MORTALITY OF ASPEN ON THE
GROS VENTRE
ELK WINTER RANGE
R. G. Krebill
USDA Forest Service Research Paper INT-129, 1972
INTERMOUNTAIN FOREST AND RANGE
EXPERIMENT STATION
Ogden, Utah 84401
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
USDA F o r e s t Service
Research Paper INT- 129
November 1972
MORTALITY OF ASPEN ON THE
GROS VENTRE
ELK WINTER RANGE
R. G. Krebill
INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STATION
F o r e s t Service
U. S. Department of Agriculture
Ogden, Utah 84401
Robert W. H a r r i s , Director
R. G. KREBILL, Plant Pathologist, stationed at the Forestry Sciences
Laboratory, Logan, Utah, joined the Intermountain Station staff
in 1962. He holds the bachelor of science degree in forestry
from the University of California and the doctoral degree in plant
pathology from the University of Wisconsin. He has published
many articles on rusts of conifers, and recently has undertaken
research on diseases of trees and shrubs important to wildlife
and recreation.
Special recognition is extended to George E. Gruell, Teton National Forest, and Norbert V. DeByle, Intermountain Station, for their
encouragement and advice on aspen ecology and the ways of wildlife; to
Chester E. Jensen, Intermountain Station, for his help in sampling
design; to T. E. Hinds, Rocky Mountain Station, for training in recognition of aspen diseases; and to Jack D. Rogers, Washington State University, and Flora G . Pollack, USDA. Agricultural Research Service,
Beltsville, who helped identify specimens of some of the problem fungi.
To these and other helpful friends, the author is deeply grateful.
CONTENTS
Page
................
METHODS . . . . . . . . . . . . . . . . . . .
RESULTS . . . . . . . . . . . . . . . . . . .
Age of Overstory Trees . . . . . . . . . . . .
Mortality of Overstory Trees . . . . . . . . .
Aspen Sprouts . . . . . . . . . . . . . . . .
INTRODUCTION
Miscellaneous Information on Injury.
Pathogens. and Insects
..........
Associated Understory Vegetation . . . . . . .
DISCUSSION AND CONCLUSION . .
.. . .
LITERATURE CITED . . . . . . . . . . . . . .
1
4
6
6
6
8
ABSTRACT
Stands of aspen on the Gros Ventre elk winter range of northwestern
Wyoming a r e suffering high mortality and are not regenerating satisfactorily. If the 1970 mortality rate (3.6 percent) continues, about a two-thirds
reduction in the numbers of tree-sized aspen can be expected by year
2000. Collected evidence suggests that the mortality rate i s unusually
high because of a combination of pathogenic fungi, injurious insects, and
physiological stress that follow bark wounding of tree trunks. Elk and
possibly moose a r e suspected of causing most of these severe trunk injuries. Sooty bark canker, Cytospora canker, sllld stem-boring insects
were the most common pests associated with tree mortality. The prospect for aspen on the elk winter range i s especially critical because of the
heavy impact of browsing and pests on aspen sprouts which prohibits
natural replacement of the dying aspen overstory.
INTRODUCTION
Aspen (PopuZus trernuloides) provide b e a u t i f u l scenery and important w i l d l i f e
h a b i t a t on the Gros Ventre e l k winter range of t h e Teton National Forest i n northwestern Wyoming. Although aspen occupies only about 4 percent of the winter range, t h e
type u s u a l l y i s i n a conspicuous p o s i t i o n i n t h e lower and middle elevations where it
o f t e n forms t r a n s i t i o n zones between shrub rangelands and conifer f o r e s t s . Ecological
succession has not been s a t i s f a c t o r i l y documented f o r t h e Gros Ventre, but t h e aspen
type of t h e general a r e a i s now mostly considered a s being sera1 t o conifers and t o have
been perpetuated by w i l d f i r e (Gruell 1970; and Loope 1971). Fires of t h e 1870's and
1880's a r e thought t o have been p a r t i c u l a r l y important i n shaping t h e current vegetat i o n , but more r e c e n t l y " . . . f i r e p r o t e c t i o n has allowed many aspen stands t o be invaded
by c o n i f e r species" (Gruell 1970). Some -2,000 t o 5,000 e l k (Cervus canadensis nelsoni)
a r e thought t o winter i n t h e Gros Ventre area. Supplemental winter feed is supplied a t
t h r e e Gros Ventre s t a t i o n s , but e l k a l s o feed on n a t i v e t r e e s , shrubs, sedges, grasses,
and forbs. Heavy e l k use of aspen has, f o r several decades, been noted by t h e occurrence of "highlined" t r e e s and browsed s p r 0 u t s . l Elk and also moose (AZces aZces
s h i r a s i ) apparently contribute t o t h e conversion of aspen stands t o conifers (Beetle
1968; and Gruell 1970). These animals cause i n j u r i e s ( f i g . 1 ) t h a t presumably induce
s u s c e p t i b i l i t y of aspen t o disease (Loope 1971) and they consume sprouts t h a t might
otherwise replace dying overstory t r e e s . In numerous stands, aspen a r e obviously dying
a t an alarming r a t e ( f i g . 2 ) . When conifers a r e absent, it appears t h a t d e t e r i o r a t i n g
aspen stands w i l l r e t r o g r e s s t o a t r e e l e s s rangeland type (fig. 3 ) .
