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USDA yA, United States

Department of

er oS Agriculture

Forest Service

Pacific Northwest
Research Station

Research Note
PNW-RN-523
December 1997

Abstract

Introduction

Understory Plant Diversity

Riparian Alder-Conifer Stands.
After Logging in Southeast= ~
Alaska A, ae

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Robert L. Deal ee ote
ee

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Stand structure, tree height growth, and understory plant diversity were assessed in
five mixed alder-conifer stands after logging in southeast Alaska. Tree species com-
eosin ranged from 7- to 91-percent alder, and basal area ranged from 30 to 55
m/ha. The alder exhibited rapid early height growth, but recent growth has slowed
considerably. Some conifers have been suppressed, but some spruce are now nearly
as tall as the overstory alders. The four stands with the most alder had high species
richness of shrubs, herbs, ferns, and mosses, but the predominantly spruce stand
had slightly fewer species of shrubs and ferns, and considerably fewer herbs. Mixed
alder-conifer stands have maintained species-rich understories for 45 years after log-
ging, and stands with conifers and alders of relatively equal stocking contained the
largest diameter conifers. Riparian alder-conifer stands maintain plant diversity and
also will provide some large-diameter conifers for large woody debris for streams.

Keywords: Riparian stands, understory plant diversity, southeast Alaska, red alder,
Sitka spruce, large woody debris, stand structure.

In southeast Alaska, new stands of Sitka spruce (Picea sitchensis (Bong.) Carr.)

and western hemlock (Tsuga heterophylla (Raf.) Sarg.) develop in clearly defined
stages (Alaback 1982, Deal and others 1991). After disturbances such as extensive
windthrow or clearcut logging, natural regeneration is rapidly established. Both spe-
cies are prolific seed producers (Harris 1967, 1969), and conifer regeneration is often
overabundant after logging (Harris 1967, Harris and Farr 1974). The forest canopy
closes when stands are 20 to 30 years old. These dense young-growth stands then
enter an intense and long-lasting (50 to 100 or more years) stem-exclusion stage
(Oliver and Larson 1990) that shades out virtually all understory vegetation (Alaback
1982).

Maintaining biodiversity in young stands established after clearcutting is a challenge
to foresters and wildlife biologists. Efforts to maintain or reintroduce understory vege-
tation for wildlife forage have been mostly unsuccessful (USDA Forest Service 1991).
Thinning of stands either releases advance hemlock regeneration or promotes new
conifer regeneration (Deal and Farr 1994) to the general exclusion of other vegeta-
tion. Ongoing research on ways to maintain biodiversity after logging, such as alter-
natives to clearcutting (USDA Forest Service 1994), may prove useful, but current
information on stand structures that provide understory plant diversity after logging
would be extremely valuable for managers.

ROBERT L. DEAL is a forester, Forestry Sciences
Laboratory, 2770 Sherwood Lane, Suite 2A, Juneau,
AK 99801.

Study Area

Methods

Sitka spruce and red alder (A/nus rubra Bong.) are common associates in southeast
Alaska riparian forests. Red alder may be beneficial in riparian areas after logging by
increasing soil nitrogen through nitrogen fixation and providing greater stand structural
diversity than pure conifer stands. Alder, however, is a relatively small-diameter tree
that decomposes rapidly in streams and generally is considered inadequate to provide
the large woody debris needed for anadromous fish habitat (Bisson and others 1987).
Sitka spruce is common in riparian areas, but managers are concerned about whether
it can compete with alder and produce large-diameter trees soon enough to provide
large woody debris for streams. The objective of this study was to assess stand struc-
tures, tree growth rates, and understory plant diversity of mixed alder-conifer stands
to provide information for managers who are interested in improving riparian areas for
anadromous fish and wildlife habitat.

