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Mean reproductive traits of fungal assemblages are correlated with resource availability

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Organisms have evolved a fascinating variety of strategies and organs for successful reproduction. Fruit bodies are the reproductive organ of fungi and vary considerably in size and shape among species. Our understanding of the mechanisms underlying the differences in fruit body size among species is still limited. Fruit bodies of saprotrophic fungi are smaller than those of mutualistic ectomycorrhizal fungi. If differences in fruit body size are determined by carbon acquisition, then mean reproductive traits of saprotrophic and ectomycorrhizal fungi assemblages should vary differently along gradients of resource availability as carbon acquisition seems more unpredictable and costly for saprotrophs than for ectomycorrhizal fungi. Here, we used 48 local inventories of fungal fruit bodies (plot size: 0.02 ha each) sampled along a gradient of resource availability (growing stock) across 3 years in the Bavarian Forest National Park in Germany to investigate regional and local factors that might influence the distribution of species with different reproductive traits, particularly fruit body size. As predicted, mean fruit body size of local assemblages of saprotrophic fungi was smaller than expected from the distribution of traits of the regional species pool across central and northern Europe, whereas that of ectomycorrhizal fungi did not differ from random expectation. Furthermore and also as expected, mean fruit body size of assemblages of saprotrophic fungi was significantly smaller than for assemblages of ectomycorrhizal species. However, mean fruit body sizes of not only saprotrophic species but also ectomycorrhizal species increased with resource availability, and the mean number of fruit bodies of both assemblages decreased. Our results indicate that the differences in carbon acquisition between saprotrophs and ectomycorrhizal species lead to differences in basic reproductive strategies, with implications for the breadth of their distribution. However, the differences in resource acquisition cannot explain detailed species distribution patterns at a finer, local scale based on their reproductive traits.
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Mean reproductive traits of fungal assemblages are
correlated with resource availability
Claus B
assler
1
, Hans Halbwachs
2
, Peter Karasch
3
, Heinrich Holzer
3
, Andreas Gminder
4
,
Lothar Krieglsteiner
5
, Ramiro Silveyra Gonzalez
6
,J
org M
uller
1
& Roland Brandl
7
1
Bavarian Forest National Park, Freyunger Str. 2, 94481 Grafenau, Germany
2
German Mycological Society, Danziger Str. 20, 63916 Amorbach, Germany
3
Bavarian Mycological Society, Section Bavarian Forest, Ablegweg 9, 94227 Rabenstein, Germany
4
German Mycological Society, Dorfstrasse 27, 07751 Jenaprießnitz, Germany
5
German Mycological Society, Konrad-Adenauer-Str. 32, 73529 Schw
abisch Gm
und, Germany
6
Chair of Remote Sensing and Landscape Information Systems, University of Freiburg, 79106 Freiburg, Germany
7
Animal Ecology, Department of Ecology, Faculty of Biology, Philipps-Universit
at Marburg, 35037 Marburg, Germany
Keywords
Assemblage composition, elevation gradient,
fruit body, null model, sporocarp.
Correspondence
Claus B
assler, Bavarian Forest National Park,
Freyunger Str. 2, 94481 Grafenau, Germany.
Tel: +49 8552 9600 157;
Fax: +49 8552 9600 100;
E-mail: claus.baessler@npv-bw.bayern.de
Funding Information
This research was supported by the Bavarian
State Ministry of the Environment, Public
Health, and Consumer Protection.
Received: 1 September 2015; Revised: 1
December 2015; Accepted: 3 December
2015
doi: 10.1002/ece3.1911
Abstract
Organisms have evolved a fascinating variety of strategies and organs for suc-
cessful reproduction. Fruit bodies are the reproductive organ of fungi and vary
considerably in size and shape among species. Our understanding of the mecha-
nisms underlying the differences in fruit body size among species is still limited.
Fruit bodies of saprotrophic fungi are smaller than those of mutualistic ectomy-
corrhizal fungi. If differences in fruit body size are determined by carbon acqui-
sition, then mean reproductive traits of saprotrophic and ectomycorrhizal fungi
assemblages should vary differently along gradients of resource availability as
carbon acquisition seems more unpredictable and costly for saprotrophs than
for ectomycorrhizal fungi. Here, we used 48 local inventories of fungal fruit
bodies (plot size: 0.02 ha each) sampled along a gradient of resource availability
(growing stock) across 3 years in the Bavarian Forest National Park in Germany
to investigate regional and local factors that might influence the distribution of
species with different reproductive traits, particularly fruit body size. As pre-
dicted, mean fruit body size of local assemblages of saprotrophic fungi was
smaller than expected from the distribution of traits of the regional species pool
across central and northern Europe, whereas that of ectomycorrhizal fungi did
not differ from random expectation. Furthermore and also as expected, mean
fruit body size of assemblages of saprotrophic fungi was significantly smaller
than for assemblages of ectomycorrhizal species. However, mean fruit body
sizes of not only saprotrophic species but also ectomycorrhizal species increased
with resource availability, and the mean number of fruit bodies of both assem-
blages decreased. Our results indicate that the differences in carbon acquisition
between saprotrophs and ectomycorrhizal species lead to differences in basic
reproductive strategies, with implications for the breadth of their distribution.