The study described herein w a s i n i t i a t e d t o provide an up-to-date p r e d i c t i o n of
t h e f u t u r e of aspen i n t h e Gros Ventre area. Emphasis was placed on determining t h e
r a t e of m o r t a l i t y of t h e aspen overstory, i t s causes, and t h e likelihood of overstory
replacement by n a t u r a l sprouting. Hopefully, these findings w i l l inform resource
managers about t h e s e v e r i t y of t h e aspen d e t e r i o r a t i o n problem so t h a t they might b e t t e r
determine whether modifications i n current management might be necessary t o maintain
t h e pleasing d i v e r s i t y of f o r e s t types now present i n t h e Gros Ventre.
llnspection r e p o r t s of Teton National Forest a s relayed by George E. Gruell,
Wildlife Biologist, Teton NF, Jackson, Wyoming.
Figure 1 .--Excessive game use of aspen i s especiaZly evident i n close proximity t o elk
feedgrounds where trees have suffered severe bark damage and sprouts are
browsed down t o the snow ZeveZ.
-~
..
~
Figure 2.--Late stages of aspen deterioration i n a stand south of Upper SZide Lake.
Areas such as t h i s are currentzy changing from an aspen type t o a grass-forb
parkZand with scattered conifers. Many other deteriorating stands succeed more
rapidZy toward a conifer type, but some cuZminate i n shrub, grass, and forb
communities.
Eigure 3.--Deteriorated stand of aspen on Breakneck Ridge east of Upper Slide Lake.
Although ,it once supported a nearly continuous aspen c r m n canopy over several
acres, t h i s s i t e has reverted t o a rangeland dominated by shrubs, forbs, grasses,
and decaying aspen remnan5s.
METHODS
The 84,000 a c r e s of t h e middle Gros Ventre drainage selected f o r t h i s study contained most of t h e aspen stands influenced by wintering e l k ( f i g . 4 ) . Within t h i s a r e a
100 sample p l o t s were located by a method i n which one p l o t occurred a t random within
each 33.3 acres of t h e 3,330 a c r e s of aspen type delineated on t h e Forest Service 1956
Timber Type Map. Each p l o t was c i r c u l a r and 1/20 acre i n s i z e . Aspen ages were
estimated by increment cores extracted a t t h e 10-inch height from two t r e e s containing
wood sound enough f o r r i n g a n a l y s i s and located nearest t h e p l o t centers. Ring counts
were made i n t h e laboratory with t h e a i d of t h e acid-phloroglucinol s t a i n i n g technique
of Patterson (1959). A l l aspen t r e e s (individual stems t a l l e r than 10 f e e t ) within
p l o t s were t a l l i e d by 6-inch d.b.h. s i z e c l a s s e s and condition c l a s s e s : i . e . , l i v e ,
died i n t h e current season, o r died before t h e current season. Trees were assigned t o
c u r r e n t year m o r t a l i t y i f unweathered dead leaves s t i l l remained i n t h e crown. Dead
t r e e s t h a t had f a l l e n t o t h e ground were not included i n t h e sample unless they were
windfall of t h e current season.
Sampling began i n August 1970 and was completed i n September, before f a l l colorat i o n . This s h o r t time span insured a f a i r l y comparable estimation of current m o r t a l i t y
on a l l p l o t s . Special e f f o r t s were made t o record i n d i c a t o r s t h a t might suggest t h e
cause of m o r t a l i t y i n both c l a s s e s of dead t r e e s and t h e presence of i n j u r y o r disease
on a l l t r e e s . Aspen sprouts within t h e p l o t s were t a l l i e d , t h e i r heights determined,
and any evidence of i n j u r y from browsing, i n s e c t s , o r diseases was recorded. The
presence and ground coverage provided by young conifers, conspicuous shrubs, grasses,
and f o r b s were a l s o noted s i n c e understory vegetation can provide clues a s t o t h e
degree o f inherent stand s t a b i l i t y and disturbance.
