JEC_613.fm Page 920 Wednesday, November 21, 2001 1:11 PM
Journal of
Ecology 2001
89, 920 – 929
Relationships between productivity, number of shoots and
number of species in bryophytes and vascular plants
Blackwell Science Ltd
A. BERGAMINI, D. PAULI*, M. PEINTINGER* and B. SCHMID*
Institut für Systematische Botanik, Universität Zürich, Zollikerstrasse 107, CH-8008 Zürich, Switzerland, *Institut
für Umweltwissenschaften, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
Summary
1 We measured species density, biomass and shoot density for both bryophytes and
vascular plants in 90 small plots in 18 calcareous fens. In addition, we recorded leaf area
index and litter mass of vascular plants. Our goals were: (a) to compare the relationship
between biomass and species density for the two taxonomic groups, (b) to test whether
biomass and species density of bryophytes and vascular plants are related to their shoot
density, and (c) to assess the degree to which biomass, shoot and species density of
bryophytes are correlated with characteristics of the vascular plant layer.
2 For bryophytes there was a positive linear relationship between biomass and species
density. Vascular plant species density was not related to biomass. Furthermore, bryophyte
biomass and species density were linearly and positively related to bryophyte shoot density.
For vascular plants, only biomass but not species density was related to shoot density.
3 We concluded that a bryophyte favourability gradient existed along which biomass
and shoot and species density increased. This gradient was attributed to positive interactions within dense bryophyte stands, high clonal fragmentation, absence of competitive
hierarchies and to the limited ability of larger bryophyte species to replace small species
along this favourability gradient.
4 Since species density for vascular plants varied independently from biomass and
shoot density, there was no such favourability gradient as for bryophytes. Large size
variation, predominantly negative interactions between species, and clonal integration
of species (e.g. tussock-forming grasses and sedges) may be responsible for the different
behaviour of the two taxonomic groups.
5 Bryophyte favourability decreased with increasing vascular plant biomass. Concerning
light availability, we found highest bryophyte favourability at intermediate levels where the
combination of radiation and moisture seems to be optimal for bryophytes. No relationship
was found between bryophyte favourability and vascular plant shoot density and litter mass.
6 The negative relationship between bryophyte favourability and vascular plant biomass
is important for bryophyte conservation. Stands of low vascular plant production are
those with the potential for highest species richness, and should therefore receive conservation priority.
Key-words: above-ground biomass, favourability gradient, positive interaction, species
diversity, wetlands
Journal of Ecology (2001) 89, 920–929
Introduction
Bryophytes are a major component of many plant
communities in terms of both biomass and species
© 2001 British
Ecological Society
Correspondence: Ariel Bergamini, Institut für Systematische
Botanik, Universität Zürich, Zollikerstrasse 107, CH-8008
Zürich, Switzerland (tel. + 41 1634 84 11; fax + 41 1634 84 03;
e-mail [email protected]).
diversity (e.g. Vitt & Pakarinen 1977; Rieley et al. 1979;
Longton 1984; Russel 1990). For example, in wet habitats
such as fens and bogs, their biomass may exceed that of
vascular plants (e.g. Longton 1984; Wheeler & Shaw
1991). Above-ground biomass or ‘productivity’ has been
considered to be one of the most important determinants
of species richness (Grime 1973, 1979; Rosenzweig &
Abramsky 1993), and while for vascular plants this relationship has received much attention since the studies
JEC_613.fm Page 921 Wednesday, November 21, 2001 1:11 PM
921
Productivity, shoot
number, and species
number
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
of Grime (1973, 1979) and Al-Mufti et al. (1977), it is
poorly understood for bryophytes with the notable
exception of rheophytic communities ( Muotka &
Virtanen 1995; Virtanen et al. 2001). We therefore focus
upon the relationship between species number, biomass
and shoot density of both bryophytes and vascular
plants in 18 Swiss montane calcareous fens.
Species richness per unit area (hereafter called
species density following Magurran 1988) of vascular
plants at the level of small plots (≤ 1 m2 ) is often reported
to be highest at intermediate levels of biomass. This
leads to a ‘hump-shaped’ relationship between species
density and biomass (reviewed in Grace 1999). Although
this pattern is perhaps not as widespread as was previously thought (Waide et al. 1999), there is good evidence
to suggest that very high biomass values are antagonistic
to high species densities (Marrs et al. 1996; Grace 1999).