Study sites were located in the coastal hemlock/spruce zone of southeast Alaska.
This region is characterized by rugged, thickly forested islands, cool temperatures,
and substantial precipitation throughout the year (Harris and others 1974). The re-
search area was along lower Noxon Creek on Baranof Island, about 25 km north of
Sitka, Alaska. Noxon Creek has a total stream length of about 20 km and drains a
watershed of nearly 2600 ha. The river terrace slopes are less than 5 percent, and
soils are well drained. Much of the lower 9 km of the stream was tractor logged in the
mid-1940s and again in the mid-1960s (Bryant and others 1992). The former stands,
before logging, were moderately productive, with timber harvest averaging about 4.0
m*?/ha. Information on species composition of former stands is unknown; however,
new stands have various mixtures of red alder, Sitka spruce, and western hemlock.

During August 1992, five 0.05-ha plots were established in riparian stands along the
river terrace of lower Noxon Creek. Stands were selected to encompass the range

of tree species composition from predominantly red alder to various alder-conifer mix-
tures to mostly Sitka spruce. Stands were naturally regenerated, unthinned, and free
of alder girdling. A 20- by 25-m plot was established within each stand, beginning at
the forest edge of Noxon Creek and extending 20 m deep into the stand.

Tree species and crown class were identified, and diameter at breast height (d.b.h.;
1.3 m) was measured for all trees with d.b.h. greater than 5 cm. Total tree height and
height to live crown were measured for 15 to 20 trees per plot, selected at random
from across the range of tree diameters. Increment cores were taken at breast height
from all trees measured for height and were used to determine tree age and diameter
growth. Tree ages were taken from all plots, and tree heights were measured from all
plots except plot 4. The four largest diameter trees (two alders and two spruces per
plot) were selected for destructive sampling to reconstruct height growth.

Within each plot, 20 understory vegetation subplots were installed within a grid, with
four parallel transects each with five sampling points placed at 5-m intervals. Tree
seedlings, shrubs, herbs, ferns, mosses, lichens, and liverworts were identified to
species on each subplot. A 1- by 1-m frame graduated every 0.1 m was used to
demarcate the subplot, and cover class for each plant species was visually estimated.

Table 1—Overstory stand structure and understory species richness for 5 stands at Noxon Creek

Overstory stand

Largest trees

Trees Basal area (top 40/ha) Understory plants
Alder Spuce Alder Spruce

Stand Age* Alder Spruce Hemlock Alder Spruce Hemlock d.b.h. d.b.h. height height Shrubs Herbs Ferns Mosses?

a Nowoustems naa ie pa Mm Mm m m -—----- No. of species — — — — —
1 45 820 60 20 32.98 0.53 3.41 448 123 24.1 7.6 4 17 5 8
2 45 560 160 20 27.46 1.63 37 404 156 21.8 11.3 3 16 4 7
3 45 400 340 180 17.26 19.11 2.60 370 524 21.4 15.8 7 17 5
4 45 280 760 180 17.58 24.97 1.84 403 488 — _— 6 12 5 10
5 26 260 2,960 660 5.67 43.33 6.24 209 251 15.0 18.0 3 6 3 9

@ Stand age is breast height (1.3 m) age.
Includes all mosses, lichens, and liverworts.

Results
Overstory Stand

Stand composition and structure varied greatly among stands. Species composi-

tion (stems/ha) ranged from 7 to 91 percent alder, and most conifers were spruce
(table 1). The basal areas of stands 1 and 2 were 89 to 93 percent alder, stands 3
and 4 were 40 to 44 percent alder, and stand 5 was 10 percent alder. The first four
stands (alder stands) were all 45 years old and were lightly stocked with only about
30 to 45 m*/ha. Stand 5 (spruce stand), although only 26 years old, had a basal area
of over 55 m*/ha and was heavily stocked with small spruce and hemlock.