Figure 4.--Boundaries of the Gros Ventre study area, Teton NationaZ Forest, Teton
County, Wyoming.
RESULTS
Age of Overstory Trees
A s determined by r i n g c o u ~ l t s , t h e t r e e s sampled i n t h e Gros Ventre were r e l a t i v e l y
o l d ; none were l e s s than 40 years of age, and t h e majority were between 81 and 120 years.
The following t a b u l a t i o n shows t h e age d i s t r i b u t i o n of sample t r e e s :
Age cZass
Years
Frequency
Percent
Mortality of Overstory Trees
The combined sample from a l l p l o t s t o t a l e d 2,332 l i v e t r e e s and 1,684 dead t r e e s ,
of which 86 had apparently died during t h e 1970 season. These data a r e transformed t o
a per-acre b a s i s (mean number of t r e e s p e r acre) and presented i n g r e a t e r d e t a i l i n t h e
following t a b u l a t i o n :
Size cZass, i n . d.b. h.
Combined
Live trees
Dead trees
Died i n 1970
Wind breakage, sooty bark canker, and llbarking" complemented by weak parasites and
stem-boring insects accounted for most of the aspen mortality on the sample plots, as
shown in the following tabulation:
Size class
i n . d.b. h.
Combined
Wind breakage
Percent
2
Sooty bark canker
Percent
35
"Barking, " weak
parasites, pests, e t c .
Percent
63
Death resulting from wind breakage or windfall was infrequent in small trees but
rather common in the larger tree classes.
Cenangiwn singutare, cause of sooty bark canker, was the single most injurious pathogen
found on dead trees (fig. 5), and it too was especially prevalent in the larger tree
classes. The third group was composed of trees that seemed to have died from a mixture
of the following: bark wounding by elk or moose; infestation by stem-boring insects
(probably species of Agrilus) (fig. 6); infection by bark fungi (including Cytospora
sp.); suppression; and occasionally,, sunscald. Mortality caused by this mixed category
was especially prominent among the smaller trees.
:gure 5. --Sooty bark canker,
caused by the fungus Cenangium
singulare i n game-wounded port i o n of the aspen trunk on the
l e f t . The t r e e on t h e r i g h t
was a l s o scarred by game, b u t
otherwise has remained healthy
Note the advanced degree o f
stand d e t e r i o r a t i o n and the
absence of v i s i b l e sprouts.
Figure 6. --GaZZeries of
stem-boring insects,
preswnabzy Agrilus sp.
i n cambia2 zone as see
when dead bark i s
stripped from aspen
trunk.
Aspen Sprouts
Even though generally hidden by grasses, forbs, and shrubs ( f i g . 7 ) , aspen sprouts
were found i n 96 of t h e 100 p l o t s . Expanded t o a per-acre b a s i s and expressed a t the
0.05 confidence i n t e r v a l , t h e mean number of aspen sprouts ranges from 463 t o 843 per
a c r e , but 20 p l o t s had l e s s than 100 aspen sprouts per acre. Sprouts of 84 p l o t s
showed indications of having been browsed down annually t o l e s s than 2 f e e t i n height.
Sprouts surv'ived and grew beyond a height of 3 f e e t i n only t h r e e p l o t s .
Miscellaneous Information on Injury, Pathogens, and Insects
Although diagnosis of m o r t a l i t y was emphasized during f i e l d evaluation of p l o t s ,
t h e more obvious i n j u r i e s , pathogens, and i n s e c t s were noted and recorded. The
Figure 7 . --Aspen sprout
browsed o f f t o about 1 4
inches high. There were
an average of 653 sprouts
per acre on s q Z e pZots,
b u t most were overtopped
by forbs as i s shown i n
t h i s photo. Although
aspen sprouts were
present on 96 pZots,
they were actuaZZy
rep lacing dying t r e e s
on only three plots;
aZZ other aspen sprouts
were held i n check by
browsing and pest
damage.
incidence of disease and i n s e c t problems i n stems expressed as a percentage of t h e
aspen t r e e sample (n=4,016) is shown i n t h e following t a b u l a t i o n :
Prob Zem
Causa Z organism
Bark wounds
Stem-boring i n s e c t s
Cytospora canker
Sooty bark canker
Cryptosphaeria canker
Heartrot
Ceratocystis canker
Rough stem g a l l s
Roundheaded borer g a l l s
Buttrot
do,.
Agri Zus spp .
Cytospora chrysospema
Cenangiwn singuZare
Cryptosphaeria popu Zina
Fomes igniarius
Ceratocystis fimbriata
D i p Zodia twnefaciens
Saperda sp.
Fomes app lanutus
PhoZiota squarrosa
2 ~ d e n t i f i e dby symptoms only.
