indicate that 76783°—19—Bull. 7412 10 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. controlled cattle grazing, such as has been carried out in the vicinity of these plots, and the perpetuation of the aspen stand are not neces- sarily antagonistic. The young, tender aspen shoots are browsed more or less by cattle, however, and the damage inflicted may increase in proportion to the density of the reproduction. Hence, where sprouts are produced in sufficient numbers to attract stock and accustom the animals to browse upon aspen, a greater per cent of damage may occur than where the stand of sprouts is sparse, as in standing timber. A maximum number of sprouts invariably follows clear-cutting, and accordingly the extent of injury by grazing to dense stands of repro- duction can best be determined on clear-cut plots. TABLE 6.—Total number of sprouts the laterals and terminals of which were removed by catile browsing on pes in standing timber; data grouped according to intensity of grazing and height classes. Lightly grazed. Moderately grazed. Season and height class. siichy | Number | Per cent ee Number | Per cent sprouts. | browsed. | browsed. | sprouts. browsed. | browsed. EE era rk ee N ed Be fae pais bye | 150) | 82325 255.|. - 22a 37 3 ie ee eee 0. 78 eee ee ey re Re eee ee 166 5 3: 2 311 13 4, 20 Heighé class: | | 6 inches to 1.5 feet ---.------..--------- = | ee = owen ao 41 2 4. 87 tS ieeb tee 5 leekrs. 5522 2st weer ye _ oe } a ee ee $3 2 2.41 PUICC’ LO a eOb 5.2 5 -5- ae t aen At eS 2 eee eee 78 2 2. 56 5 feet te4 aioee eee ae eee ee eae | S| Pete, eel Ak oo ee 52 | 1 1.92 | te All intensities of Heavily grazed. | zrazing. Season and height class. ne ae otal | wz ae ota | Number Per cent Per cent number | ~ number sprouts. | browsed. | browsed. sprouts. browsed. (CT Sh ee, ibaa ee: |. ee ee 40 | 7 17.50 10 1.75 BIG ILE, cgi 3. SA Th ee | 36 | 13 36. 10 31 6.20 Height class: Gunehes Lol Freche. 2) Ss ses) Sa | Gece eres fon ots 2 2. 60 Se CCR LONE Eek ste ee ee 15 | 4 | 26. 60 6 | 3.20 Dr eebin Dileebn oss. 1s ea ks | 7 |e ae bua 98 ee zoe oy Meni ce 2 Sento Poteet 2 ae s.t see sa | ry OS eae oe — 1 1.60 I ! INJURY TO ASPEN REPRODUCTION BY GRAZING ON CLEAR-CUT LANDS. EFFECT OF SHEEP BROWSING. Because of the high mortality due to natural causes the most authentic data showmg the effect of grazing on the stand and the condition of the sprouts can probably be obtained by comparing the stands on similar fenced and unfenced clear-cut plots. Four plots were selected on typical aspen range and clear-cut in the fall of 1912; two of these were fenced against stock and the other two left unienced. The data recorded on the two sets of plots are summarized in ficures 2 and 3. Figure 2 shows for each season during which the observations were made (1) the rate of sprout production on normal - EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 11 erazed clear-cut aspen plots, (2) the number of sprouts (a) injured and (b) killed by browsing, (3) the number uninjured, and (4) the number injured, by causes other than grazing. Figure 3 shows seasonally (1) the sprouting ability of clear-cut aspen protected from grazing, (2) the number of sprouts injured (a) by barking by rodents and (b) by frost, and. (3) the number killed by (a) barking by rodents and (b) by frost. In 1913 practically the same number of sprouts per acre were pro- duced on the protected and on the unprotected plots—namely, about 80,000 per acre. In the summer of 1914, when the sprouts were retallied, the number had increased to 105,589 per acre on the unprotected plots and to 90,480 on the protected plots. In the fall of 1914, 103,241 specimens were recorded on the unfenced plots—a slight decline over that of the previous count—while on the fenced areas the number had decreased to 58,324. From that time on there was a sharp but uniform decline on both sets of plots until in the fall of 1916 not one living sprout remained on the grazed plots (Plate III), though 2,646 vigorous sprouts per acre were recorded on the protected plots, most of which had attained a height beyond _ that at which sheep browse (Plate IV, fig. 1). While this number is ample for the establishment of a full aspen stand, a great many more sprouts have been found on plots established elsewhere. The number of uninjured sprouts was notably greater on the ungrazed than on the grazed plots (Figs. 2 and 3). On the grazed plots there was a drop in the number of uninjured sprouts in the | summer of 1914, due probably to slightly more intensive grazing at that time. This was followed by an increase and then from the fall of 1914 to the summer of 1916 the number of sprouts declined rather uniformly to zero. On the protected plots the number of uninjured sprouts decreased at approximately the same ratio as the total number produced. On the grazed plots there were no unin- jured sprouts in the summer of 1916, but on the plots protected from grazing there remained, 2,646 sprouts per acre. The rate of mortality even on the ungrazed plots is surprisingly high. Usually a large proportion of the sprouts are killed, often within a season, by the injuries caused by bark-eating rodents, chiefly field mice, gophers, and rabbits (Plate IV, fig. 2). Besides, a very large proportion of the specimens recorded as merely injured by rodents and by frost died later from such injuries. The repro- duction is often completely girdled, and not uncommonly several belts of bark an inch or more in width are removed. While the adverse factors of inclement weather and bark-eating rodents are active in the elimination of reproduction on the fenced and unfenced plots alike, such injury is insignificant compared with the injury chargeable to sheep grazing on the unfenced plots. 12 BULLETIN 741, U. 8S. DEPARTMENT OF AGRICULTURE. s “ Gy & 1S) XS 1G Q Ww wR > S a Q Y re S & ly Q = > = 4 \ x i / \ = 5 : ad Abra es Ee ~ oun -<. —_——— 5 Fall Summer Fal/ Summer fal/ Summer fal/ KK 19/3 19/4 1914 IGQhS 19/5 1316 19/6 : Total Number Of Sprouts Per Acre Wek BAN pa ee a Uninjured Sprouts eA inal ete Injured By Grazing MUNG eer — Kilted By Grazing ays pein en no haan Injured By Causes Other Than Grazing . Fic, 2.