However, the differences in resource acquisition cannot explain detailed species
distribution patterns at a finer, local scale based on their reproductive traits.
Introduction
Sexual reproduction is essential for many organisms to
guarantee the segregation and recombination of genes for
maintaining genetic diversity (Stearns 1988). For this,
organisms produce propagules (e.g., spores, seeds, eggs)
in specialized organs. The production of these organs is
often costly; therefore, sexual reproduction leads also to
trade-offs among traits related to reproduction (e.g.,
between size and number of flowers) as well as between
reproductive traits and traits related to other biological
functions (e.g., between reproduction and growth). Such
trade-offs and selection by the environment should lead
to a reproductive syndrome characterized by a specific
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This is an open acc ess article under the terms of the Creative Commons Attribution License, which permit s use,
distribution and reproduction in any medium, provided the original work is properly cited.
1
combination of traits that allows effective reproduction at
minimal costs in a given environment. As a consequence,
the reproductive syndromes of species co-occurring in
assemblages should show predictable relationships to
environmental gradients (for reviews, see e.g., Clutton-
Brock 1991; Roff 1992; McGill et al. 2006).
There are two ways to approach such predictions.
First, measurements of reproductive traits across species
can be correlated with environmental variables measured
in the habitats of the considered species. Second, the
mean value of traits across all species co-occurring
within an assemblage can be analysed. Such an “assem-
blage approach” retrieves even subtle differences among
assemblages (e.g., Gossner et al. 2013; Zeuss et al. 2014)
and offers two tests of the influence of the environment
on the (reproductive) trait composition of assemblages.
If a (reproductive) trait affects the distribution and
occurrence of species, we would expect that the mean
value of the trait of an assemblage differs from that of
the species pool, i.e., the set of species with the potential
to colonize the considered area or habitat. Therefore,
one test is to evaluate whether the mean value of a trait
within assemblages differs from an expectation derived
by an appropriate null model (Ulrich and Gotelli 2013).
In another test, the means of (reproductive) traits across
species within assemblages can be correlated with envi-
ronmental variables. For example, the mean reproductive
characteristics of plant assemblages depend on elevation:
with increasing elevation, plant assemblages consist of
more species with capsules carrying numerous tiny seeds
(Pellissier et al. 2010).
Fungal spores are produced in fruit bodies. Fruit bodies
show a fascinating variation in size, form, and color that
rivals the morphology of flowers of angiosperms (Hibbett
and Binder 2002). There is no hard evidence that this
morphological variation of fruit bodies is adaptive (cf.
Gould and Lewontin 1979). However, a recent cross-spe-
cies analysis showed that the size of the fruit body is cor-
related with the trophic lifestyle; specifically, free-living
species of saprotrophic fungi produce smaller fruit bodies
than ectomycorrhizal fungi (B
assler et al. 2015).
One hypothesis argues that both the availability and
distribution of resources cause this surprising difference
between the two fungal guilds (B
assler et al. 2015). Both
guilds need a carbon source for vegetative growth and
sexual reproduction. However, the carbon sources of
saprotrophic fungi and ectomycorrhizal fungi differ.
Saprotrophs have to exploit a suitable substrate, which is
often scarce, patchily distributed and in some cases even
ephemeral (e.g., substrates of coprophilous species).
Moreover, saprotrophic species need to break down the
carbon of the resource with extracellular enzymes, whose
production is costly (Baldrian 2008). Such limitations in
the access to carbon might also explain the trade-off
between size and number of fruit bodies of saprotrophs
(B
assler et al. 2015). By contrast, ectomycorrhizal fungi
receive carbon from the host plant and therefore have
reliable access to carbon (termed “carbon excess”, see
Correa et al. 2011) in most temperate and boreal forest
biomes. Thus, differences in the life history with respect
to C acquisition between saprotrophs and ectomycorrhizal
fungi could be interpreted within the framework of the
r-K continuum (Pianka 1970), with saprotrophs (r-strat-
egy) exploiting variable and unpredictable resources and
ectomycorrhizal fungi adapted to a predictable resource
(K-strategy). This carbon excess for ectomycorrhizal fungi
might provide degrees of freedom for reproduction have
allowed the evolution of large fruit bodies and have
freed these species from other environmental constraints.