Big game and rodents
Plot
incidence
The incidence of diseases and i n s e c t s i n aspen f o l i a g e i n 1970 expressed i n terms
of presence i n a l l p l o t s (n=100) i s shown i n the following t a b u l a t i o n :
Prob Zern
Causa Z organism
Aspen l e a f miner
Aspen l e a f b l i g h t
Aspen shoot b l i g h t
Phy ZZocnistis popuZie ZZa
Marssonina popuZi
PoZZaccia radiosa
PZot
incidence
53
13
8
Inasmuch a s only t h e more obvious i n j u r i e s and p e s t s were recorded, t h e frequency
and various types of t r e e Bamage problems probably a r e underestimated i n these tabulat i o n s . For example, conks (used t o i n d i c a t e incidence of h e a r t and b u t t r o t ) were evident
on l e s s than 2 percent of t h e t r e e s sampled, but h e a r t r o t was present i n 33 percent of
t h e increment cores examined f o r age data. Furthermore, some types of aspen damage i n
t h e Gros Ventre were not included because they occurred outside the 100 sample p l o t s .
For example, beaver (Castor canadensis) had damaged t r e e s immediately adjacent t o one
p l o t but t h e r e was no beaver damage i n any of the p l o t s .
Many of t h e problems l i s t e d i n t h e t a b u l a t i o n s on pages 9 and 10 a r e combined i n
t h e m o r t a l i t y f i g u r e s shown i n t h e t a b u l a t i o n on page 7. For instance, Fomes appZanatus
had r o t t e d t h e r o o t s of a t l e a s t t h r e e t r e e s t h a t succumbed t o windfall. Likewise,
h e a r t r o t was present i n many o t h e r t r e e s t h a t were included i n "wind breakage," and a t
l e a s t one t r e e broke o f f a t a Ceratocystis canker.
In addition t o t h e d i r e c t e f f e c t s of bark i n j u r y caused by game and rodents, it
was a l s o noted t h a t 90 percent of t h e sooty bark cankers occurred i n the lower p a r t of
t r e e s where t h e trunks had been scarred by animals. Elk were responsible f o r most of
these bark s c a r s , but moose, mice, and hares a l s o contributed.
Of t h e f o l i a g e p e s t s , shoot b l i g h t was limited t o sprouts, but l e a f b l i g h t and
t h e aspen l e a f miner occurred both on sprouts and on overstory leaves. Aspen l e a f
hoppers were r a r e , but occurred on sprouts i n a t l e a s t one p l o t .
Associated Understory Vegetation
The mean understory vegetation was composed of about 49 percent forbs, 36 percent
grasses and occasional sedges, 12 percent shrubs, and 3 percent conifer saplings. Each
of these groups contained a d i v e r s i t y of species. The frequency of occurrence of t h e
more conspicuous t r e e s , shrubs, and forbs a r e presented i n t a b l e 1. Grasses and sedges
were not i d e n t i f i e d except f o r pinegrass (CaZamagrostis rubescens), which on an average
occupied 66 percent of t h e ground cover i n the eight p l o t s i n which it occurred.
Although forbs were present i n a l l p l o t s and grasses o r sedges were found i n a l l but
one p l o t , shrubs were absent from 10 p l o t s and conifer saplings were missing i n 27 p l o t s .
53
5
5
7
7
41
7
54
Shepherdia canadensis
Symphoricarpos oreophiZus
Berberis repens
Artemisia tridentata
PotentiZZa fruticosa
Juniverus communis
ArctostaphzlZos uva-ursi
: No. of
: plots
ROS~
spp.
Shrubs
: No. o f
: plots
.
Geranium richardsonii
85
Lupinus arqenteus
80
ThaZictrwn occidentale
65
GaZium boreale
49
Fragaria sp.
46
AchiZ Zea miZZefoZium
38
Senecio spp.
34
Aster spp.
32
EpiZobiwn angustifo Ziwn
30
Lathyrus sp. and AstragaZus sp. 23
PotentiZZa graciZis
22
PedicuZaris bracteosa
14
CastiZZeja suZphurea
14
HeZianthe ZZa unifZora
14
Forbs
t h e r e were 16 s p e c i e s of f o r b s not l i s t e d because t h e y were noted i n fewer than 10 p l o t s . This included
p o s s i b l e i n d i c a t o r s p e c i e s such a s Frasera speciosa, BaZsamorhiza sagittata, Perideridia gairdneri, and
PyroZa secunda. There were some f o r b s i n t h i s group (such a s AZZiwn s p . , Tarmacwn s p . , and Agoseris
gZauca) t h a t probably were more f r e q u e n t b u t were i n a d v e r t e n t l y overlooked because of t h e i r decadent
c o n d i t i o n a t t h e time of sampling.
