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Oecologia (2006) 149: 158–164
DOI 10.1007/s00442-006-0437-9
COMMUNITY ECOLOGY
Catherine A. Gehring · Rebecca C. Mueller
Thomas G. Whitham
Environmental and genetic effects on the formation
of ectomycorrhizal and arbuscular mycorrhizal
associations in cottonwoods
Received: 8 September 2005 / Accepted: 3 April 2006 / Published online: 27 April 2006
© Springer-Verlag 2006
Abstract Although both environment and genetics
have been shown to aVect the mycorrhizal colonization
of host plants, the impacts of these factors on hosts that
can be dually colonized by both ectomycorrhizal (EM)
and arbuscular mycorrhizal (AM) fungi are less under-
stood. We examined the inXuence of environment and
host crosstype on the EM and AM colonization of cot-
tonwoods (Populus angustifolia and natural hybrids) by
comparing levels of colonization of trees growing in
common gardens that diVered in elevation and soil type.
We also conducted a supplemental watering experiment
to determine the inXuence of soil moisture on AM and
EM colonization. Three patterns emerged. First, garden
location had a signiWcant impact on mycorrhizal coloni-
zation, such that EM colonization was 30% higher and
AM colonization was 85% lower in the higher elevation
garden than the lower elevation garden. Second, cross-
type aVected total (EM + AM) colonization, but did not
aVect EM or AM colonization. Similarly, a signiWcant
garden £crosstype interaction was found for total colo-
nization, but not for EM or AM colonization. Third,
experimental watering resulted in 33% higher EM colo-
nization and 45% lower AM colonization, demonstrating
that soil moisture was a major driver of the mycorrhizal
diVerences observed between the gardens. We conclude
that environment, particularly soil moisture, has a larger
inXuence on colonization by AM versus EM fungi than
host genetics, and suggest that environmental stress may
be a major determinant of mycorrhizal colonization in
dually colonized host plants.
Keywords Arbuscular mycorrhiza · Crosstype ·
Ectomycorrhiza · Populus · Soil moisture
Introduction
Although most plant species generally form only one of
several possible types of mycorrhizal association, mem-
bers of a few plant families, including the Fagaceae,
Myrtaceae and Salicaceae, routinely form functional
mycorrhizal associations with ectomycorrhizal (EM)
fungi and arbuscular mycorrhizal (AM) fungi simulta-
neously (Molina et al. 1992). In singly colonized plants,
the extent of colonization of plant root systems by
mycorrhizal fungi is inXuenced by both environmental
factors (Smith and Read 1997; van der Heijden and
Sanders 2002) and host plant genetics (Barker et al. 2002;
Linderman and Davis 2004). Similarly, in dually colo-
nized host plants, colonization by AM versus EM fungi
is inXuenced by the local soil environment (Smith and
Read 1997) and water availability (Lodge 1985, 1989),
and recent studies also suggest that host plant genetics
may play a role in determining the dominant mycorrhi-
zal type in dually colonized hosts (Walker and McNabb
1984; Tagu et al. 2001; van der Heijden and Kuyper
2001; Khasa et al. 2002; Tagu et al. 2005). However, few
studies have simultaneously examined host plant genet-
ics and environment to determine their relative inXuence
on mycorrhizal colonization.
The purpose of this study was to examine the degree
to which environmental parameters and host crosstype
inXuenced the mycorrhizal relationships of narrowleaf
cottonwood (Populus angustifolia James) and its natu-
rally occurring hybrids with Fremont cottonwood (P.
fremontii S. Wats). We compared colonization by EM
and AM fungi on the root systems of 15 diVerent cotton-
wood clones growing in two contrasting common gar-
dens established in natural riparian sites. We also
manipulated soil moisture levels in the Weld using paired
watered and control seedlings to determine if variation in
Communicated by Jim Ehleringer
C. A. Gehring (&) · R. C. Mueller · T. G. Whitham
Department of Biological Sciences, Merriam-Powell
Center for Environmental Research,
Northern Arizona University, FlagstaV, AZ 86011-5640, USA
E-mail: catherine.gehring@nau.edu
Tel.: +1-928-5239158
Fax: +1-928-5237500
159
soil moisture would result in shifts in EM and AM colo-
nization. We addressed three speciWc questions. First, do
levels of total (AM + EM) mycorrhizal colonization and
the prevalence of AM versus EM associations vary with
environment as determined by common garden location?