—Number of sprouts per acre on clear-cut plots and their injury and mortality. (Area unfenced.) Bul. 741, U. S. Dept. of Agriculture. PLATE III. re ox BEEN UNRESTRICTED. In 1914 there were 103,241 specimens of aspen sprouts per acre; in 1916 not a specimen remained. The shrubby vegetation is mountain elder (Sambucus microbotrys). Fia. 2.—CLEAR-CuT ASPEN PLOT No. 2, UPON WHICH, LIKE PLOT 1, THE TIMBER WAS REMOVED IN THE FALL OF 1912 AND THE AREA SUBSEQUENTLY GRAZED NORMALLY BY SHEEP. The density of the reproduction on this plot was practically the same as on plot 1 the first two Seasons after the timber was removed, but in the fall of 1916 not a living specimen was to be found. A luxuriant stand of the palatable mountain brome grass (Bromus marginatus seminudus), has occupied the soil since clear cutting. The luxuriance of the brome-grass cover is evidence of the fact that the aspen reproduction was not destroyed as a result of overgrazing. a Bul. 741, U. S. Dept. of Agriculture. Plate IV. ae es Biel 2 Fic. 1.—FENCED PLOT, COMPLEMENT TO UNFENCED PLOT SHOWN IN PLATE III, Fic. 1. Atithe end of the third season following cutting there remaimed 3,800 Vigorous Sprouts per acre ci an average height of approximately 4 feet. This number three years afier cutting is more than ample for the establishment of a full aspen stand. Fig. 2.—THREE-YEAR-OLD ASPEN SPROUTS ON CLEAR-CUT FENCED PLOT KILLED BY BARK-EATING RODENTS. Noite extent of removal of bark at various places along main stem and at base of large lateral branch. EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 13 r Acre Sprouts Fe fall Summer fall Summer Falf Summer fa// 1913 19/4 19/4 1915 19015 19/6 IIE Tota! Number Of Sprouts Fer Acre Uninjured Sprouts se I RO ae A -- /njured By Barking SU eC — Killed By Barking eS Bata Injured By Frost ee ied By. Frost, Fig. 3.—Number of sprouts per acre on clear-cut plots and their injury and mortality due to causes other than grazing. (Area fenced.) 14 BULLETIN 741, U. 8S. DEPARTMENT OF AGRICULTURE. On the clear-cut lands protected from grazing and where the mor- tality of the reproduction was not abnormally heavy, no new sprouts ~ appeared, after two years following clear-cutting (fg 4). On the clear-cut unprotected plots, on the other hand, where a large pro- portion of the sprouts were destroyed by browsing, considerable reproduction was produced not only in the first two seasons but in the third season following cutting, though not subsequently. The removal of practically the entire aerial portion of the sprouts by browsing prevents the transfer and utilization of the large stores of plant food deposited in the elaborate root system of the parent plant, and this nutri- tive material appar- ently tends to stimu- late the production of sprouts in the third season following the removal of the tim- ber. The majority of these third-year sproutswereproduced between 2 and 5 weeks later than those sent up during the first two seasons after cutting, and instead of indi- vidual specimens ap- pearing more or less uniformly over the area, as in the case of sprouts produced dur- ing the first two years, they appeared in Fic. 4.—Number of sprouts produced on clear-cut aspen plots bunches offrom about computed on acreage basis, 1913 to 1916 inclusive. 2 to 15 spe cimens. The sprouts produced during the third season distinctly lacked vigor and were, for the most part, killed by frost or other unfavorable climatic factors. Only avery small percentage, even when not injured by inclement weather, possessed sufficient vitality to become perma- nently established. Thus only a sparse, uneven, and weak aspen stand may be expected from sprouts originating on grazed lands dur- ing the third season after cutting, even though grazing is discon- tinued the second year after the timber has been removed. NUMBER OF SPROUTS PER ACRE ‘ : - ae E . " 4 i t . : es > i SS a QR ae peer Fall Summer Falf Summer fall Summer Falf — 1913 _1914 1914 1915 1915 19161916. __ Number Of Sprouts Froduced Fer Season (Frotected Area) ~——— Number OF Sprouts Produced Per Season (Unprotected Area) __._.. Jota Number Of Sprouts Remaining (. Protected Area) ____.__. /ofa/ Number Of Sprouts Remasning ( Unprotected Area) EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 15 EFFECT OF CATTLE BROWSING. Injury to reproduction on clear-cut aspen plots chargeable to browsing, trampling, and rubbing by cattle was observed during the seasons of 1915 and 1916 in the same detail as in the case of the plots located on the sheep allotments. Early in the spring of 1915 two representative areas located at an elevation of approximately 8,300 feet were cleared of timber, and sample plots and quadrats were established, by means of which the rate of sprouting and the exact character of damage caused exclusively by cattle were noted. The clear-cut areas were comparable in every way, except that one was subject to normal grazing while the other was fenced against stock. In recording the damage to the repro- duction, account was taken of the injury and mortality due both to the presence of cattle and to causes other than j or azin g. The data Lightly Grazed Moderately Grazed Hea vily Grazed Percentage were grouped accord- ing to grazing inten- Tee stby. he results are )aq77> 70> Sprouts Injured By Grazing summarized in Table —--—~ Sprov7s Killed By Gr azing 7 ) and for purposes of Fic. 5.—Relation of cattle grazing to number of uninjured and rea dy comp arison the killed sprouts, season 1915. data recorded in 1915, which are practically the same as for 1916, are shown graphically in figure 5. In 1915 the number of uninjured sprouts was highest on the lightly and moderately grazed plots (72.7 and 74.4 per cent, respectively) and notably lower on the heavily grazed areas (40 per cent). In 1916 the relation was similar. There was practically no difference in the per cent of injured and killed sprouts on the lightly grazed and on the moderately grazed plots, while on those heavily grazed there was an appreciable increase in the percentage of both injured and killed sprouts. A considerable proportion of the sprouts on these plots, as in the case of sprouts on plots previously reported on sheep range, were injured or killed by causes other than grazing. Eight per cent of the sprouts died and 5.9 per cent were injured by causes other than Sprouts Uninsured by Grazing 16 BULLETIN: 741, U. S. DEPARTMENT OF AGRICULTURE. grazing, while only 2.6 per cent were killed and 5.7 per cent injured by grazing. TABLE 7.