This hypothesis, however, assumes implicitly that large
fruit bodies offer advantages, such as increased successful
dispersal (reviewed in B
assler et al. 2015; see also
Discussion).
To test whether the observed differences in fruit body
size between the guilds is of ecological relevance, we anal-
ysed assemblages of fungi in the Bavarian Forest. This
area is generally nutrient poor with a low productivity
(Heurich and Neufanger 2005). If the size of fruit bodies
of species of saprotrophic fungi constrains distribution,
mean fruit body size of assemblages of this guild should
be smaller than expected from random draws of the
regional species pool (B
assler et al. 2015). B
assler and co-
workers analysed only a pooled list of species recorded
along an elevation gradient. However, the deviation from
the expectation should decrease along gradients of
resource availability, which also exist in the investigated
area. In our study presented here, we used detailed infor-
mation on the distribution of species across the gradient
and determined expectations by using null models that
consider only the species recorded along the gradient.
Such null models consider differences in the occupancy of
species and therefore allow correcting for possible statisti-
cal biases. We analysed data on the composition of fungal
assemblages along resource availability in the Bavarian
Forest National Park (Germany) and specifically tested
the following hypotheses:
1 The mean fruit body size and mean number of fruit
bodies produced should differ between the two guilds
across all sites. In particular, the mean fruit body size
of saprotrophs should be smaller than that of ectomyc-
orrhizal fungi and the mean number of fruit bodies
produced by saprotrophs should be larger.
2 Both the mean fruit body size and the number of fruit
bodies of saprotrophic assemblages should increase
with increasing resource availability, but those of ecto-
mycorrhizal fungi should not.
2 ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Ecological importance of fruit body size C. B
assler et al.
3 The mean fruit body size of assemblages of ectomycor-
rhizal fungi should not deviate from the expectation
derived from the regional species pool, but that of
saprotrophic fungi should be smaller than expected and
the deviation should decrease with increasing resource
availability.
Material and Methods
Fungal and environmental data
To analyse patterns of reproductive traits of assemblages,
we sampled fungi across a resource availability gradient
within the Bavarian Forest National Park in south-east-
ern Germany (48°54
0
N, 13°29
0
E). The Bavarian Forest
lies in the south-western part of the Bohemian Massif,
which is formed of granite and gneiss. Acidic sand and
loamy soils prevail. Elevations range between 650 and
1350 m a.s.l. At 650 to 1150 m a.s.l., forests are domi-
nated by Norway Spruce (Picea abies) admixed with
European Beech (Fagus sylvatica) and Silver Fir (Abies
alba). Above this elevation, forests are dominated by
Norway Spruce and Mountain Ash (Sorbus aucuparia).
This area is characterized by harsh climatic conditions
(Walentowski et al. 2004; see also Fig. 1). At higher ele-
vations, mean annual temperature regularly drops below
3.5°C. As a result, net primary production decreases with
elevation; the growing stock of the living stand decreases
from approximately 350 to 150 m
3
ha
1
(Heurich and
Neufanger 2005).
From 2009 to 2011, we sampled soil-related (terri-
colous) macrofungi from 48 circular plots covering the
available elevational gradient in stands dominated by
mature spruce (Fig. 1). Plots had an area of 200 m
2
and
were surveyed weekly between June and November, i.e.,
during the main period of fruit body production in the
study region. We counted fruit bodies at the species level
and removed all fruit bodies from the plots after each
survey. It has been shown that removing fruit bodies
from an area has no effect on the future production of
the species (Egli et al. 2006). During these field studies,
259 species were recorded. For all recorded species, we
extracted from published records the trophic strategy of
the species (Rinaldi et al. 2008; Tedersoo et al. 2010;
Comandini et al. 2012) and the mean cap diameter d of
mature fruit bodies (Knudsen and Vesterhold 2008). The
size of the fruit bodies of each species was estimated as
d². This index is a reliable measure of the biomass of
macrofungal fruit bodies, even though it is measured in
mm² (T
oth and Feest 2007; B
assler et al. 2015). From our
field data, we estimated the total number of fruit bodies
produced by each species by summing up the number of
fruit bodies produced by each species in each year and
dividing each sum by the number of plots on which we
recorded that species.