In stands of the same age (stands 1-4), the heights and diameters of the largest
alders were similar, but the size of the largest spruces differed greatly (table 1). The
diameters of the largest alders ranged from 37.0 to 44.8 cm, and the height of the
tallest trees ranged from 21.4 to 24.1 m. Conversely, the diameters of the largest
spruces ranged from 12.3 to 52.4 cm, and the height of the tallest trees ranged from
7.6 to 15.8 m. The tallest alders in these stands were consistently taller than the
tallest spruces. The largest diameter trees were usually alder; however, the largest
trees found in stands 3 and 4 were spruce. The spruce stand was a young, dense,
mostly conifer stand, and dominant trees were all about the same diameter and
height.

Spruce and alder in the alder stands had very different height growth patterns. The
alder exhibited rapid early height growth and were about 4 to 6 m tall at age 10, 10
to 15 m tall at age 30, and 17 to 21 m tall at age 45 (fig. 1). Alder height growth has
slowed considerably during the last 15 years. The spruce showed variable height
growth. Some spruce were suppressed and only 1 to 3 m tall. Some of the larger
spruce have shown steady growth and are now nearly as tall as the overstory alders
(fig. 1).

Understory Plants

= Alder, stand 1
—&- Alder, stand 1
— Alder, stand 2
—* Alder, stand 4
“®- Spruce, stand 1

-©: Spruce, stand 4

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=
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<<
=
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ae

1940 19505 OOO a 970 er OCOl a SOO se2Z000

Year

Figure 1— Reconstructed height growth patterns for destructively sampled alder and spruce trees found in
the alder stands (stands 1-4) at Noxon Creek.

Seven species of shrubs were found in the study area. The most common shrubs
were Oplopanax horridum (Smith) Miq., Ribes bracteosum Dougl., and Rubus
spectabilis Pursh. Tree seedlings were sparse with some Tsuga heterophylla and
Picea sitchensis, and very few Alnus rubra present. Six fern species were found.
Athyrium filix-femina (L.) Roth. and Gymnocarpium dryopteris (L.) Newm. were
present in all stands. Twelve mosses, lichens, and liverworts were identified. The
most common genera of mosses were Hylocomium, Pleuroziopsis, Plagiomnium,
Pogonatum, and Rhytidiadelphus. Twenty-six herb species were found on the study
area, and the percentage cover was compared for the 10 most common herbs (fig. 2).

The four alder stands had similar numbers of species of shrubs, herbs, ferns, and
mosses with 3 to 7 shrubs, 12 to 17 herbs, 4 to 5 ferns, and 7 to 10 mosses (table 1).
The spruce stand had slightly fewer shrubs (3) and ferns (3), about the same number
of mosses (9), and considerably fewer herbs (6) than the alder stands. Most under-
story plant species had much higher percentage cover in the alder stands than in the
spruce stand. The number of herbs found on the spruce stand was less than the
numbers of herbs on the alder stands, but the small sample size of five stands gave
a nonsignificant result (p = 0.11, chi-square test for differences among groups). Also,
only 4 of the 10 most common herbs were found on the spruce stand. The percent-
age cover of the 10 most common herbs also was consistently lower for the spruce
stand than for the alder stands (fig. 2).

Discussion

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Percent Cover
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SSS

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Figure 2—The mean percentage cover for the 10 most common herb species found in the five stands at
Noxon Creek. Herb species are Cial—Circaea alpina L., Elmi—Elymus hirsutus Presl, Hela—Heracleum
lanatum Michx., Madi—Maianthemum dilatatum (Wood.) Nels. & Macbr., Osch—Osmorhiza chilensis H. &
A., Pral—Prenanthes alata (Hook.) D. Dietr., Stam—Streptopus amplexifolius Michx., Titr—Tiarella trifoliata
L., Vevi—Veratrum viride Ait., and Vigl—Viola glabella Nutt.