—Summary of aspen sprouts killed and injured by cattle grazing on clear-cut plots. Total "Gor dean | Ei Grazing intensity and year of | °\" | Uninjured | Killed by | Injured b pte jury not athe number pate A Fie Y{sproutsnot| due to examination. sprouts.| Sprouts. | grazing. | grazing. a pay t ieeae tae Num-| Per |Num-| Per |Num-| Per |Num-| Per |Num-| Per rently, grazed: ber. | cent. | ber. | cent.| ber. | cent.| ber. | cent.| ber. | cent. BS py trmen bea a auiis eeeibaaiaptl aaa 4 150 | 109 | 72.7 Cal iMASOM eee eo ee eee 188) 14 4 1918 ae te Ae a Sea Ie LCG A 4ONNKS4.45 oo Paes anes GHiSaG i, 16 9.6 Bioderately, grazed: Ble Sy ee ene AME 379 | 282 | 74.4 12] 3.2 15.| .3.9 46 | 12.2 24 6.3 916 Le Ge SYS Nt uae a AT SHE PEM See a RSIS 14} 4.5 13} 4.2 10 3.2 Heavily grazed: POU EER eR OL em au 40 16 | 40.0 8 | 20.0 15 | 37.5 nM Pia? |e at IS at AGIG a eee en eee pees ales 36 21 | 58.3 1| 2.8 12 | 33.3 29 ust. OaNe sae | cme NOY 2) Eat eRe age 1082") e842 eee cee Pde esa eae G25 eee s 8672 see Gao eas Mean of all grazing intensities...|........).....- UidaSisl apace GAG | Ima fa Oslo Meenas Se Os piesa 5.9 COMPARATIVE INJURY TO ASPEN SPROUTS BY SHEEP AND CATTLE. The damage to reproduction on sheep range, on the plots estab- lished both in standing timber and on clear-cut lands, was consider- ably different in character and extent from that on cattle range. This difference in the degree of damage was evident in all localities where the study was conducted, and especially where plots had been grazed to practically the same degree of intensity. On plots in standing timber the comparative damage by sheep and by cattle may be summarized as follows (data given in Tables 1, 2, and 6): (1) The average per cent of reproduction injured by sheep ate from 1912 to 1916, inclusive, was 27.3, and from 1915 to 1916, in- clusive, 19.9. During the latter period the average per cent of 1 ie on cattle range, where the forage had been cropped to practically the same extent, was but 3.97. (2.) The average per cent of reproduction killed on the plots scat- tered over the sheep allotments from 1912 to 1916, inclusive, was 17.1, and during 1915 and 1916, 24.8. During 1915 and 1916 less than 0.5 per cent of the reproduction on the plots grazed by cattle was killed by browsing. On the clear-cut plots the damage to reproduction by sheep as compared with’ that by cattle is proportionately much more serious than on plots in standing timber. The seriousness of normal sheep erazing on clear-cut aspen lands may be summarized in the statement that in the third year following the removal of the standing timber it may be expected that no sprouts whatsoever will be found on the lands. If any sprouts remain, they are usually so mutilated as either to die after a season or two or so deformed as to produce timber of — EFFECT OF GRAZING UPON ASPEN REPRODUCTION. ay questionable value. Reproduction on clear-cut lands located on cattle range, on the other hand, while damaged more or less. by browsing, appears to occur in cient density over the plots under observation to insure a maximum stand of first quality mature timber. Practically without exception there are no young aspen sprouts on range grazed annually by sheep during the period that the sprouts are being produced. The few sprouts found are almost invariably lacking in vigor and are often more or less seriously diseased. On the range used exclusively by cattle it is the exception not to find at least a partial stand of sprouts varying in age, most of which are vigorous and healthy. Cattle naturally prefer the leafage of herbs, especially erasses, to shrubs and other woody plants, and while nee browse aspen. reproduction, the damage they do seldom endangers the per- manent establishment of the stand unless the range is stocked with cattle beyond its natural carrying capacity. An analysis of the character of the injuries showed that the pro- portion of terminal and lateral shoots browsed was practically the same on cattle and on sheep allotments. It was quite evident, however, that the cattle browsed the foliage more and the woody — tissue less than sheep; consequently the complete removal of ter- minal and lateral shoots was less commonly observed on the cattle ranges than on the sheep ranges. The difference would appear to account for the more rapid and complete recovery of Hee sprouts on.cattle allotments. Practically no damage is caused to aspen reproduction by rubbing and trampling by cattle. Rubbing is generally confined to young conifer saplings characteristically scattered through the aspen type, the needles and bark of which afford the friction desired, or to aspen specimens of about pole size. Young aspen sprouts are so limber that stock seldom break the branches or otherwise distort them by rubbing. Sheep, of course, not bemg addicted to the rubbing habit like cattle, cause virtually no damage in this way. Trampling by either class of stock causes very shght mortality or permanent injury. On sheep ranges the young sprouts are either killed or seriously damaged long before the formation of prominent trails which might otherwise ‘result in trampling out the reproduction. On cattle allotments there is occasionally a small amount of damage to young sprouts by tram- pling, portions of the bark bemg removed along the main stem or the specimen being broken; but such injury is negligible on lands stocked according to their actual carrying capacity and on which the animals are properly distributed. Where cattle have a tendency to congregate, however, near watering and salting places, for mstance, both browsing and feet have a telling ae on the density and vigor of the reproduction.. 