For an estimate of the traits that one can expect in the
regional species pool, we used a published database of
fruit body size occurring in central and northern Europe
(see B
assler et al. 2015 for a detailed description). This
database consists of ~600 saprotrophic and ectomycor-
rhizal terricolous Agaricomycetes (Agaricales, Russulales,
and Boletales) across 91 genera. The taxa were randomly
selected on the basis of page numbers in the Funga Nor-
dica (Knudsen and Vesterhold 2008) to approximately
represent the proportional number of species within gen-
era and sections of the species described in this source.
According to our hypotheses, both mean fruit body size
and the number of fruit bodies of saprotrophic assem-
blages should be correlated with resource availability.
Saprotrophic fungi rely on dead organic matter as a car-
bon resource for growth and reproduction. The amount
of organic matter and hence resource availability for
saprotrophic fungi is closely correlated with net primary
productivity (Carlile et al. 2001b). However, direct esti-
mates of net primary productivity for our area are not
available. Nevertheless, net primary productivity is corre-
lated with aboveground biomass, which is estimated by
the volume of growing stock of forests (m
3
ha
1
) (Kim-
mins 2002). We therefore measured the following growing
stock variables on the study plots: number of trees, vol-
ume (m
3
), basal area (m
2
), and mean diameter at breast
height (DBH; m). To estimate these variables, we used
full-wave LiDAR data from airborne laser scanning (Rie-
gel LMS Q-560 system at a point density of 25 points
m
2
) within circular subplots of 0.1 ha at the centre of
the plots used for sampling fungi. For final analysis, all
values were expressed on a 1 ha basis. Laser scanning data
allow discrimination between broad-leaved and coniferous
tree species and derivation of measures such as the height
(m) and DBH (m) of each tree from the laser point
clouds (Yao et al. 2012). From the height and DBH mea-
sures, the volume (m³) of each tree within a sample plot
was calculated using indices that consider the decrease in
diameter with tree height according to Heurich (2008).
The algorithms used to derive these variables were devel-
oped and calibrated within our study area (for more
details see Heurich 2008; Yao et al. 2012). We also visu-
ally estimated the vegetation cover on each plot (0.02 ha):
% cover of shrub layer (>15 m), lower tree layer (>5
15 m), and upper tree layer (>15 m). We considered the
age of the stand in a plot based on tree ring analysis of
forest inventories (Heurich and Neufanger 2005). Vari-
ables such as growing stock, mean DBH and canopy
cover are often closely correlated. We therefore subjected
all variables to a principle component analysis (PCA)
based on the correlation matrix. The first component
ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 3
C. B
assler et al. Ecological importance of fruit body size
accounted for 52% of the total variance; volume of the
growing stock (m
3
ha
1
) had the highest correlation with
this component (r = 0.96). Basal area, mean DBH and
upper tree layer were also closely correlated with the first
component (r = 0.90). The second component accounted
for 17% of the variability; the lower tree layer and forest
age showed the highest loadings on this component. We
therefore used the scores of the first component as a mea-
sure of resource availability (large scores indicate a high
level of resource availability) and the second component
as a covariate representing forest stand succession (large
scores indicate old forest stands; note that 88% of all
plots are characterized by stands with a mean age of
70 years).
Statistical analyses
We only considered species in our analysis that occurred
on at least four plots to obtain a reliable measure of the
mean number of fruit bodies for each species. Before we
calculated the mean trait values of the assemblages, fruit
body size and number of fruit bodies of each species were
log
10
transformed. In all subsequent analyses, we treated
the sampling year (2009, 2010, and 2011) as a factor to
account for variability among years.
For comparison of the mean trait values of local assem-
blages with the regional (central and northern Europe)
species pool, we divided the data set according to the
trophic guilds. Subsequently, we randomly selected the
same number of species as observed for each assemblage
from each pool and calculated the mean fruit body size
across species in that random draw. We repeated this
procedure 100 times for each plot and calculated the stan-
dardized effect size (SES) by subtracting the expected
mean from the observed mean and dividing the difference
by the standard deviation across the random draws for
each plot. Values < 2 and >2 indicate significant devia-
tions from the expectation. For the number of produced
fruit bodies, only data for the local data set were avail-
able. This test was therefore only possible for fruit body
size. Note that this null model simply tests whether the
species recorded in the investigated area produce fruit
bodies that differ in size from what one would have
expected from the fruit body size of species occurring
across Europe. This null model ignores factors that could
influence the probability that a species colonizes a partic-
ular area (e.g., regional abundance).