Furthermore, plot biomass depends on the number of
individuals (or ramets for clonal plants) growing in a
plot. Fisher et al. (1943) and Preston (1962), by linking
the number of species to the number of individuals,
proposed that the number of species should increase
with the number of individuals by a probabilistic effect
of drawing different numbers of individuals from a
single pool. However, does this hold for systems, such as
calcareous fens, where most plant species grow clonally
and for bryophytes as well as for vascular plants?
Most bryophytes are ectohydric (Buch 1947), i.e.
they neither possess roots nor an internal vascular
system, and since water and nutrient uptake occurs
over the whole shoot surface, their size is restricted.
Also, since bryophytes cannot regulate water loss, they
frequently dry out and enter a physiologically inactive
state and, thus, are considered to be poikilohydric
plants (Walter 1962). Physiological activity is prolonged
where shoot density is high, because evaporative water
loss is reduced (Proctor 1982) and growth is therefore
often best in dense stands (Bates 1988; Økland &
Økland 1996; but see also Zamfir & Goldberg 2000).
Such a positive effect of density on plant performance
is less common for vascular plants and may occur mostly
in communities growing in rather harsh environments
(Callaghan 1987; Bertness & Callaway 1994).
Many studies have examined interactions among
bryophytes (recently reviewed by Rydin 1997) and effects
of bryophytes upon seedling emergence (e.g. Zamfir
2000) but very few studies have considered how the
performance of the bryophyte layer is dependent upon
structural properties of the vascular plant layer (Watson
1960; Sveinbjörnsson & Oechel 1992; Økland 1994).
Nevertheless, structural properties of the vascular plant
layer, such as biomass, shoot density, leaf area index
(LAI) and litter mass, seem likely to be important since
bryophytes are much smaller than most vascular plants
and interactions for light are thus asymmetric (Rydin
1997). Light absorption of the vascular plant layer
depends mainly on the cumulative leaf area (Monsi &
Saeki 1953) and, in addition to the effect of living tissues,
a thick, persistent litter layer will inhibit the development
of a vigorous bryophyte layer (Cornish 1954; Wheeler
& Giller 1982; van Tooren et al. 1988), mainly by heavy
shading (Sveinbjörnsson & Oechel 1992). Dense stands
of vascular plants could, however, benefit bryophyte
growth if they reduce evaporative water loss by increased
shading. Such contrasting effects could produce a
unimodal relationship between biomass and shoot
density of bryophytes and variables related to vascular
plant productivity. Moreover, if bryophyte species
density is unimodally related to bryophyte biomass, then
bryophyte species density will be lowest at intermediate
vascular plant productivity.
We asked the following specific questions:
1 Is their biomass an adequate predictor of the species
density of bryophytes and vascular plants, and what is
the shape of each relationship?
2 Do biomass and species density of bryophytes and
vascular plants depend upon their shoot density?
3 What is the relationship between the properties of
the bryophyte community (biomass, shoot density and
species density) and those of the vascular plant layer
(biomass, shoot density, LAI and litter mass)? Which
variable or combination of variables best explains
bryophyte variation?
Methods
   
We studied 18 montane wetlands located in the pre-Alps
of north-eastern Switzerland. The sites were randomly
selected from the inventory of Swiss fenlands (BUWAL
1990). They are situated between 800 and 1400 m a.s.l.
and distributed over an area of 3500 km2. The use of
extensive management practices (mowing once a year
in September) means that these montane wetlands can
be considered to be semi-natural communities. Annual
precipitation is relatively high throughout the study
region (1500–2800 mm, Uttinger 1967). Sites have predominantly north to north-western aspects and range
in size from 1 to 9.1 ha. The underlying bedrock consists of various calcareous sediments of tertiary and
mesozoic age (Spicher 1972). Soil pH ranges from 5.4 to
7.2 (mean 6.2) and the vegetation belongs mainly to the
Caricion davallianae alliance (vegetation classification
after Ellenberg 1996).
Species richness of both bryophytes and vascular
plants is high (Peintinger 1999; Bergamini et al. 2001).
Graminoids, such as Carex davalliana Sm., C. panicea
L., Molinia caerulea Moench and Festuca rubra L., and
forbs, such as Potentilla erecta (L.) Räuschel, Trifolium
pratense L., Lotus corniculatus L.s.l. and Succisa pratensis
Moench, are common (Plattner 1996; Peintinger 1999),
and the bryophytes are usually dominated by pleurocarpous mosses, with Calliergonella cuspidata (Hedw.)