The alder stands contained lightly stocked mixtures of alder, spruce, and hemlock.
Typical well-stocked hemlock/spruce stands in southeast Alaska at similar sites and
ages have basal areas of 50 to 70 m*/ha, | or nearly twice the density of these alder
stands. These lightly stocked alder stands had high plant diversity in the understory
and exceptionally high herb-species richness compared with typical, heavily stocked
conifer stands of the same age. Alaback (1982) reported that understory biomass in
hemlock/spruce stands peaked at a stand age of 15 to 25 years, and rapidly declined
once the canopy closed. After canopy closure, these stands enter an intense stem-
exclusion stage (Oliver and Larson 1990), with the least diverse understories occur-
ring in stands between 40 and 150 years of age (Alaback 1982). The alder stands in
this study were about the same age as typical 40- to 50-year-old hemlock/spruce
stands that regenerate after clearcutting. Young-growth hemlock/spruce stands of this
age are in their most intense period of stem exclusion and contain the least diverse

’ Farr, W.A. 1976. The effects of stand density on growth
and yield of hemlock-spruce stands in coastal Alaska.
Revised study plan. On file with: Forestry Sciences
Laboratory, 2270 Sherwood Lane, Suite 2A, Juneau, AK
99801.

Acknowledgments

English Equivalents

plant understories. These alder stands, however, currently provide abundant and di-
verse understory plant communities, a rare condition in young-growth stands in south-
east Alaska. High plant species richness in alder stands also has been reported in
coastal Oregon (Franklin and Pechanec 1968), and more comparative work among
different regions and stand structures is needed to clarify the underlying causes.

Different alder-conifer mixtures in the alder stands provided favorable conditions for
understory plant development; however, relatively light stocking for conifers may be
critical to maintain plant understories. The spruce stand, although almost 20 years
younger than the alder stands, already had more basal area than any of the alder
stands. Stocking was comparable to typical hemlock/spruce stands of the same age
throughout the region. The densely stocked spruce stand has already entered the
stem-exclusion stage, and increased competition and decreased understory plant
diversity is expected during the next few decades. Although the spruce stand was
somewhat younger, it already had less understory vegetation than the alder stands,
and the differences in understory plant diversity among stand types is expected to
become greater with time. The most intense period of the stem-exclusion stage for
the spruce stand is still to come, and plant diversity likely will continue to decrease
as the spruce stand matures.

Large-diameter conifers provide large woody debris in streams, which is necessary to
sustain rearing habitat for fish and other aquatic organisms (Bisson and others 1987,
Franklin and others 1981). The lightly stocked alder stands contained spruce of rela-
tively large diameter, including the largest diameter trees in the study area. Diameter
growth is expected to increase even more if the spruce can emerge from beneath the
overstory alder canopy. Some of the largest spruces are now nearly as tall as the
dominant alders, and limited data suggest that the tallest spruces will reach the upper
canopy in 10 to 20 years. Stand species composition appears to be critical to the de-
velopment of large conifers. Stands with stocking greater than 75 percent alder, con-
tained suppressed spruce and hemlock with relatively small diameters. The spruce
stand was densely stocked and had excessive numbers of small-diameter trees.
Stands with conifers and alders of relatively equal stocking and basal area, however,
contained the largest diameter conifers. These equal-stocking stands may be the best
for producing large-diameter conifers in the shortest time. If producing large-diameter
conifers is desirable for large woody debris inputs in forested streams, then creating
relatively open alder-conifer stands may be the most desirable management strategy
in riparian areas.

Funding for this study was provided by the Chatham Area of the Tongass National
Forest, USDA Forest Service. The author gratefully acknowledges David Bassett,
David D’Amore, and Scott Moore for field assistance and Buck Bryant, Paul Alaback,
and Martha Brooks for their many useful comments.

When you know: Multiply by: To find:

Centimeters (cm) 0.394 Inches

Meters (m) 3.281 Feet

Hectares (ha) 2.471 Acres

Kilometers (km) 0.621 Miles

Square meters per hectare (m?/ha) 4.355 Square feet per acre
Cubic meters per hectare (m°/ha) 14.292 Cubic feet per acre

Literature Cited Alaback, P.B. 1982. Forest community structural change during secondary suc-
cession in southeast Alaska. In: Means, J.E., ed. Forest succession and stand
development research in the Northwest: Proceedings of the symposium; 1981
March 26; [Corvallis, OR]. Corvallis, OR: Forestry Research Laboratory, Oregon
State University: 70-79.