l ~ h eshrubs Lonicera invoZucrata, Prunus virginiana, SaZix scouZeriana, Ribes spp , AmeZanchier
aZnifoZia, Sambucus s p . , and Pachistima myrsinites were p r e s e n t i n fewer than f i v e of t h e p l o t s . Also,
2
16
16
39
35
: No. o f
: plots
Picea engehannii
Pinus fZexiZis
Pinus contorta
Abies Zasiocarpa
Pseudotsuga menziesii
Trees
Table 1.--Constancy of trees, shrubs, and forbs comonZy found in the 200 sample p ~ o t s l
DISCUSSloN AND CONCLUSION
Since aspen generally a r e known t o be short-lived t r e e s (Strothmann and Zasada
1957), it i s n a t u r a l t h a t some aspen stands a r e becoming decadent i n t h e Gros Ventre.
bleinecke (1929) noted f o r nearby Utah t h a t s e n i l i t y occurs with aspen a t about 120
years of age and t h a t the "wild aspen f o r e s t as a whole does not reach much beyond
130 years." I f t h i s i s t r u e f o r t h e Gros Ventre, we can expect a marked change i n
much of t h e aspen type within t h e next few decades s i n c e about two-thirds of t h e
sampled t r e e s a r e already i n the 81- t o 120-year age c l a s s . Furthermore, a d r a s t i c
decline i n t h e aspen type i s eminent i f game browsing continues t o p r o h i b i t replacement
of dying overstory t r e e s and i f exclusion of f i r e continues t o allow invasion by
conifers
.
Even i f t h e aspen s e n i l i t y and decadence s i t u a t i o n s of Utah do not apply d i r e c t l y
t o t h e study area, t h e Gros Ventre m o r t a l i t y data (see page 6, second tabulation)
provide a dismal view. Using those data we can estimate t h a t i n 1970 t h e r e were
approximately 1,500,000 aspen t r e e s i n t h e 3,330 acres of aspen type i n t h e Gros Ventre
sample area. I f the 3.6 percent m o r t a l i t y r a t e of 1970 remains s t a b l e i n t h e f u t u r e ,
aspen would decline t o about 500,000 t r e e s by year 2000, and t o only 40,000 by year
2070. Although t h e 3.6 percent r a t e was a f a i r estimate3 based on t h e c r i t e r i a used
t o estimate current year mortality, it probably was conservative since it could not
have included t r e e s which died i n 1970 during winter dormancy. Certainly, these d a t a
i n d i c a t e t h a t unless t h e r e is replacement of overstory t r e e s , the aspen overstory i s
due f o r a marked decline.
Although few comparative data a r e a v a i l a b l e , aspen m o r t a l i t y appears t o be somewhat
g r e a t e r on the Gros Ventre than i n o t h e r aspen areas. For instance, Hinds (1964) found
t h a t only 9 percent of t h e standing t r e e s were dead i n sample p l o t s s c a t t e r e d over a
wide a r e a of Colorado, whereas almost 42 percent i n t h e Gros Ventre sample were dead.
Assuming t h a t most aspen t r e e s remain standing f o r about 10 years a f t e r death (which
approximates the measured 1970 Gros Ventre m o r t a l i t y r a t e ) , t h e r e appears t o be l e s s
3 5 ~ hconfidence
e
i n t e r v a l a t t h e 0.05 l e v e l was only 2.6 t o 4.6 percent as determined from a standard e r r o r computed as described by Freese (1962) f o r a t t r i b u t e s from
unequal-sized c l u s t e r s .
than 1 percent annual m o r t a l i t y i n Colorado as compared t o t h e 3.6 percent r a t e i n t h e
Gros Ventre. Furthermore, t h e Gros Ventre r a t e i s perhaps two t o four times g r e a t e r
than reported r e c e n t l y by Manion and Valentine (1971) i n New York. Also, i n Ephraim
Canyon of c e n t r a l Utah, D r . K. T. ~ a r ~ found
e r ~about 2.7 percent annual m o r t a l i t y i n
a sample of 2,091 aspen l a r g e r than 4 inches d.b.h. Harper's technique involved reexami n a t i o n i n 1970 of t r e e s o r i g i n a l l y examined and tagged as being dead i n 1969. When
examining Harper's sample t r e e s i n autumn of 1970, we found t h a t many would not have
been t a l l i e d a s 1970 m o r t a l i t y by the c r i t e r i o n used i n t h e Gros Ventre study; these
were t r e e s t h a t had not maintained dead leaves and were assumed t o have succumbed during
t h e 1969-1970 dormant season. From t h i s we conclude t h a t t h e 3.6 percent annual mort a l i t y measured i n t h e Gros Ventre probably is an underestimate of actual mortality.