Second, does the tendency of cottonwood ramets to form
EM versus AM associations have a signiWcant genetic
component? Third, does increasing soil moisture lead to
changes in overall rates of mycorrhizal colonization and/
or levels of AM and EM colonization? Understanding
the roles of environmental factors and host genetics in
determining the relative mycorrhizal colonization of
dually colonized plant species may provide valuable
insights into the functioning and importance of these
mutualisms not only for species capable of forming
mycorrhizal association with both AM and EM fungi,
but also for plants that form a single type of mycorrhizal
association.
Materials and methods
Host plant hybridization and common garden
environment
This study was conducted along the Weber River in
north-central Utah, USA. Within this watershed, P.
angustifolia occupies higher elevation riparian habitats
(1,400–2,300 m), while P. fremontii is found at lower ele-
vations (1,300–1,500 m). These species naturally cross to
produce F1 hybrids which then backcross only with nar-
rowleaf cottonwood, an example of unidirectional intro-
gression (Keim et al. 1989; Martinsen et al. 2001). This
results in a 13-km hybrid zone consisting of parental spe-
cies, F1 hybrids and a variety of complex backcross
hybrids (Keim et al. 1989; Whitham 1989).
We compared levels of AM and EM colonization on
cottonwoods planted as cuttings at the same time in two
common gardens, one at lower elevation (lower garden)
in the P. angustifolia £P. fremontii hybrid zone and one
at higher elevation (upper garden) in the pure P. angusti-
folia zone. Cottonwood ramets within both gardens were
generated from cuttings collected in the late winter from
parental trees, rooted in a greenhouse, and transplanted
into the common gardens as 1- to 1.5-m stecklings in the
spring of 1985. The crosstype (i.e., P. angustifolia, F1 and
backcross hybrids) of all study trees had been previously
determined by Martinsen et al. (2001) based on the pro-
portion Fremont markers using 35 RFLP markers (1
cpDNA, 1 mtDNA, 33 nDNA) that were diagnostic of
the parental species. In both gardens, trees were planted
parallel to the Weber River in the lowest terrace of the
natural riparian zone. During establishment, trees at
both sites were watered weekly from May to October.
In addition to elevation, the common gardens also
diVered in soil type, soil moisture, total N and C
(Schweitzer 2002), and tree growth (Table 1). Soil in the
upper garden was classiWed as loamy-sand, and soil in
the lower garden was sandy-loam. The percentage
sand (particles 0.05–2.0 mm) and silt-clay (particles <
0.05 mm) was signiWcantly greater in the upper garden
than the lower garden, while the percentage gravel
(particles >2.0 mm) was signiWcantly greater in the
lower garden than the upper garden. Particle size was
based on USDA soil classiWcations. In addition, gravi-
metric soil moisture averaged more than twofold higher
at the upper elevation garden than at the lower elevation
garden at the time of root sampling. The pH of the upper
garden soil was slightly, but not signiWcantly, higher than
that of the lower garden soil. These soil diVerences were
reXected in tree performance; tree height and trunk
diameter were nearly threefold higher in the upper
garden than the lower garden (T.G. Whitham and
K.M. Floate, unpublished data) (Table 1).
InXuence of environment and host genetics on
mycorrhizal colonization
To determine if environment and/or host plant genetics
inXuenced mycorrhizal relationships among P. angustifo-
lia, and hybrids, we collected root samples from cotton-
wood ramets of the 15 clones growing in each of two
common gardens. One to three stecklings of each clone
were sampled, and mycorrhizal colonization was aver-
aged when more than one replicate was sampled. The
ramets from which roots were collected included pure P.
angustifolia, F1 hybrids, and complex backcross hybrids.