18 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. HEIGHT AT WHICH REPRODUCTION IS EXEMPT FROM GRAZING INJURIES. The factor which chiefly determines the time of exemption of the reproduction from destructive browsing is the height of the sprouts. Observations have shown clearly that as soon as the terminal shoots and some of the lateral branches have attained a growth beyond which stock generally browse, the reproduction is no longer in danger of serious damage. Since sheep are particularly destructive to young aspen reproduction, detailed observations as to the height at which they browse were recorded both on the open range and on controlled plots. The height to which cattle browse was observed under range conditions only. HEIGHT TO WHICH SHEEP BROWSE. The height to which sheep browse depends, of course, upon the palatability of the plants that grow within the reach of the animals, as well as upon the particular breed of sheep. Sheep relish woody plants, and they browse the aspen reproduction at as great a height as other highly palatable species. The sheep under observation consisted of about equal numbers of ewes and lambs of the Ram- bouillet and merino breeds, chiefly the former. The animals of both breeds were of about average size. i In observing the height of grazing on the range it was necessary to adhere almost entirely to reproduction in standing timber, as rela- tively little aspen timber on the sheep allotments has as yet been clear-cut or heavily thinned. For this reason the height of the sprouts, the stand of which was usually quite sparse and scattered, was uneven and often considerably below the maximum height at which sheep may browse. Accordingly, to determine the height of aspen browsed with unquestionable accuracy, and to secure a basis for future management of grazing in the aspen type, the observations on the range were supplemented by a carefully planned experiment on a clear-cut fenced inclosure where the sprouts varied in height from 20 to 70 inches, the majority having a height of about 45 inches. The plot was one-fourth of an acre in area. Two 5-year old ewes and three lambs 3 months old were grazed on the plot as long as the feed, consisting of a luxuriant undergrowth of grasses, pea vines; a variety of weeds, and some shrubby growth, was sufficient to sustain the animals at their original weight. Accordingly, the plot was grazed much more closely than would be possible over the range generally where the lands are handled on the basis of a sustained annual yield. The plot in question was clear-cut late in the autumn of 1913 and the sheep were placed on it in August, 1916, so that the major portion of the reproduction was about 3 years of age. At the time the sheep were turned on there was a stand of 30,056 sprouts e EFFECT OF GRAZING UPON ASPEN. REPRODUCTION. 19 per acre (Plate V, fig. 1). The condition of the sprouts before and after grazing was noted on a transect 24 feet wide and 40 feet long running across the plot. The results are given in Table 8. ‘ TasLE 8.—Character and extent of sheep browsing on clear-cué aspen plot and height at which injuries were inflicted. : P Condi- Height | Height . ovals Sprout No. of sprout: Enon cedn a Character of injuries. Laterals grazed. Do. Do. Laterals and terminal grazed. Laterals grazed. 0. 0. Valerals and terminal grazed. 0. Laterals grazed. 0. paterals and terminal grazed. 0. 2) + IDO}. Laterals grazed. Do. Do. Lateralsand terminal grazed. Laterals grazed. Do. Do. Do. d Laterals and terminal grazed. 0. Laterals grazed. 0. Laterals and terminal grazed. Hgtersis grazed. 0. paterals and terminal grazed. 0. Laterals grazed. Laterals and terminal grazed. Laterals grazed. . Laterals and terminal grazed. 0. Laterals grazed. Do. Do. Laterals and terminal grazed. Paterals grazed. 0. Do. Laterals and terminal grazed. Laterals grazed. Laterals and terminal grazed. Laterals grazed. Do. Do. Laterals and terminal grazed. Laterals grazed. 0. 0. Laterals and terminal grazed. Laterals grazed. 0. 44dddddaddddddeededdddddqseeseedddqaddaddd44d4444eeessedddqseqqaaesec 1 1'V indicates that the sprout was in vigorous and normal condition after grazing. W signifies that it was weak and likely subsequently to die from the injuries caused by browsing. 20 BULLETIN 741, U. 8S. DEPARTMENT OF AGRICULTURE. Table 8 shows that while the plot was grazed much more closely than would be warranted on the basis of sustained forage yield, only 8 of the 69 sprouts on the transect (11.6 per cent) were seriously grazed and thus likely subsequently either to die or become very deformed. All the other sprouts were in vigorous condition; and though they were browsed slightly, the damage was not such as to manifest itself in the future development of the stand. In most in- stances the browsing was confined to the lateral branches, only 20 of the 69 sprouts (29 per cent) having both the terminals and the laterals removed. Only 3,483 of the 30,056 sprouts to the acre were seriously weakened 70 ; 2 : Leet te soe | | Z | 26,571 vigorous speci- mens for the estab- lishment of the stand, a number exceeding by more than 24,000 the quantity neces- sary to restock the land fully. The most signifi- cant fact brought out by the experiment is ed ss eM thatthe maximum height of sheep brows- ie bi cae ing was 47 inches, and this occurred only Average tHeight OF Sprouts in the case of a single Height Of Sprouts Exempt Fromspecimen. To show fe ersous Browsing more clearly than in Table 8 the average height at which the Fic. 6.—Heights at which sprouts are browsed. brow sing oecurre d,th e sprouts were grouped into 6-inch height classes and the average height of the browsing in the case of each height class recorded. The figures - are given in Table 9 and the results are shown graphically in figure 6. 40 ~ i) Height Of Sprouls In Inches Se Ree | Average freight Browsed TABLE 9.