Loeske, Climacium dendroides (Hedw.) Web. & Mohr,
Campylium stellatum (Hedw.) J. Lange & C. Jens, Bryum
pseudotriquetrum (Hedw.) Schwaegr., Drepanocladus
cossonii (Schimp.) Loeske and Plagiomnium elatum
JEC_613.fm Page 922 Wednesday, November 21, 2001 1:11 PM
922
A. Bergamini et al.
(B. & S.) T. Kop. being prominent components (Bergamini
et al. 2001).
Data were collected during August 1997 (i.e. at peak
vascular plant biomass) in five randomly selected plots,
each of 20 × 20 cm (0.04 m2), at each site (total n = 90).
Leaf area index (LAI) of vascular plants was used as an
indicator of light availability for bryophytes and was
measured, using a LAI-2000 Plant Canopy Analyser
(LI-COR Inc., Lincoln, Nebraska, USA), at four points
for each plot: one just above the vascular plant canopy
and three within the canopy just above the bryophyte
layer. LAI measurements are only available for 84 plots
due to technical problems. We then clipped vascular
plants just above the bryophyte layer and recorded the
number of species and total shoot number (ramets
rather than shoots were counted for clonal plants and,
as these formed the majority of vascular plants, shoot
number did not equal genet number). Next, all bryophytes were collected, including their brown parts, and
also the litter of vascular plants. Litter was defined as
unattached above-ground vascular plant material. A
provisional list of bryophyte species was prepared in
the field and species identity was subsequently checked
in the laboratory after separation of litter and bryophyte
material. The dry weight of bryophytes (hereafter ‘bryomass’), vascular plants and litter was recorded after
drying for at least 24 h at 70 °C.
After drying, a random subsample (compasing 100–
130 shoots) was taken from each bryophyte sample
from each plot. Shoot number, including lateral branches
of indeterminate growth (offshoots, equivalent to the
main shoot) greater than 10 mm, was counted before
weighing and extrapolating to the whole sample (to
give bryophyte shoot density in each plot).
Peak biomass (excluding the bottom layer of approx.
3 cm) was used as a measure of vascular plant productivity. Although bryomass is the result of accumulation
and decomposition of biomass over several years, there
is good evidence that it is closely related to bryophyte
productivity (Wielgolaski et al. 1981; see also van Tooren
et al. 1988). This was confirmed by our analyses of the
data of Rieley et al. (1979) and Pande & Singh (1988),
both of which resulted in a highly significant positive
linear regression of bryophyte production on bryomass
(R2adj. = 87.2%, P < 0.001, n = 14, and R2adj. = 85.3%,
P < 0.001, n = 18, respectively).
 
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
We used regression models in which ‘site’ was included as
block factor. Plot values were thus adjusted for differences between sites. Plots without LAI values (see above)
were omitted from analyses of the effect of vascular
plant variables on bryophyte variables, but the whole data
set was used otherwise. Variables were log-transformed
(indicated in tables) whenever necessary to obtain
normally distributed residuals and/or to achieve homoscedastic distributions of points around regression
lines. Both linear and quadratic terms were fitted but
only linear regressions are presented where the quadratic
term was not significant (P > 0.05). Irrespective of
significance, the quadratic term is then included to assess
whether the widely cited ‘hump-shaped’ relationship
applies to bryophytes and vascular plants in this study.
To find the most parsimonious regression explaining
variation in bryophyte variables (bryomass, species
density or shoot density), we used the forward selection
strategy after Payne et al. (1993) with the inratio set to 4
(criterion for including a variable in the multiple regression equation). Since bryomass and log (bryophyte shoot
density) were unimodally related to LAI (see Results),
the variables LAI and LAI2 were treated as if they were
one variable in the forward selection. Data were evaluated
for collinearity by the standard procedure provided by
GENSTAT 5.0 program, release 3.2 (Payne et al. 1993).
As a guide to the fit of the regression models we used
the adjusted R2 statistic after Payne et al. (1993), expressed
as a percentage:
R2adj. = 100 × (1 − (residual mean squares/total mean
squares))
All analyses were carried out with the GENSTAT
5.0 program, release 3.2 (Payne et al. 1993).
Results
The bryophyte layer of the studied wetlands was generally well developed, forming a continuous cover with
bare soil rarely visible. Bryophyte species density per
plot varied from 3 to 18 (mean 8.5 ± 0.26, plot area =
0.04 m2) and bryomass from 0.04 g to 13.2 g (6.3 g ± 0.39).