The ramets in both gardens were 8 years old at the time
of root collections in June 1992. Because trees sampled
were the same age, diVerences in EM and AM coloniza-
tion due to successional changes (e.g., Dhillion 1994;
Chen and Brundrett 2000; dos Santos et al. 2002) were
likely minimal. In addition, by collecting roots from
ramets of the same genetic makeup in both gardens, we
were able to examine the inXuence of environment while
holding plant genetics constant.
Table 1 The upper and lower gardens diVered in soil properties and
tree growth
Data presented are means or means§1SE
a Wilks’ Lambda=0.148, F=11.55, p=0.007. Mean gravel, sand and
silt-clay fractions based on Wve samples per site
b Gravimetric soil moisture based on Wve samples per site
c From Schweitzer 2002
Site and tree parameters Garden P
Upper Lower
Elevation (m) 1,582 1,381
Soil compositiona
% Gravel fraction 3.62 (1.79) 48.47 (7.43) <0.001
% Sand fraction 92.0 (2.05) 48.47 (7.43) 0.001
% Silt-clay fraction 4.38 (0.90) 1.45 (0.31) 0.015
Soil pH 8.46 (0.10) 8.04 (0.17) 0.075
% Soil moisture (g/g)b26.17 (1.18) 12.97 (3.33) <0.001
Soil total C (%)c3.9 2.6
Soil total N (%)c0.17 0.10
Tree height (m) 8.5 (0.03) 2.9 (0.2) <0.001
Trunk diameter (cm) 14.4 (1.3) 5.0 (0.4) <0.001
160
Root samples were collected using a trowel to dig to a
constant maximum depth of 15 cm. Most of the Wne
roots of the trees were located in the upper 15 cm of the
soil proWle. Roots were traced to their origin to ensure
that they were from the desired tree. Roots were heated
for 1 h in 10% potassium hydroxide, bleached in dilute
hydrogen peroxide, and stained in acid fuchsin (Korma-
nik and McGraw 1982). Percentage mycorrhizal coloni-
zation was assessed using a dissecting microscope at 40£
and a gridline intersect method (Giovanetti and Mosse
1980). Root intersections containing arbuscules, vesicles,
or internal hyphae connected to one of these two struc-
tures were scored as AM while root segments covered by
a fungal mantle were scored as EM. A subset of the sam-
ples were examined using a compound microscope at
200£ at a later date to verify that AM fungal structures
were accurately identiWed. Hand sections were made and
examined on microscopes slides with a compound micro-
scope at 100£ to verify the presence of a Hartig net in a
subset of the EM samples. Because we never observed
fungal structures of both symbionts in the same root
intersection, we were able to calculate overall levels of
mycorrhizal colonization and the percentage of that
amount attributable to EM and AM fungi. Percent colo-
nization by AM and EM were calculated as the number
of cross-sections with AM or EM fungal structures pres-
ent divided by the total number of cross-sections exam-
ined.
To determine the inXuence of environment as deter-
mined by common garden location and crosstype as
determined by the proportion Fremont alleles (Martin-
sen et al. 2001) on mycorrhizal colonization, EM and
AM colonization of the two gardens were analyzed using
a multivariate analysis of variance with the location of
the common garden and proportion Fremont alleles as
the treatment factors and overall levels of mycorrhizal
colonization, colonization by AM fungi, and coloniza-
tion by EM fungi as response variables. All percentage
data were arcsin-square root transformed prior to analy-
sis (Zar 1984). Non-transformed data are presented in
the Wgures.