Bisson, P.A.; Bilby, R.E.; Bryant, M.D. [and others]. 1987. Large woody debris in
forested streams in the Pacific Northwest: past, present, and future. In: Salo, E.O.,;
Cundy, T.W., eds. Streamside management: forestry and fishery interactions.
Seattle, WA: College of Forest Resources, University of Washington: 143-190.

Bryant, M.D.; Deal, R.L.; Wright, B.E. [and others]. 1992. Watershed rehabilitation
and restoration: survey and assessment of selected streams on northwest Baranof
Island; progress report for Chatham Area, Tongass National Forest. 38 p. Available
from: U.S. Department of Agriculture, Forest Service, Alaska Region, Juneau, AK.

Deal, R.L.; Farr, W.A. 1994. Composition and development of conifer regeneration
in thinned and unthinned natural stands of western hemlock and Sitka spruce in
southeast Alaska. Canadian Journal of Forestry Research. 24(5): 976-984.

Deal, R.L.; Oliver, C.D.; Bormann, B.T. 1991. Reconstruction of mixed hemlock-
spruce stands in coastal southeast Alaska. Canadian Journal of Forestry
Research. 21(5): 643-654.

Franklin, J.F.; Cromack, K.; Denison, W. [and others]. 1981. Ecological
characteristics of old-growth Douglas-fir forests. Gen. Tech. Rep. PNW-118.
Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest
Forest and Range Experiment Station. 48 p.

Franklin, J.F.; Pechanec, A.A. 1968. Comparison of vegetation in adjacent alder,
conifer, and mixed alder-conifer communities. |: Understory vegetation and stand
structure. In: Trappe, J.M.; Franklin, J.F.; Tarrant, R.F.; Hansen, G.M., eds.
Biology of alder. Portland, OR: U.S. Department of Agriculture, Forest Service,
Pacific Northwest Forest and Range Experiment Station: 37-42.

Harris, A.S. 1967. Natural reforestation on a mile-square clearcut in southeast
Alaska. Res. Pap. PNW-52. Portland, OR: U.S. Department of Agriculture, Forest
Service, Pacific Northwest Forest and Range Experiment Station. 16 p.

Harris, A.S. 1969. Ripening and dispersal of a bumper western-hemlock-Sitka
spruce seed crop in southeast Alaska. Res. Note PNW-105. Portland, OR: U.S.
Department of Agriculture, Forest Service, Pacific Northwest Forest and Range
Experiment Station. 11 p.

Harris, A.S.; Farr, W.A. 1974. Forest ecology and timber management: the forest
ecosystem of southeast Alaska. Gen. Tech. Rep. PNW-25. Portland, OR: U.S.
Department of Agriculture, Forest Service, Pacific Northwest Forest and Range
Experiment Station. 109 p.

Harris, A.S.; Hutchison, O.K.; Meehan, W.R. [and others]. 1974. The setting: the
forest ecosystem of southeast Alaska. Gen. Tech. Rep. PNW-12. Portland, OR:
U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and
Range Experiment Station. 40 p.

Oliver, C.D.; Larson, B.C. 1990. Forest stand dynamics. Getz, Wayne M., ed.
Biological Resource Management. New York: McGraw-Hill, Inc. 467 p.

U.S. Department of Agriculture, Forest Service. 1991. Tongass land management
plan revision. Misc. Publ. R-10-MB-146. Juneau, AK. 820 p.

U.S. Department of Agriculture, Forest Service. 1994. Alternatives to clearcutting
in the old-growth forests of southeast Alaska: a collaborative effort in ecosystem
management research by the Pacific Northwest Research Station and the Alaska
Region. Juneau, AK: U.S. Department of Agriculture, Forest Service. 69 p.

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