The use of mean values across species and correlation
of these values with other variables extracted from the
matrix of species occurrences (e.g., species richness)
might lead to spurious correlations (Zeleny and Schaffers
2012). The use of variables not extracted from the spe-
cies-by-site matrix also could lead to spurious results
because we expect that species richness co-varies with the
resource gradient. Therefore, we additionally used a null
model that randomizes species occurrence across sites but
fixes both marginal sums for sites (i.e., species richness of
sites) and marginal sums for species (i.e., occupancy of
sites across the 48 plots). As described above, we first
divided the data set according to the trophic guilds and
calculated the observed mean size and number of fruit
bodies for each plot. We then randomized the community
(A) (B)
Figure 1. (A) Map showing the position of
the study area within Europe. Colors indicate
the mean annual temperature (derived from
WorldClim database, www.wordclim.org). (B)
Study area (Bavarian Forest National Park)
showing the sampling transects (gray lines) and
study plots (black dots; see B
assler et al. 2008
for details on the study design). Colors reflect
mean annual temperature (19882007) for the
study area. Inset shows the relationship
between mean annual temperature and
resource availability. Note that the
temperatures shown in the map are outputs
from a smoothing algorithm and the values
may differ somewhat from the measured
values of a particular site.
4 ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Ecological importance of fruit body size C. B
assler et al.
data matrix 100 times for each guild with the indepen-
dent swap algorithm (Gotelli 2000). We calculated for
each randomized community matrix the mean fruit body
size as well as the mean number of produced fruit bodies
of the randomized assemblages. Finally, we calculated the
standardized effect size by subtracting the expected mean
(mean across all randomizations) from the observed mean
and dividing the difference by the standard deviation
across the randomizations for each plot (see above). Note
also that the use of separate null models for the two
guilds for calculating effect sizes also removes differences
in the mean fruit body size between guilds (see Table 2).
By contrast, if our null model with regional data pro-
duced deviations from the expectation only for sapro-
trophs, we would expect differences in effect sizes
between the two guilds.
We used the (raw) mean fruit body size and mean
number of fruit bodies as well as the standardized effect
sizes to test for the influence of resource availability (first
principle component, see above) on the reproductive
characteristics of the fungal assemblages by applying lin-
ear mixed-effects models using the add-on package nlme
in R (R Development Core Team, 2015). In these models,
we considered both the forest stand succession (second
principle component, see above) and the factor sampling
year as covariates. This enabled us on one hand to quan-
tify the relative importance of resource availability relative
to each covariate, and on the other hand to assess the
variability of the reproductive traits of the different fungal
guilds among years. We found no significant three-way
interaction between guild, resource availability and year
(results not shown). Within all models, variance was
weighted based on the guild using the varIdent function
(form =~1|guild) to account for within-group
heteroscedasticity (Pinheiro and Bates 2000). We further-
more used the plot as a random factor to account for
repeated measurements. For all comparisons among the
models, we used standardized effect sizes of the parameter
estimates using an expected mean of 0 (z-values = esti-
mates divided by the respective standard error; see Bring
1994).
Results
In the three years of our sampling, we collected 172,176
saprotrophic fruit bodies representing 100 species, and
24,435 ectomycorrhizal fruit bodies representing 150 spe-
cies (for descriptive data, see Table 1).
The means of fruit body size and number of produced
fruit bodies of the two guilds clearly differed. On average,
the saprotrophs co-occurring on a site produced smaller
but more fruit bodies than the ectomycorrhizal species
(Table 2). Furthermore, analysis of the effect size calcu-
lated using random draws from the regional data set
demonstrated clear differences between the two guilds,
which indicated that the two guilds differ in their devia-
tion from the regional expectation. This is also underlined
by our finding that standard effect sizes of mean fruit
body size were less than 2 for most of the assemblages
of saprotrophic fungi, whereas effect sizes of mean fruit
body size of ectomycorrhizal fungi species on most plots
were between 2 and 2 (Fig. 2). These results were con-
sistent across years (Fig. 2).
For all models and of all predictors, resource availabil-
ity showed the largest effect sizes on our metrics describ-
ing both the mean fruit body sizes and the mean number
of fruit bodies produced by species across sites. For both
guilds, mean fruit body size increased with resource avail-
ability, and the mean number of produced fruit bodies
decreased (Table 2, Fig. 3A,B). When we used the raw
means, however, the slope between the two guilds differed
(Table 2). This difference in the slope disappeared when
Table 1. Number of species and number of fruit bodies of mutualistic ectomycorrhizal (ECM) and saprotrophic (ST) fungal species and mean
values across 48 plots sampled in the years 2009, 2010 and 2011.