Watering experiment
To determine if the diVerences we observed between gar-
dens in the amount of AM versus EM colonization could
be partially attributed to variation in soil moisture, we
performed a watering experiment using naturally estab-
lished backcross hybrid seedlings growing in a hybrid
zone. The environmental conditions at this site were sim-
ilar to those of the lower garden; soil was classiWed as
sandy-loam, and total C and N in these soils were 2.9 and
0.15%, respectively (Schweitzer 2002). Although several
environmental features varied between the common gar-
dens that could have contributed to the diVerences we
observed in mycorrhizal colonization, including eleva-
tion, tree size, soil moisture, and soil particle size distri-
butions (Table 1), we selected soil moisture for further
study for two reasons. First, mean soil moisture was
more than twofold higher at the upper elevation site than
at the lower elevation site (Table 1), and second, a study
by Lodge (1989) of P. deltoides suggested that EM versus
AM colonization of poplars may be linked to soil mois-
ture.
Within a 50-m transect parallel to the Weber River on
the lower Xoodplain, 22 cottonwood seedlings were
selected and paired for size (height and number of
branches) and location. Members of a pair were no more
than 3 m apart. One randomly selected member of each
pair received 7.5 L of supplemental water on a weekly
basis for 6 weeks beginning in early July and extending
through mid-August 1996. Seedlings were watered slowly
and surrounded by a rubber dam placed 10 cm beyond
the drip line and buried approximately 5 cm. This dam
ensured that the root system of the target seedling was
well watered and minimized runoV toward neighboring
plants. Control seedlings received no supplemental
water. During the 6 weeks of watering, the daytime high
temperatures averaged 33.2°C and the sites received
24.1 mm of rain.
Two to three days after the last watering treatment,
root samples were collected from the watered and con-
trol seedlings as described above for the common garden
trees. Levels of mycorrhizal colonization were deter-
mined as described above. Data were arcsine-square root
transformed and analyzed using a multivariate analysis
with watered and control trees as treatment groups and
overall levels of mycorrhizal colonization, colonization
by AM fungi, and colonization by EM fungi as response
variables. Data are presented as means§1SE.
Results
Environment, crosstype and mycorrhizal colonization
Common garden environment had a signiWcant eVect on
the overall levels of mycorrhizal colonization in cotton-
wood ramets (Fig. 1). The results of the MANOVA
showed that the two sites diVered signiWcantly in their pat-
terns of mycorrhizal association (Wilks’ lambda=0.088,
F3,12=41.691, P<0.0001), and a univariate F test demon-
strated that total mycorrhizal colonization contributed
signiWcantly to this diVerence (F1,28=73.458, P<0.001).
Ramets growing in the upper garden had signiWcantly
higher levels of mycorrhizal colonization than those in the
lower garden. Total mycorrhizal colonization of the upper
garden was 97.9§0.69%, compared with 91.1§2.26% in the
lower garden. In addition, univariate F-tests showed that
patterns for both EM and AM colonization contributed
signiWcantly to the overall MANOVA (EM: F1,28=33.281,
P<0.0001; AM: F1,28=15.610, P=0.001). Ramets grow-
ing in the lower elevation garden had 30% lower levels of
EM colonization and 85% higher levels of AM coloniza-
tion than ramets growing in the higher elevation garden
(Fig. 1a). Ectomycorrhizal colonization in the upper and
lower gardens was 94.1§1.99 and 65.6§4.62%, and AM
161
colonization of these sites was 3.8§1.66 and 25.5§4.09%,
respectively.
In addition to the impact of environment on mycor-
rhizal colonization, we found a signiWcant overall eVect
of crosstype (Wilks’ lambda=0.059, F=3.292,
P=0.001). However, although we did Wnd a signiWcant
eVect of crosstype on the total mycorrhizal colonization
of cottonwoods (F=14.236, P< 0.001), we found no
crosstype eVect on either EM colonization (F=1.217,
P=0.354) or AM colonization (F=0.943, P=0.495) of
host plants (Fig. 1b).