—Summary of heights at which aspen sprouts are browsed by sheep. Number of Height class. Height | specimens “2 eee class No. iMG a! | 4 height class.| Average.| Mean. ee | | C00 STD Or > CO ND Ft _ (=) ct ° f4 ~] rs or 10} 66to71 69 / EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 21 The height of browsing increases gradually with the increase in the height classes of the reproduction up to the fifth class; 1. e., from 21 to 41.linches. From this point on up to the ninth class the height of the browsing varies slightly above or below the 40-inch mark. In the ninth class occurs the maximum average height of 41.8 inches. The average height at which the 69 grazed sprouts in the transect were grazed was 38.1. This is about the height of the top of the heads of mature Rambouillet and merino sheep. Sprouts the terminal shoots of which have attained a height of 40 inches or so are apt to suffer only slight permanent injury, and extensive observations on the range and on the various check plots have shown that reproduction having an average height of 45 inches is practically exempt from serious injury. Hence lands supporting reproduction averaging 45 inches in height may be grazed with practically no injury to the terminal shoots. Moderate browsing of some of the lateral branches will occur, of course, as long as they are easily within the reach of sheep, but the effect of such browsing is insignificant so far as concerns the development and health of the specimen. HEIGHT TO WHICH CATTLE BROWSE. It has been pointed out that reproduction only a few inches or so in height is subject to about the same degree of browsing by cattle as is reproduction of greater heights. Thus, contrary to what might be expected, sprouts two or three feet high which by virtue of their ereater conspicuousness might be presumed to afford more con- venient browsing than the shorter specimens are nevertheless dam- aged as little as any of their younger associates. Considerable variation is observed in the height of the browsing by cattle. Isolated instances have been recorded of the removal of leafage to a height of about 70 inches, but browsing at such a height is exceptional and of little economic significance. Damage of greater severity occurs between heights of 55 and 60 inches, the latter figure, however, being about the minimum at which twigs and stems are eaten. Thus in localities where cattle are apt to drift and linger and where it is desired to reforest the lands fully the reproduction should be permitted to attain a growth of not less than 5 feet prior to heavy stocking. Obviously, then, lands which are to be reforested should not be used as salting grounds until a sufficient portion of the reproduction has attained a height beyond which destructive brows- ing is likely to occur. RATE OF GROWTH OF ASPEN REPRODUCTION. In the management of grazing on cut-over lands it is necessary to know, in order to make the most judicious use of the knowledge of the height which aspen reproduction must attain to be exempt from 22 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. serious damage by sheep and cattle, (1) the rate of growth of the reproduction and the age at which the major portion of the stand is exempt from serious browsing, and (2) the density of the stand that may be expected, and accordingly the extent of thinning, if any, Average Height In Inches 90,000 80,000 70,000 60,000 50,000 40,000 30,000 20,000 | Number Of Living Aspen Sprouts Fer Acre fall Spring Fall ~— Spring ‘Fall ~— Spring ‘Fall ~—“Spring ‘Fall 1912 1913 1913 I91¢ (9/4 1915 1915 AWE 1916 woke Bem Bae Spring Cutting pea@er tis Beni —. Summer Cutting Fall € uiting Fic. 7.—Number of sprouts and average height growth according to season of cutting. that may be permitted by browsing and that will leave enough reproduction to secure the number of trees which the lands will well support. To determine the sprouting capacity of aspen and the corresponding vigor and rate of growth of the resulting sprouts when the landsare clear-cut in different seasons, the timber was removed on three plots of even-aged aspen, each one-half acre in size. One of the plots was clear-cut in the fall of 1912, another early in the spring of 1913, and the third in midsummer 1913. In Table 10 issummarized the number of living sprouts per acre on these plots and the height growth per season according to the time of cutting. To facilitate comparison of results the data are assembled graphically in figure 7. fe EFFECT OF GRAZING UPON ASPEN REPRODUCTION. 23 TaBLE 10.—Sprouting capacity and height growth of aspen reproduction on clear-cut plots. : Fall cutting, 1912. Spring cutting, 1913. | Summer cutting, 1913. Date of examination. Number of Number of Number of living Average living Average living Average sprouts height. sprouts height. sprouts height. per acre. per acre. per acre. Inches Inches Inches Summer, 1913...-.------------|-------+----[------------ WGN) joe csesuos scales oneauoosballocucesaosae6 DEVIL, MONRO Seeannekscdoadsaao AOD) ese cere BeneOU NN Sagoac BOSH GaSe hse aaeeFeSSaour Spring Ola ae eee Le eee cin. « 32,670 8.6 27, 590 UB Nas Sa SRN Re a ga VORNUL ACH Ne Na ee ae oe 34, 700 23. 4 85, 520 1245) 50, 960 16.9 Sorin el Ola ease aera ceo) 32, 810 25.5 62, 580 1 fs 44,140 20.3 DEUS iS USI SERS es AR a 22, 210 39. 