In t h e Gros Ventre, it appears t h a t increased m o r t a l i t y has been caused by big game
"barking" combined with pathogens and i n j u r i o u s i n s e c t s t h a t invade t h e wounded t r e e s .
Most of t h e 1,061 t r e e s included i n t h e "Barking, weak p a r a s i t e s , p e s t s , etc." category
of t h e t a b u l a t i o n on page 7 had s c a r s r e s u l t i n g from b i g game chewing and some had been
completely girdled. "Barking" damage was most i n j u r i o u s t o t h e smaller t r e e s ; t h i s
p a r t i a l l y accounts f o r t h e e s p e c i a l l y high r a t e of m o r t a l i t y i n t h e 0- t o 6-inch c l a s s .
Probably though, t h e higher m o r t a l i t y i n smaller t r e e s was caused mostly by p a r t i a l
suppression which reduced t h e i r r e s i s t a n c e t o weak pathogens and stem-boring i n s e c t s t h a t
Cytospora chrysospema probably i s a primary
c h a r a c t e r i s t i c a l l y follow "barking."
c o n t r i b u t e r t o the demise of such t r e e s . This fungus was thought t o be a major f a c t o r
i n t h e aspen dieback associated with s i m i l a r e l k wounding of .trees i n Rocky Mountain
National Park (Packard 19421, and " . . . i t i s generally considered a weak p a r a s i t e on
declining o r dying t r e e s " (Hinds 1964). Although encountered infrequently, Cryptosphaeria popuZina seemed t o replace Cytospora a s an invader of wounded bark i n t h e
dead aspen t r e e s encountered i n some of t h e Gros Ventre p l o t s . I t is not known whether
t h i s fungus is p a r a s i t i c on aspen, but these observations suggest t h a t it may be a weak
p a r a s i t e on t h i s species and s i m i l a r i n aggressiveness t o Cytospora.
The sooty bark fungus (Cenangiwn sing~lZarel i s an aggressive pathogen t h a t can
e n t e r wounded t i s s u e s and g i r d l e and k i l l aspen t r e e s within 2 t o 4 years (Hinds 1962).
In t h e Gros Ventre, t h i s fungus appears t o be frequently aided by AgriZus b e e t l e s .
Although Hinds (1964) found t h a t e l k wounds on aspen i n Colorado seldom served as
entrance p o i n t s f o r canker organisms, t h e sooty bark fungus i n the Gros Ventre was more
frequently associated with.big game wounds than would be expected by mere coincidence.
Perhaps t h i s r e s u l t s from more severe wounding i n t h e Gros Ventre, o r possibly wounding
occurs a t a more favorable time f o r fungus i n f e c t i o n . In t h i s respect, it should not
be overlooked t h a t moose a r e abundant i n the Gros Ventre and might be "barking" some
aspen t r e e s during t h e growing season. In t h i s study, a few f r e s h "barking" wounds
were found i n l a t e September ( f i g . 8) i n an area where only moose sign was f r e s h . I n
c o n t r a s t , o t h e r observations i n t h e Gros Ventre sample area suggest t h a t most of t h e
e l k "barking" occurs i n winter between December and May.
Harmful fungi and i n s e c t s were not p a r t i c u l a r l y severe on aspen f o l i a g e during
1970, but it i s suspected t h a t i n years when they a r e epidemic they might complement
t h e aggressiveness of other pathogens, or a s Mielke (1957) suggested f o r Marssonina,
some aspens might even d i e from heavy a t t a c k s . In s p i t e of t h e appeal of t h i s l o g i c
t h e Gros Ventre data f a i l e d t o support a r o l e by Marssonina i n aspen mortality; t h e 13
p l o t s showing l e a f b l i g h t had s i m i l a r mean numbers of l i v e and dead t r e e s (20.5 and
14.8, respectively) a s d i d p l o t s without Marssonina (23.7 and 17.1, r e s p e c t i v e l y ) . The
hypothesis might b e t t e r be t e s t e d during and a f t e r epidemic conditions.
&personal communication, D r . K. T. Harper, University of Utah, S a l t Lake City,
based on research while employed by Intermountain S t a t i o n .
13
Figure 8 . --Big game scars
on a 9-inch-diameter
aspen trunk. The fresh
toothmarks near the
center and Zower r i g h t
of t h i s Zate-summer
photo are suspected t o
have been made by a
moose. The scar on the
upper Z e f t ZikeZy was
from e l k chewing of the
previous winter and
t h a t on t h e r i g h t from
eZk a year e a r l i e r .