We also found a signiWcant overall eVect of
garden £crosstype on mycorrhizal colonization (Wilks’
lambda=0.035, F=4.316, P< 0.001), but univariate
tests showed that only the total mycorrhizal colonization
was signiWcantly aVected (F=10.258, P< 0.001). No sig-
niWcant interaction eVect was detected for either EM
colonization (F=1.112, P=0.404) or AM colonization
(F=0.663, P=0.681).
Watering experiment
Watering resulted in major quantitative and qualitative
changes in mycorrhizal colonization, a statistically
signiWcant overall eVect (Wilks’ lambda=0.365,
F3,18=10.433, P<0.001) (Fig. 2). However, we found no
diVerences in overall mycorrhizal colonization between
watered and control trees (F1,20=1.241, P=0.278). In
contrast to the overall levels of mycorrhizal colonization,
AM and EM colonization showed signiWcant responses
to experimental watering. Ectomycorrhizal colonization
increased by 33% in response to the experimental
increase in soil moisture (F1,20=26.121, P<0.001). Ecto-
mycorrhizal colonization of cottonwood seedlings
receiving supplemental water was 62.6§2.88%, while EM
colonization of control seedlings was 41.5§2.95%. In
addition, although colonization by EM fungi increased
in response to increased soil moisture, watering resulted
in a 45% decline in AM colonization (F1,20=22.049,
P<0.001). Colonization by AM of watered seedlings was
19.5§1.99%, compared to 36.0§2.88% for control seed-
lings.
Discussion
Soil moisture eVects on EM versus AM colonization
The results of the watering experiment showed that
increasing soil moisture caused rapid, but opposite shifts
Fig. 1 Percent EM (black portion of bar) and AM (gray portion o
f
bar) colonization of cottonwoods (Populus angustifolia and natural
hybrids). The height of the bar represents the mean total % mycor-
rhizal colonization attributed to both types of symbionts. a Percent
EM colonization was signiWcantly higher in the upper garden, while
percent AM colonization was signiWcantly higher in the lower gar-
den. Total mycorrhizal colonization was signiWcantly greater in the
upper garden than the lower garden. b Percent EM and AM coloni-
zation of diVerent crosstypes growing in both gardens was not sig-
niWcantly diVerent, but total colonization varied by crosstype
Narrowleaf F1 hybrid Backcross
% Mycorrhizal colonization
0
20
40
60
80
100
120
B
Upper Lower
% Mycorrhizal Colonization
0
20
40
60
80
100
120
A
Fig. 2 Percent ectomycorrhizal (black portion of bar) and arbuscu-
lar mycorrhizal (gray portion of bar) colonization of cottonwood
seedlings that either received supplemental water or served as non-
watered controls. The height of the bar represents the mean total
percent mycorrhizal colonization attributed to both types of symbi-
onts. Percent ectomycorrhizal colonization of watered trees was sig-
niWcantly higher than control trees, while percent AM colonization
of watered trees was signiWcantly lower than control trees. Total
mycorrhizal colonization of watered and control trees were not sig-
niWcantly diVerent
Watered Control
% Mycorrhizal Colonization
0
20
40
60
80
100
162
in the relative colonization of cottonwood seedlings by
AM and EM fungi.
Previous studies have implicated other factors, includ-
ing fungal inoculum potential (Baon et al. 1992; van der
Heijden and Votsaka 1999; Trowbridge and Jumpponen
2004), litter accumulation and quality (Chilvers and Pryor
1965; Reddell and Malajczuk 1984; Read 1991; Conn and
Dighton 2000), and host vigor (Power and Ashmore 1996;
Swaty et al. 2004), in inXuencing both AM and EM colo-
nization of host plants. However, although all of these
factors could have contributed to the diVerences observed
in mycorrhizal colonization between the upper and lower
gardens, these variables were held constant for the water-
ing experiment, where seedlings were of uniform size and
age and were growing under similar environmental condi-
tions. The rapid shift in mycorrhizal colonization in
response to water addition, in agreement with the Wndings
of Lodge (1989), demonstrates that soil moisture has a
strong inXuence on patterns of mycorrhizal colonization
of cottonwoods. Under conditions of high water avail-
ability, EM colonization increases, while AM colonization
declines. Similar patterns of EM versus AM colonization
of P. deltoides were observed by Lodge (1989) and Lodge
and Wentworth (1990).