2 39, 920 31.4 43, 850 34.0 Sorts, WONG se cp cesdoseseeoace 24, 100 39.8 37, 609 3255 42,730 34.1 AIO IG Se ise cee ae cen elaine 21,920 43.1 29,910 41.2 31,070 46.6 Table 10 shows that the production of sprouts is heavy on all areas, regardless of season of cutting, and that in general the maximum number of sprouts is in evidence within a season following the removal of the timber. Considering the number of mature, well-developed trees that may occupy the ground, more than enough sprouts are produced, on the respective plots regardless of season of timber removal. The most vigorous sprouting occurred on the plot the timber of which was removed in the spring, and the least vigorous on the autumn-cut plot. On the spring-cut plot, however, the death rate of sprouts was greatest. The rate of growth was relatively uniform and approximately the same on each plot, and the annual increment averaged, about 15 inches, the height growth being practi- cally the same from season to season. From the above facts it may be concluded that (a) regardless of season of cutting a sufficient number of aspen sprouts is produced on clear-cut lands to restock the lands fully, provided grazing is properly controlled; and that (6) in general the reproduction will have attained sufficient height by the end of the third year of growth to be exempt from destructive browsing by sheep, and by the end of the fourth, or more certainly the fifth year, to be out of danger from cattle. SILVICULTURAL MANAGEMENT OF ASPEN. METHODS OF CUTTING. Experimental evidence gathered from variously treated plots points to the fact that if the lands are to be devoted permanently to the production of aspen, clear-cutting or repeated heavy thinnings are the best means by which to secure vigorous sprouting and an evenly distributed stand. As is shown by the sparse stand of sprouts on the plots established in the standing timber, aspen is practically unable to reproduce 24 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. under its own shade. Light thinnings are of some value, but a very large percentage of the sprouts soon succumb because of insufficient — sunlight. The surviving sprouts are forced into intensive competi- tion with various shade-enduring, aggressive, shrubby, and herba- ceous species. This, coupled with inadequate light, renders the sprouts weak and not uncommonly diseased. If the slender speci- mens are not killed outright by fungous attacks, sooner or later they fall easy victims to the wind. The average stand of prop timber, the diameter breast-high of which does not exceed 10 inches, consists of about 480 trees per acre. To insure a stand of this number of trees at the average rate of mortality of the sprouts, a stand of 2,500 specimens per acre the third year after cutting is sufficient, even though the lands are moderately grazed by cattle or sheep after the terminal shoots are no longer subject to browsing. In practically any type of aspen properly protected from stock, the stand following clear-cutting will generally be 2,500 specimens per acre. Thus in the case of the plot pictured in figure 2 of Plate V, representing the sprouting capacity following the clear-cutting of an 80-year old stand, there are more than 30,000 specimens at the end of the third year. The sprouting appears to be quite as vigorous when younger stands are clear-cut. METHODS OF BRUSH DISPOSAL. Various methods of disposing of the brush are in practice; some of which tend to expose the sprouts unduly and others to protect them. Piling and burning the brush is the most popular; but this method, owing to the complete opening up of the lands, is responsible for highly destructive browsing, especially by sheep, the result being that the stand is materially thinned and correspondingly mutilated. The method appears to endanger the establishment of the stand approximately in proportion to the number of spaces burned. Experiments have been made in scattering the brush over the cut- _ ting without lopping the nonmerchantable parts. The method which has given the best results, and which at the same time lends itself to general field practice, is that of scattering the unlopped ones about the stumps, the butts of the discarded portions being placed next to the stumps in such a manner as to have the branches extend out in all directions from the stump. Since the major portion of the reproduction originates from superficial roots near the parent plant, the tops are located where they will afford the greatest possible protection to the new sprouts. This light screen of unlopped branches, arranged as described, is surprisingly effective against repeated visitations by sheep during the first three seasons after the cutting, which is the most critical] period. While, to be sure. there is usually not a sufficient supply of Bul. 741, U.S. Dept. of Agriculture. PLATE V. Se Lea Speer Fic. 1.—VIEW SHOWING STAND AND HEIGHT OF ASPEN SPROUTS ON CLEAR-CUT PLOT ADJACENT TO AREA FENCED OFF AND GRAZED BY SHEEP. Photographed three years after clear cutting and one month after the compietion of the grazing experiment on control plot. Fic. 2.—LUXURIANT STAND OF ASPEN REPRODUCTION AS IT APPEARS ON FENCED PLoT THREE YEARS AFTER CLEAR CUTTING. over 30,000 specimens, approximately 75 per cent of the terminal shoots of which are beyond the peiene of sheep browsing, are growing on the area. At this stage of growth there is very little Tee of ae oe ae Pevoud ue ane timber-carrying capacity of the lands. rt of such a luxurian i isti 2 BAderstOr Glhiorkacis stand by browsing would be a distinct benefit. The us vegetation is luxuriant and consists of the choicest of forage species. EFFECT OF GRAZING UPON ASPEN REPRODUCTION. | 25 tops to cover effectively the entire ground surface and thus prevent the entrance of stock, the method permits the establishment of a fairly good stand of vigorous, well-formed trees. Observations covering a five-year period have made it evident that the tops of aspen scattered over a clear-cutting in such a manner as to protect the sprouts are well along in decay in the third year after scattering; and usually by the end of the fifth year only the central axis of the top remains, this portion often being pretty thoroughly decayed. The absence of turpentine and other volatile and highly inflammable oils, coupled with the relatively high precipitation. in the aspen type and the tendency of the species to absorb and retain a large percentage of moisture when lying on the ground, makes the fire risk due to the scattering of the brush practically negligible. Another means of protecting the sprouts from browsing is to pile the tops in windrows, as it were, thereby fencing out the stock during the period required for partial decay of the nonmerchantable parts This method, however, is not particularly applicable to field