Unfortunately, m o r t a l i t y tends t o open t h e crown canopy which can i n i t i a t e a chain
of events t h a t r e s u l t i n rapid stand d e t e r i o r a t i o n . Graham, Harrison, and Westell
(1963) noted t h a t d e t e r i o r a t i o n following t h e opening of aspen crown canopies can somet o a worthless
times render "...within a 5- o r 10-year period a valuable stand
condition." Presumably, increased exposure t o sunlight and associated sunscald t h a t
r e s u l t s from opening t h e canopy increases t h e s u s c e p t i b i l i t y of remaining t r e e s t o harmf u l i n s e c t s and diseases. This is an alarming h p o t h e s i s f o r t h e Gros Ventre e l k winter
range where most stands of aspen a r e already i n a t l e a s t t h e e a r l y stages of d e t e r i o r a t i o n . I t a l s o seems reasonable t o suspect, a s Beetle (1968) suggested, t h a t remaining
t r e e s become more "vulnerable t o wind" as stands a r e opened up through overstory
mortality.
...
I t should n o t be overlooked t h a t some m o r t a l i t y i n a r e l a t i v e l y dense aspen overs t o r y might improve w i l d l i f e h a b i t a t by increasing forage production i n the understory.
Concerning t h e Coconino National Forest, Reynolds (1969) reported t h a t thinning aspen
from 229 s q . f t . basal area per acre t o 42 s q . f t . per acre s i g n i f i c a n t l y increased t h e
production of forbs, perennial grasses, and aspen sprouts. In the Gros Ventre, l i v e
aspens averaged only about 138 s q . f t . per acre i n t h e 1970 p l o t s , but 16 percent of t h e
p l o t s had l e s s than h a l f t h a t amount. Natural m o r t a l i t y seldom r e s u l t e d i n f a l l e n t r e e s
i n numbers s u f f i c i e n t t o c r e a t e b a r r i e r s t o animal movement such as Reynolds had noted
and it i s assumed t h a t increased forage is r e a d i l y a v a i l a b l e t o big game.
(r
In most Gros Ventre p l o t s t h e r e were numerous aspen sprouts
= 653 per a c r e ) ,
but f a r too few escaped browsing and p e s t s f o r successful replacement of overstory
m o r t a l i t y . In f a c t , replacement was occurring on only t h r e e of t h e 100 sample p l o t s .
Insects and diseases were i n j u r i o u s t o sprouts i n addition t o the detrimental e f f e c t of
browsing by e l k , moose, deer, c a t t l e , and rodents. The aspen l e a f miner was prevalent
and Marssonina leaf b l i g h t and l e a f hoppers were observed. On some of t h e wetter s i t e s ,
PoZZaccia shoot b l i g h t was devastating. Sprouts t h a t were s t r i p p e d of leaves by moose
l a t e i n t h e growing season were p a r t i c u l a r l y s e n s i t i v e t o f r o s t i n j u r y , which may part i a l l y account f o r t h e severe impact of browsing.
A study of t h e associated vegetation i n t h e aspen sample p l o t s reinforces t h e
conclusion t h a t much of t h e Gros Ventre aspen i s changing. Of the 100 sample p l o t s ,
only 13 p l o t s showed a species composition t h a t was e s s e n t i a l l y i d e n t i c a l t o t h e s t a b l e
PopuZus tremuloides-Symphoricarpos oreophi Zus h a b i t a t type described by Reed (1971) .
Nevertheless, t h e r e is some encouragement from these 13 p l o t s because t h e i r 1970 mort a l i t y r a t e was only about two-thirds of t h a t i n the remaining 87 p l o t s . Also, these
p l o t s contained a g r e a t e r proportion of t r e e s i n t h e smallest s i z e c l a s s , i n which mort a l i t y was only about one-third t h a t of t h e remaining 87 p l o t s . This supports general
observations suggesting t h a t some stands of aspen w i l l p e r s i s t much longer than o t h e r s .
Forty-seven of t h e 100 p l o t s had spruce o r f i r saplings i n t h e i r understory; t h i s , and
t h e abundance of p l a n t s such a s buffaloberry (Shepherdia canadensis), i n d i c a t e t h a t
perhaps h a l f of t h e aspen i n t h e Gros Ventre occupy s i t e s t h a t a r e somewhat l i k e t h e
Abies Zasiocarpa-PyroZa secunda h a b i t a t type of Reed (1969). I t could a l s o be argued,
however, t h a t t h e occurrence of p l a n t s such a s pinegrass, balsamroot (BaZsamorhiza
s a g i t t a t a ) , and sagebrush (Artemisia t r i d e n t a t a ) , along with t h e presence of conifer
saplings i n 73 p l o t s , i s i n d i c a t i v e of a s t a t e of browsing disclimax ( i n t h e sense of
Beetle 1968) i n otherwise s t a b l e aspen. In e i t h e r case, i t i s evident t h a t i f t h e
current trends continue, perhaps one-half t o three-fourths of t h e a r e a now i n aspen
w i l l succeed t o conifers. Some of t h e remaining w i l l go t o shrub, grass, and f o r b
communities, and a t b e s t , only a token amount w i l l remain as a dominant aspen type.