Genetic inXuences on AM versus EM colonization
Mycorrhizal responses to host plant genetics have been
demonstrated in numerous ecosystems and with diverse
host plants. Plant genotypes have been shown to diVer in
both their ability to form mycorrhizae, and in the relative
beneWt received from mycorrhizal colonization (Smith
and Read 1997; Barker et al. 2002). Studies of Populus
have also shown that EM formation is under genetic con-
trol (Tagu et al. 2001, 2005). In addition, a study by
Khasa et al. (2002) in a single common garden found that
the relative susceptibility to colonization by AM versus
EM fungi varied among diVerent species and hybrids of
Populus, suggesting that mycorrhizal status of dually col-
onized host plants can also be inXuenced by host plant
genetics. In the present study, we detected a signiWcant
inXuence of crosstype on total mycorrhizal colonization,
but allocation to EM versus AM was unaVected by cross-
type. Lack of consistent results between the present study
and the Wndings of Khasa et al. (2002) could be due to
diVerences in the scale of examination; we examined only
a single species of Populus and various hybrids, while the
study by Khasa et al. (2002) included 28 clones of various
Populus species and hybrids, and did not include P.
angustifolia or P. fremontii.
Similarly, the interaction between garden £crosstype
signiWcantly aVected total mycorrhizal colonization, but
did not inXuence allocation to EM versus AM fungi. This
lack of signiWcant interaction shows that even in vastly
diVerent environments, the inXuence of crosstype on col-
onization by EM versus AM fungi was limited. In other
words, regardless of environmental conditions, crosstype
did not have a signiWcant impact on the patterns of colo-
nization by the two types of mycorrhizal fungi.
Although we detected no inXuence of crosstype on
colonization by EM versus AM fungi, diVerences in
ramet crosstype may have a greater impact on the com-
munity composition of either ectomycorrhizal or AM
fungal species than on whether EM or AM associations
dominated at a given site. For example, Wimp et al.
(2005) found that cottonwood crosstype did not inXu-
ence the richness or abundance of arthropods, but it did
signiWcantly aVect arthropod community composition.
Similarly, genetic diversity of cottonwood host plants
accounted for nearly 60% of the variation in arthropod
diversity (Wimp et al. 2004). The eVects of host plant
genetics on mycorrhizal community composition is not
well understood, but diVerences in host speciWcity of
both AM and EM fungi (Molina et al. 1992) suggest that
the composition of mycorrhizal fungi is likely to be
responsive to diVerences in host plant genetics.
Stress, competition and mycorrhizal colonization
The shifts in mycorrhizal colonization in response to
supplemental water we observed were consistent with the
Wndings of Lodge (1989), who found that EM fungi colo-
nized roots in moist but well drained soils, while AM
fungi had higher levels of colonization in Xooded and
very dry soils (Lodge 1989). We did not have the oppor-
tunity to sample Xooded soils, but both very dry and
Xooded soil can represent stressful conditions for host
plants (Entry et al. 2002). The shifts observed by Lodge
(1989) and the Wndings of this study suggest that AM
respond positively to stressful conditions, while EM are
negatively aVected by stress.