opera- tions, and instead of fostering the sprouts by protecting them from frost, the sun’s excessive heat, evaporation, and other adverse factors, exposes the reproduction to the elements in much the same way as when the brush is piled and burned. SUMMARY. (1) Aspen, a tree of high commercial value on many National For- ests in the West and on some of the farm woodlots and lands adjacent thereto in northeastern United States, is often reproduced with diffi- culty where the lands are made to serve the double purpose of timber and meat production. (2) The leafage, young twigs, and branches of the reproduction are browsed with varying degrees of relish by both cattle and sheep. Over 90 per cent of the damage inflicted by stock is chargeable to browsing, the injury due to trampling, rubbing, and similar causes being negligible. (3) Sheep are responsible for severe damage to the reproduction, both as it occurs in standing timber and on clear cuttings, regardless of the variety and supply of choiceforage. Cattle cause some damage, but the extent of injury is usually slight, except where the lands are overerazed or where the animals are inclined to congregate for more or less lengthy periods. (4) The injury and mortality chargeable to the presence of live stock is roughly proportional to the closeness to which the lands are grazed. Observations covering a 5-year period in standing timber on sheep range showed that 27.2 per cent of the reproduction was either injured or killed on lightly grazed plots, 31.8 per cent on mod- erately grazed areas, and 65 per cent on heavily grazed plots. A 26 BULLETIN 741, U. § DEPARTMENT OF AGRICULTURE. - large proportion of the nonbrowsed sprouts are killed by causes other than grazing. In standing timber on cattle range also the injury varied according to grazing intensity, but was less than on the sheep range. During 1915 and 1916 the average percentage of injured and killed sprouts by cattle browsing was 1.6, 2.4, and 26.8 on lightly, moderately, and heavily grazed plots, respectively. (5) On clear-cut lands, where the reproduction is conspicuous and the stand even, the annual mortality due to sheep grazing is exceed- ingly heavy. As a rule three years of successive sheep grazing on such lands results in the destruction of the entire stand. Some injury is also caused by cattle on clear-cut areas, but unless the range is stocked with cattle beyond its normal carrying capacity there is little danger of the reproduction being destroyed beyond the requirements necessary for the establishment of a full commercial stand. (6) Only slight difference is recorded in extent and character of browsing either by sheep or by cattle on different height classes of reproduction, so long as the total height growth of the sprouts has not passed the limit at which stock find the food accessible. (7) A comparison of the character and intensity of browsing shows that a notably greater proportion of the woody stems is consumed by sheep than by cattle. Even in the autumn after the leaves have dropped sheep devour a considerable quantity of the stems of a single season’s growth regardless of the presence of an abundance of choice forage. In the case of cattle, however, the naked stems are practi- cally untouched. : (8) Aspen sprouts are not necessarily permanently injured, nor will the mature tree be lacking in form or symmetry as a result of the removal once or twice of the terminal shoot. Nearly any one of the lateral branches which grow near the terminal shoot appears to be a potential terminal and may readily assume the function of the ter- minal itself. The destroyed leader is very commonly and promptly replaced by shoots originating from adventitious buds near the ter- minal. On the other hand, the removal of both the lateral branches and the terminal shoot to such an extent as to interfere appreciably with photosynthesis and the nutrition of the specimen readily weak- ens and decreases its subsequent rate of growth materially. If the normal leaf surface is not readily replaced and then maintained, death is the inevitable result. | (9) On lands protected from grazing aspen sprouts are produced only during the first two seasons after cutting. On grazed lands a considerable number of sprouts are sent up for three successive sea- sons following the removal of the timber. The third year’s repro- duction, however, appears from two to five weeks later than that produced in the two previous seasons and is, for the most part, EFFECT OF GRAZING UPON ASPEN REPRODUCTION. . Patt eliminated shortly after its appearance by adverse climatic factors, chiefly frost. (10) A surprisingly large proportion of the reproduction produced even on the most favorable sites is killed during the first three years of its growth by causes other than grazing. Frost and bark-eating mammals, notably gophers, field mice, and rabbits, are mainly respon- sible for such mortality. Much of the damage caused by gophers and mice is done under the snow during winter or early spring. (11) The factor that chiefly determines the time of exemption of the reproduction from destruction by sheep and cattle is the height of the sprout. The average maximum height at which sheep browse is approximately 42 inches. Sprouts averaging 45 inches in height are found to be exempt from destructive browsing by sheep. In the case of cattle there is some damage to sprouts between 55 and 60 inches in height, but seldom to those of any greater height. Repro- duction averaging 5 feet in height, therefore, is practically free from damage by cattle browsing. (12) The annual rate of height increment of the aspen reproduction averages about 15 inches. Hence sprouts 3 years of age are exempt from serious injury by sheep, and those from 4 to 5 years of age are free from serious injury by cattle. (13) Aspen is practically unable to reproduce under its own shade, and the best means of obtaining vigorous and dense reproduction, and at the same time of harvesting the timber economically, is to clear-cut the lands or to thin the stand heavily. (14) In