The high r a t e of m o r t a l i t y i n t h e aspen overstory on t h e ,Gros Ventre e l k winter
range appears t o be caused mostly by a mixture of pathogenic fungi and i n s e c t s which
invade t r e e s wounded by b i g game. The s i t u a t i o n i s c r i t i c a l because the combination of
heavy browsing and p e s t s prevents aspen sprouts from replacing dead t r e e s . I f current
t r e n d s continue, most of t h e aspen type w i l l u l t i m a t e l y be eliminated from t h e Gros
Ventre winter range. Additional d a t a about t h e p o t e n t i a l f o r regenerating these aspen
stands a r e needed before an aggressive and intensive management program can be i n i t i a t e d
t o preserve both t h e aspen type and t h e b i g game herds.
LITERATURE CITED
Beetle, A. A.
1968. Range survey i n Teton County, Wyoming. P a r t 111. Trends i n vegetation.
Univ. Wyo., Agric. Exp. Stn., Laramie, Res. J. 26, 16 p.
Freese, F.
1962. Elementary f o r e s t sampling. USDA Agric. Handb. 232, 91 p.
Graham, S. A., R. P. Harrison, Jr., and C. E. Westell, Jr.
1963. Aspens--phoenix t r e e s of t h e Great Lakes Region. 272 p. Ann Arbor: Univ.
Mich. Press.
Gruell, G. E.
1970. Elk h a b i t i n v e s t i g a t i o n s , Teton National Forest. In: Jackson Hole Coop.
Elk Stud. Advis. Counc. Meeting, March 12, 1970, 24 p.
Hinds, T. E.
1962. Inoculations with t h e sooty-bark canker fungus on aspen. Plant D i s . Rep.
46 :57-58.
Hinds, T. E.
1964. D i s t r i b u t i o n of aspen cankers i n Colorado. Plant D i s . Rep. 48:610-614.
Loope, L. L.
1971. Dynamics of f o r e s t communities i n Grand Teton National Park. N a t u r a l i s t
22 ~39-47.
Manion, P. D . , and F. A. Valentine
1971. Diseases of trembling aspen i n t h e Adirondack Region of New York. Plant
D i s Rep. 55 :662-665.
Meinecke, E. P.
1929. Quaking aspen--a study i n applied f o r e s t pathology. USDA Tech. Bull. 155,
34 p.
Mielke, J. L.
1957. Aspen l e a f b l i g h t i n t h e Intermountain Region. USDA For. Serv. Intermt.
For. and Range Exp. Stn. Res. Note 42, 5 p.
Packard, F. M.
1942. Wildlife and aspen i n Rocky Mountain National Park, Colorado. Ecology
23 ~478-482.
Patterson, A. E.
1959. Distinguishing annual r i n g s i n d i f f u s e porous t r e e species. J. For. 57 :126.
Reed, R. M.
1969. A study of t h e f o r e s t vegetation i n the Wind River Mountains, Wyoming.
77 p. Ph.D. t h e s i s , Wash. S t a t e Univ., Pullman.
Reed, R. M.
1971. Aspen f o r e s t s of t h e Wind River Mountains, Wyoming. The Am. Midl. N a t . 86:
327-343.
Reynolds, H. G.
1969. Aspen grove use by deer, elk, and c a t t l e i n southwestern coniferous f o r e s t s .
USDA For. Serv. Res. Note RM-138, 4 p.
Strothmann, R. O., and Z. A. Zasada
1957. S i l v i c a l c h a r a c t e r i s t i c s of quaking aspen. USDA For. Serv. Lake S t a t e s For.
Exp. Stn., Stn. Pap. 49, 26 p.
.
Headquarters for the Irxtermountain Forest and
Range Experiment Station are in Ogden, Utah.
Field Research Work Units are maintained in:
Boise, Idaho
Bozeman, Montana '(in cooperation with
Montana State University)
Logan, Utah, (in cooperation with Utah
State ~ k v e r s i t ~ )
Nlissoula, Montana (in cooperation with
University of Montana)
Moscow, Idaho (in cooperation with the
University of Idaho)
Provo, Utah (in cooperation with Brigham
Young University)
Reno, Nevada (in cooperation with University of Nevada)
GPO 782-215