Dwarf mistletoe infection from residual western hemlock on cutover stands

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FOREST AND RANGE
EXPERIMENT STATION

-USDA FOREST SERVICE RESEARCH NOTE|

PNW-278 October 1976

DWARF MISTLETOE INFECTION FROM RESIDUAL
WESTERN HEMLOCK ON CUTOVER STANDS

FOREST AND RANGE
IPERIMENT STATION

{MARS 1977 by

J. L. Stewart V

ION LIBRARY COPY

ABSTRACT

The percentage of dwarf mistletoe-infected hemlock trees
within 30 ft of an infected residual overstory tree increased
rapidly with age of the stand. Infected unmerchantable trees
should therefore be destroyed during or as soon after harvest
as is possible.

KEYWORDS: Parasites (plant) (- forest damage, dwarf
mistletoe, Arceuthobtum tsugense, western
hemlock.

infested with dwarf mistletoe from.
residual trees. Smith (1966) stated that

Until recent years, hemlock dwarf
mistletoe was generally not considered

an important problem by most forest
managers. Infected residual trees were
not removed during or after clearcutting.
As a result, regenerated stands on
thousands of acres of Federal, State,

and private lands are now becoming

1
a At the time this research was conducted
the author was Project Leader at the Pacific

Northwest Forest and Range Experiment Station,
He is now Director of Forest

Corvallis, Oregon.
Insect and Disease Management, USDA Forest
Service, Washington, D.C.

as few as 10 evenly spaced infected
residual trees per acre are sufficient to
reinfest the trees on an entire acre.
Land managers, responding to recent
publications (Shea and Stewart 1972,
Smith 1969) and efforts by pest control
pathologists, now want to remove resid-
This raises the
question--when do the new stands
become so infested that it no longer pays
to make a special entry into the stand
to remove the residuals?

ual infested trees.

REST SERVICE - U.S. DEPARTMENT OF AGRICULTURE - PORTLAND, OREGON - :

Hawksworth and Graham (1963),
working with lodgepole pine, showed
the percentage of understory trees
infected to be correlated with stand age.
This paper reports an attempt to corre-
late the percentage of infected young
hemlock with stand age.

Methods and Materials

Transects, 10-ft-wide by 30-ft-long,
radiating from the bases of infected
residual overstory trees, were estab-
lished in pure hemlock stands ranging
in age from 10 to 25 years. Smith
(1966) shows that over 95 percent of the
mistletoe seeds dispersed from an
overstory tree fall within 35 ft of the
tree. Density varied from 725 trees
per acre in the lightest stocked tran-
sects, to 18, 705 trees per acre in the
most heavily stocked transects. The

PERCENT TREES INFECTED
100

90
80
70
60
50
40
30
20

10

stands were located on lands managed
by the Bureau of Indian Affairs, U.S.
Forest Service, and Crown-Zellerbach ~
Corporation in western Washington and
Oregon. A total of 39 overstory trees b |
and 52 transects were used. Data were
averaged when there was more than one q
transect for an individual overstory tree,

Each tree within a transect was
examined for the presence of dwarf
mistletoe shoots. Stand age was deter- —
mined by counting annual rings of a few
trees cut at the ground line.

Results and Discussion

Within 30 ft of an infected residual
overstory tree, the percentage of trees
visibly infected with dwarf mistletoe
increases with age of the stand (fig. 1).
Although the relationship is highly

STAND AGE

Figure 1.--Percentage of infected trees within 30 ft of a

residual infected overstory tree by age of the stand.

significant, less than 50 percent of the

| variation is accounted for by stand age.
The unaccounted variation could be due

| to other factors influencing infection

such as stand density, tree size, or

/ inoculum in overstory trees. These

| factors were not included in the analysis
because of extreme variation. Although the
equation, log, Y = -4. 368 + 2. 676 (log, X),
cannot be used as a precise estimator of
percentage of trees infected, it does show
the trend of rapid increase in percentage
of trees infected with stand age. The
equation stresses the importance of
eliminating all infected residuals at or
soon after the time of harvest.

In stands 20 years and older, resid-
ual overstory trees were difficult to
locate because of the density and height
of the stand. Barnes (1962) gives the
average height of dominant and codomi-
nant trees at 20 years on site index 150
as 31 ft. In such stands, residuals
could be spotted from roads, on aerial
photos, or from opposite slopes--but
only with great difficulty from within
the stand. Under these circumstances,
residual tree removal is impractical
except when done during thinning or
other cultural operations.

Residual tree removal as a separate
operation is practical in stands 15 to 20
years of age or younger; but even in
these stands, it may be advantageous to
combine residual tree removal with pre-
commercial thinning. Priority for resid-
ual tree removal depends on the manager's
objective for the area. Youngest stands
should have priority if the goal is to
reduce dwarf mistletoe infection to a
minimal level on as many acres as
possible. Older stands should have
priority if the goal is to prevent infection
from reaching moderate levels. The
number and distribution of residuals per

acre should also be a criterion for setting

priorities in stands under 20 years of age.

Stands with a greater number of residuals
per acre should be given higher priority
than those with fewer residuals.

Acknowledgment

Appreciation is given to the Bureau
of Indian Affairs, U.S. Department of
the Interior, and to Crown- Zellerbach
Corporation for their assistance in this
study.

Literature Cited

Barnes, (Ge Hi
1962. Yield of even-aged stands of
western hemlock. U.S. Dep. Agric.
ech. (Bulll27 3, -5e ape
Hawksworth, F. G., and D. P. Graham.
1963. Spread and intensification of
dwarfmistletoe in lodgepole pine

reproduction. J. For. 61(8):587-591.

Shea ke, Re. andi. ds. wtewant.
1972. Hemlock dwarf mistletoe.
USDA For. Serv. For. Pest Leafl.
1 S55 (0) yoy.
Simith,, Re Bi
1966. Hemlock and larch dwarf
mistletoe seed dispersal. For.
Chron. 42:395-401.
Smith), oR.) Be
1969. Assessing dwarf mistletoe on
western hemlock. For. Sci. 15(3):
277-285,

aa ar oe

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