the logging operations various methods are used in dis- posing of the branches and tops, the most common practice being to pile and burn them. This method of brush disposal is not conducive to the best results on typical aspen lands, as it exposes the repro- duction unduly both to grazing and to unfavorable climatic conditions. Protection from live stock and from frost, excessive insolation, evaporation, and other adverse factors is obtained by arranging the unlopped tops about the stumps so as to protect the on-coming sprouts. In three or four years, when the reproduction is practically exempt from serious browsing, the brush is for the most part decayed and out of the way. Furthermore, such a disposition of the brush does not make the danger from fire any greater. RECOMMENDATIONS. From the results given in the preceding pages certain recommenda- tions may be made for managing grazing in the aspen type so as to - secure the highest possible economic use of the lands, both in the way of timber output and meat production. If properly handled, live stock will not prove antagonistic to the establishment of the repro- duction, the understory of palatable forage will not be wasted, and 28 BULLETIN 741, U. S. DEPARTMENT OF AGRICULTURE. by the consumption of the herbage the fire risk to the timber may be > greatly minimized. Being unable to reproduce in its own shade, aspen must be well opened up either by heavy thinnings or by clear-cutting, preferably the latter, if the lands are to be fully restocked. When the logging is done on sheep range, or on a combination sheep and cattle range, the forthcoming reproduction will be destroyed almost to the last sprout if the areas are even moderately grazed by sheep during the first three years following the cutting. To avoid destruction of the young aspen cover, then, only three courses are open: (1) Entire exclusion of grazing for three successive seasons following logging, (2) exceedingly lght grazing by sheep, and (8) moderate grazing by cattle. Obviously, the first of these possibilities does not appeal either to the timberman or the stockman. The entire exclusion of grazing animals means increased fire risk and additional fire patrol, the cost of which must be met by the timberman. For the stockman whose farm operations generally, including to a large extent the marketing of the crops produced, are dependent upon the grazing afforded in the aspen type, entire exclusion of stock may spell ruin. The second possibility, very light grazing by sheep, would upon first thought appear to meet the requirements, and guarantee estab- lishment of the reproduction. But the sheepman who will graze his sheep very lightly over the choicest of forage, such as invariably becomes established on clear-cut or heavily thinned aspen lands, has not yet been discovered. Naturally the sheepman’s interests lie in the production of the maximum mutton and wool consistent with sustained forage yield; he has little interest in the production of timber. Since the standards of grazing intensity vary with the individual, what may be declared as very-light cropping by the average stockman-might be classed as moderately heavy by the grazing expert or the silviculturist. _ Furthermore, the intensity of the grazing, at least so far as the stockman is concerned, would, of, course, be judged by the extent of cropping of the herbaceous cover rather than by the seriousness of the browsing of the aspen reproduc- tion, whereas, owing to the tendency of sheep to browse rather than graze, the aspen would probably be badly damaged before much of the herbage would be consumed. Accordingly only the very lightest grazing, coupled with the most expert handling of the flocks during the first three seasons, could be expected adequately to protect the reproduction, and exceedingly few herders would be qualified to assume such responsible management. This brings us to the third course open to avoid the destruction of the young aspen cover; viz, moderate grazing by cattle. The aspen type occupies lands of moderate elevation usually characterized by > s EFFECT, OF GRAZING UPON ASPEN REPRODUCTION. 29 gentle topography, and the forage is made up quite as much of herbaceous as of woody species, so that this type is fully as well suited to the grazing of cattle as of sheep. Moderate cattle grazing during the first three or four seasons following cutting would insure a satisfactory stand of timber, while at the same time the forage crop could be utilized to its full capacity and the profits from grazing left unimpaired. Furthermore, it is well known that a temporary change in the class of stock generally grazed on the lands materially increases and improves the forage, as the species relished by cattle may be quite different from those relished by sheep. This reverse of close cropping by cattle of species grazed lightly by sheep for many years and light cropping of the species previously weakened by continued close grazing would permit the species especially relished by sheep not only to regain their full vigor, but to reproduce abund- antly, and thus maintain themselves against competition with other species. Shifting from sheep to cattle for a three-year period, then, would accomplish two important things: (1) It would guarantee the establishment of a full aspen stand, and (2) it would improve the earrying capacity of the range for sheep grazing. Obviously on cattle range no change in grazing is required, pro- | vided the lands are not too heavily stocked. Care should be taken, however, to have the stock properly distributed over the range at all times. This may best be accomplished by the proper location of salting grounds and watering places. Where the animals are inclined to drift on the lands so early in the season as to be forced to subsist on browse of second choice, such as aspen reproduction, in the absence of more choice feed, drift and division fences should be built to facilitate judicious distribution of the stock. ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 10 CENTS PER COPY NX