2009 2010 2011
ECM ST ECM ST ECM ST
All plots
Number of species 93 54 104 70 122 72
Number of fruit bodies 9627 46,340 6211 34,365 8597 91,471
Mean across plots (0.02 ha)
Number of species 12.4 8.0 11.0 10.2 14.5 9.9
Min number of species 5 2 2 4 4 5
Max number of species 26 14 22 25 29 21
Number of fruit bodies 2001 965 129 716 179 1906
Min number of fruit bodies 18 6 3 9 21 10
Max number of fruit bodies 1325 5583 830 2269 725 11,490
ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5
C. B
assler et al. Ecological importance of fruit body size
effect sizes considered (Table 2). We were not able to
exclude the possibility that this interaction is a result of a
statistical bias (see Zeleny and Schaffers 2012); therefore,
we will ignore this interaction in our discussions. Note
also that for the null model with local data, most effect
sizes fell within the range of 2 to 2 (Fig. 3D,E), and
therefore individual effect sizes were in most cases not
significant. Nevertheless, the overall trend of effect sizes
across sites with respect to resource availability showed a
clear pattern (Fig. 3D,E).
Effects of the age of the stand were weaker than effects
of resource availability (Table 2). Nevertheless, for both
guilds, we found a significant negative relationship
between age and all metrics of the mean fruit body size
and a positive relationship between age and the metrics
of mean number of produced fruit bodies (Table 2). No
model revealed a significant three-way interaction
between guild, resource availability and year (results not
shown). Therefore, the basic patterns between guilds are
consistent across years.
Discussion
Our comparison of species or a species list across an
entire region showed that saprotrophs not only produce
smaller fruit bodies than ectomycorrhizal species, but
that this difference also holds for local assemblages and
is consistent across years. Furthermore, in local assem-
blages, species of saprotrophic fungi with small fruit
body sizes co-occur more often than expected from the
regional species pool, whereas no differences were found
for members of the ectomycorrhizal guild. This pattern
was consistent across years and therefore supports, with
some more statistical sophistication, the results of Bassler
et al. (2015).
These basic results are consistent with the expectation
that the different strategy of carbon acquisition of the
two guilds might lead to different reproductive strategies,
which in turn affects the distribution of species. If we
assume that there is an upper limit to the resources a spe-
cies can invest into reproduction, each species is faced
with the problem of either investing in large fruit bodies
or increasing the number of (small) fruit bodies. Species
with small fruit bodies have the option of a finer-grained
response in reproductive investment than species with
large fruit bodies. Small fruit bodies mature more quickly
than large fruit bodies, which might be especially impor-
tant for saprotrophic fungi (Haard and Kramer 1970).
Therefore, species with small fruit bodies are able to
adjust reproductive investment flexibly to specific local
conditions and at the same time reduce the risk of perish-
ing before sporulation. Our results can be understood in
terms of the concept of r- and K-strategies (e.g., Pianka
1970; Grime 1988). Saprotrophs follow the r-strategy in
order to exploit an unpredictable resource, whereas mutu-
alistic ectomycorrhizal fungi follow the K-strategy because
their carbon source is more reliable (Boucher et al. 1982;
Boucher 1985).
However, by following an assemblage approach and
considering a gradient of resource availability within the
Table 2. Results of linear mixed-effects models to test the importance of resource availability and age of the stand considering the three sam-
pling years (2009, 2010, 2011) on reproductive characteristics of saprotrophic (ST) and mutualistic ectomycorrhizal (ECM) fungi. Variance was
weighted within the models based on the guild to account for within-group heteroscedasticity. Plots were treated as random factors to account
for repeated measurement. For all comparisons among the models, we used standardized effect sizes (SES) of the parameter estimates using an
expected mean of 0 (z-values = estimates divided by the respective standard error). The reference group for the test of differences (between
guilds and sampling years) is indicated in italics. Significant effect sizes are in bold (*P < 0.05, **P < 0.01, ***P < 0.001). Significant differences
(interaction) between the guilds are shaded gray.
Reference
Guild
20092010 20092011 20102011
Resource availability Age Adj. R²
ST
2009 2009 2010
ECM ST ECM ST ECM ST ECM ST ECM ST ECM ST
Mean fruit
body size
39.8*** 2.69** 1.29 0.69 0.18 2.00* 1.11 4.00*** 5.24*** 1.99* 3.18** 0.21 0.27
Mean number
of fruit bodies
19.11*** 1.04 1.81 0.50 2.11* 1.54 0.30 4.19*** 5.61*** 3.02** 2.14* 0.25 0.24
SES fruit body
size regional
pool
19.05*** 2.15
* 3.37** 0.77 2.52* 1.39 0.85 4.56*** 4.96*** 2.78* 3.39** 0.22 0.31
SES fruit body
size local pool
0.14 1.83 1.74 0.87 0.84 0.96 0.90 4.27*** 5.43*** 2.29* 3.42*** 0.20 0.30
SES number
fruit bodies
local pool
0.34 0.04 1.09 0.25 0.78 0.21 0.31 4.10*** 5.94*** 3.43*** 2.36* 0.23 0.30
6 ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Ecological importance of fruit body size C. B
assler et al.