The patterns of EM versus AM colonization we
observed could result from either preferential allocation
to one type of mycorrhizal fungi, or competition between
fungal symbionts in response to environmental condi-
tions. For example, Saikkonen et al. (1999) hypothesized
that under environmental conditions that result in host
plant carbon limitation, allocation to EM may switch
from fungal species with high carbon requirements to
species with low carbon requirements. In support of this
hypothesis, Markkola et al. (2004) found that coloniza-
tion of thick-mantled EM fungal species declined in
response to simulated herbivory. Similarly, AM coloniza-
tion of oaks was positively correlated with wasp herbiv-
ory, while EM colonization was negatively associated
with herbivory (Mueller et al. 2005). Because recent stud-
ies suggest that EM and AM may confer diVerent bene-
Wts to their host plants (van der Heijden 2001; Gehring
and Whitham 2002), carbon limitation in host plants that
support both AM and EM may result in preferential allo-
cation to AM, which still function in nutrient uptake but
may have lower carbon requirements than EM (Janos
1983; Connell and Lowman 1989; Jakobsen et al. 2002).
Conversely, in favorable environments, hosts plants
may preferentially allocate to EM, which have higher
carbon requirements, but which may be more eVective at
nutrient uptake than AM (Jones et al. 1998; van der
Heijden and Kuyper 2001). In support of this hypothesis,
163
Lodge (1985) found that EM were most abundant on
members of the genera Populus and Salix in the eastern
US where conditions for plant growth are optimal. In
our system in the arid west, measures of growth showed
that the ramets in the lower garden had lower growth
rates than those found in the upper garden (Table 1).
Because trees in the upper and lower garden were the
same age, the diVerences suggest that the opposite pat-
terns of EM versus AM colonization in these gardens
could have been inXuenced by diVerential carbon alloca-
tion by host plants to the two types of mycorrhizae.
Alternatively, shifts in EM versus AM colonization
could result from competition between these two types of
mycorrhizal fungi. Several mechanisms of competition
between established mycorrhizal fungi have been pro-
posed, including diVerences in sink strength for host
plant carbohydrates (Wilson and Tommerup 1992; Dea-
con and Fleming 1992), variable levels of aggressiveness
for colonization sites (Wilson and Tommerup 1992), and
competitive and antagonistic exclusion (Deacon and
Flemming 1992; Bruns 1995). Competition between AM
and EM on a single host plant could also occur through
interactions between hyphae in the soil, or result from
limitations of habitat space within roots (Chen et al.
2000). Observations of fungal interactions in the soil and
at the root interface are necessary to evaluate these
potential mechanisms.
Conclusions
We found that, compared to environmental factors,
host crosstype played a minor role in determining levels
of EM versus AM colonization in the Weld. Although a
small percentage of plants are capable of forming dual
mutualisms with both EM and AM (Trappe et al.
1987), many of these plants are dominant species, such
as members of Eucalyptus and Quercus, and as a result,
these Wndings are applicable to numerous ecosystems.
In addition, studies of plant species such as cotton-
woods that are capable of forming both AM and EM
associations may provide valuable insights into the
functioning and importance of these mutualisms, not
only to dually colonized hosts, but also to plants that
form only one or the other association. Our research
and that of Lodge (1985, 1989) indicates that soil mois-
ture may be an important environmental attribute
determining the prevalence of AM versus EM.
Although other factors may have had some inXuence
on levels of mycorrhizal colonization, the results of our
watering experiment showed that soil moisture was a
major determinant of the levels of EM versus AM colo-
nization. We suggest that this relationship be further
explored with the view to incorporating it into the theo-
ries regarding the occurrence and importance of the
various types of mycorrhizal mutualism in ecosystems
(Allen et al. 1995) and how these patterns compare with
those of other trophic levels and taxa. Furthermore,
although the majority of studies have not examined
both environmental and genetic eVects on mycorrhizal
colonization concurrently, our Wndings suggest that
both factors are important determinants of mycorrhizal
colonization of host plants.
Acknowledgments We thank K. Floate, T. Del Vecchio Lane,
A. Gatherum, V. Meyeres, V. Oza, and S. Woolbright for Weld and
laboratory assistance, the State of Utah and the Ogden Nature Cen-
ter for providing test plots along the Weber River for the establish-
ment of the common gardens, and T. Theimer for commenting on
the manuscript. Funding was provided by USDA grant 95-37302-
1801 and NSF grants DEB-9726648 and EF-0425908.
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