Bavarian Forest, our study provided further insights into
the structure of fungal assemblages with respect to traits
involved in sexual reproduction. Across plots, all metrics
that characterize the mean trait values of assemblages
clearly responded to resource availability. However, in
contrast to our expectation, mean values of reproductive
traits of both guilds responded similarly to the local gra-
dient of resource availability. In both guilds, the mean
fruit body size increased with resource availability but the
mean number of fruit bodies decreased. Furthermore, by
considering the variability among years, we showed that
these patterns were robust across years. However, our
insights into the possible processes influencing resource
acquisition, reproductive traits and distribution are not
sufficient to understand all the patterns found in our
study, and we acknowledge that both a carbon-centred
approach and the application of the r-K continuum are
oversimplifications.
Contrary to our expectation, assemblages of both
guilds were composed of species that on average produce
larger fruit bodies in highly productive environments.
This leads to the conclusion that large fruit bodies
should have some advantages irrespective of the lifestyle.
We list five possible advantages. (1) A large fruit body
can generally produce more spores than a small fruit
body as fruit body size is correlated with the hymenial
Figure 2. Histograms of the standardized
effect sizes for each sampling year (2009,
2010, 2011) calculated from the comparison
of the mean fruit body size of local
assemblages (Bavarian Forest) with that of a
regional species pool (central and northern
Europe) of (AC) saprotrophic fungi and (DF)
ectomycorrhizal fungi. Note that for all
saprotrophic assemblages, standardized effect
sizes were in most cases less than 2 (clear
deviation from the regional pool), whereas for
ectomycorrhizal assemblages, the effect sizes
fall between 2 and 2 (no deviation from the
pool). Insets in the upper panel represent
typical species of the two guilds (a, Mycena
rorida growing on a twig; b, the infamous
ectomycorrhizal species Amanita muscaria).
ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 7
C. B
assler et al. Ecological importance of fruit body size
surface (B
assler et al. 2015) and most probably also to
longevity (Richardson 1970; Kramer 1982). Fruit bodies
sporulate as long as they remain vital, and therefore the
number of produced spores increases as the fruit body
matures (Haard and Kramer 1970; McKnight 1990;
Moore et al. 2008). Hence, fungi with large fruit bodies
might be less dispersal limited, which can become a
major factor for both the distribution of species and spe-
cies diversity (Peay et al. 2010). (2) Fruit body size of
ectomycorrhizal fungi is correlated with spore size
(B
assler et al. 2015). Large spores, like large seeds, might
offer advantages during establishment at a site by allow-
ing prolonged dormancy (Halbwachs and B
assler 2015).
(3) Spores released from larger and therefore taller spe-
cies will more easily leave the boundary layer of still air
and disperse farther than spores of shorter species
(Galante et al. 2011). (4) A large fruit body has a lower
surface-to-volume ratio; this might increase protection
against desiccation and pathogens, which generally seems
to be critical for sporulation in agarics (Buller 1909;
Cl
emenc
ß
on 1997). (5) A large fleshy trama and stipe
might act as a defence against invertebrates that feed on
fruit bodies. Furthermore, large fruit bodies might attract
animal dispersal vectors (Bunyard 2007).
The fact that the mean fruit body size of local sapro-
trophic assemblages was smaller than expected from the
Figure 3. Raw scatterplots and linear
regressions of (A) mean fruit body size, (B)
mean number of fruit bodies, (C) standardized
effect sizes (SES) of mean fruit body size
(regional species pool), (D) SES of mean fruit
body size (local species pool) and (E) SES of
mean number of fruit bodies (local species
pool) compared to resource availability (first
axis of the PCA; see Material and methods for
details). Light gray symbols: saprotrophic fungi;
dark gray symbols: ectomycorrhizal fungi.
Different symbols represent different sampling
years. Slopes (b) and adjusted R² from
univariate linear mixed effect regression
models that account for repeated
measurement are given.
8 ª 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Ecological importance of fruit body size C. B
assler et al.
regional species pool suggests that saprotrophs occurring
in the investigated area (Bavarian Forest National Park)
are generally constrained by environmental factors. By
contrast, from a local perspective, the two guilds
responded similarly to the resource availability gradient.
The reproductive traits of the assemblage of saprotrophs
were hence influenced by both regional and local environ-
mental filters, whereas ectomycorrhizal fungi were affected
only by local filters. Although ectomycorrhizal species in
the low productivity area studied did not differ from the
expectation from the regional species pool, they clearly
responded to resource availability at the local scale. We
presently are not able to offer a convincing hypothesis to
explain these interesting differences in the importance of
regional and local factors between saprotrophic and ecto-
mycorrhizal fungi.
In our study area, elevation is not only correlated with
productivity but also with climate. Mean annual tempera-
ture decreases with elevation, ranging from 6.5 to 3.5°C
in our study area (B
assler 2004). Although we followed
the reasonable assumption that resource availability
should reflect the response of reproductive syndromes
(Carlile et al. 2001a), we are not sure whether tempera-
ture or its correlates also contribute to explaining the
observed pattern. In this respect, it might be important to
consider that fungi are ectothermic organisms with meta-
bolic rates related to temperature (Carlile et al. 2001b).
Experiments are needed to disentangle confounding
effects between climate variables and resource availability.
One further important drawback of our study is that we
were not able to estimate the investment into sexual
reproduction. This would require information on the
belowground biomass of fungal individuals (genet). How-
ever, there is no feasible way to obtain such information
in ecological field studies across broad gradients (Dahl-
berg and Stenlid 1994; Guidot et al. 2001, 2004). Whether
competitive interactions might also contribute to explain
the observed pattern must be left to future studies.
Overall, our study revealed basic differences in the
reproductive syndromes between saprotrophic and ecto-
mycorrhizal fungal assemblages. We argue that these dif-
ferences can be explained in part by their resource
acquisition strategies. Furthermore, the results of our
study suggest that both regional and local environmental
filters affect saprotrophs, whereas only local environmen-
tal filters affect ectomycorrhizal fungi. Nevertheless, not
all patterns found in our study are consistent with the
strategies of resource acquisition. Still, our study provides
another aspect of mutualistic relationships. Most studies
in this field are from the perspective of the hosts e.g., that
demonstrate that plants hosting mycorrhizas are more
productive than those without mycorrhiza (e.g., Burgess
et al. 1993; Smith and Read 2008; Smith et al. 2010). Our
study reversed this perspective, and we show that evolu-
tion towards mutualism might have increased the repro-
ductive output of ectomycorrhizal fungi compared to
free-living saprotrophs. However, to deepen our under-
standing of lifestyle-specific assemblages, we need to iden-
tify the size of fungal individuals (genet) and to estimate
the true investment in sexual reproduction along environ-
mental gradients.
Acknowledgments
This research was supported by the Bavarian State Min-
istry of the Environment, Public Health, and Consumer
Protection. We are grateful to Andreas Kunze, Anna von
Steen, Sebastian Schacht and Regina Siemianowski for
help with fieldwork and to Leho Tedersoo for fruitful
comments on an earlier draft. We thank Karen A. Brune
for linguistic revision of the manuscript.
Conflict of Interest
None declared.
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... In recent years, research has come up with insights into functions of fungal morphological reproductive traits, something which had previously been largely ignored (Pringle et al., 2015). Fruit body and spore characteristics constitute a window into fungal niche occupation, lifestyles, reproductive fitness and assembly rules Bässler et al., 2016;Halbwachs et al., 2016;Dawson et al., 2018;Zanne et al., 2019). Here, we look into the reproductive traits of coprophilous taxa, assuming that the substrate may have a selective impact. ...
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The spores of most coprophilous mushrooms require a passage through a mammalian gut. Guts and faeces constitute a chemically and microbially aggressive environment. Hence, the spores need to be armed, e.g. by melanisation and thick walls, possibly leading to large spores due to volume constraints. Fruiting takes place under fierce competition by microbes and dung-dwelling invertebrates on faeces which quickly perish. Therefore, coprophilous mushrooms are forced to mature fast, implying small fruit bodies. To find evidence for our assumptions, we compiled a database of fruit body and spore sizes of mushroom genera which contain coprophilous species, comprising 633 (including ca. 20% coprophilous) species across 13 genera worldwide. The data set was subjected to a phylogenetically informed statistical analysis. Our hypotheses were confirmed though the fruit body size differences are less pronounced than differences in spore size. It would be worthwhile to further elucidate the coprophilous trait syndrome with molecular methods.
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Chapter
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