Vascular plant species density (20.3 ± 0.54; range 8–32)
was higher than that of bryophytes. Mean biomass was
also higher for vascular plants (15.9 g ± 1.27; range
4.6–92.0.g) but in 19 plots, distributed over 12 sites,
bryomass exceeded vascular plant biomass.
Shoot densities of bryophytes and vascular plants
were both very variable (8–45 946, mean 1421.4 ± 107.1
and 83–579, mean 248.7 ± 10.4, respectively). Litter
mass was low, varying from 0.7 to 6.0 g per plot (mean
2.4 g ± 0.10).
There were significant (P ≤ 0.05) or marginally significant (P ≤ 0.1) effects of site in most regressions
(Tables 1 and 2), indicating large-scale variability.
There was a positive relationship between species
density and biomass (Table 1a, Fig. 1), but it was linear
rather than unimodal (quadratic term not significant).
In contrast, vascular plant species density was unrelated
to biomass (Table 1b, Fig. 1).
Species density of bryophytes increased with increasing
shoot density, even when four outlying plots with low
densities (Fig. 1) were omitted, but species density was
unrelated to shoot density in vascular plants (Table 1,
Fig. 1).
Biomass was positively related to shoot density for
both groups (Fig. 1), although the relationship for
vascular plants should be interpreted with caution,
JEC_613.fm Page 923 Wednesday, November 21, 2001 1:11 PM
923
Productivity, shoot
number, and species
number
Table 1 Linear or quadratic regressions for the relationships between species density, biomass and shoot density for bryophytes and
vascular plants. Percentage variance accounted for by each regression is the adjusted R2 statistic. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001
Dependent variable
(a) Bryophytes
Species density
Species density
Log [Biomass]
(b) Vascular plants
Species density
Species density
Log [Biomass]
Source of variation
d.f.
SS
F-ratio
Site
Bryophyte biomass
[Bryophyte biomass]2
Residuals
Site
Log [Bryophyte shoot density]
Residuals
Site
Log [Bryophyte shoot density]
Residuals
17
1
1
70
17
1
71
17
1
71
141.60
146.59
0.05
250.15
141.60
93.15
303.65
6.09
9.53
1.98
Site
Log [Vascular plant biomass]
(Log [Vascular plant biomass] )2
Residuals
Site
Log [Vascular plant shoot density]
Residuals
Site
Log [Vascular plant shoot density]
Residuals
17
1
1
70
17
1
71
17
1
71
1135.69
36.20
40.67
1109.93
1135.69
3.24
1183.56
1.38
0.25
2.41
R2adj.
2.33**
41.02***
0.01
40.9%
1.95*
21.78***
29.3%
12.84***
341.91***
85.9%
4.21***
2.28
2.56
39.2%
4.01***
0.19
36.1%
2.39**
7.27**
25.1%
Table 2 Linear or quadratic regressions for the relationships between properties of the vascular plant layer (biomass, shoot
density, litter mass and LAI) and bryophyte variables (biomass, shoot density and species density). Percentage variance accounted
for by each regression is the adjusted R 2 statistic. †P ≤ 0.1, *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001
Dependent variable
Source of variation
d.f.
SS
F-ratio
(a) Bryophyte biomass
Site
Log [Vascular plant biomass]
Residuals
Site
Vascular plant shoot density
Residuals
Site
LAI
LAI2
Residuals
Site
Litter mass
Residuals
Site
Log [Vascular plant biomass]
Residuals
Site
Vascular plant shoot density
Residuals
Site
LAI
LAI2
Residuals
Site
Litter mass
Residuals
Site
Log [Vascular plant biomass]
Residuals
Site
Vascular plant shoot density
Residuals
Site
LAI
LAI2
Residuals
Site
Litter mass
Residuals
16
1
66
16
1
66
16
1
1
65
16
1
66
16
1
66
16
1
66
16
1
1
65
16
1
66
16
1
66
16
1
66
16
1
1
65
16
1
66
318.82
209.07
545.39
318.82
55.89
698.57
318.82
35.44
109.79
609.23
318.82
1.11
753.36
20859232
16214755
52133429
4.90
0.29
11.34
4.90
0.15
1.26
10.23
4.90
0.01
11.62
0.391
0.076
0.914
137.26
19.67
355.88
137.260
0.001
9.858
375.549
137.26
0.46
375.10
2.41**
25.30***
Bryophyte biomass
Bryophyte biomass
Bryophyte biomass
(b) Bryophyte shoot density
Log [Bryophyte shoot density]
Log [Bryophyte shoot density]
Log [Bryophyte shoot density]
(c) Log [Bryophyte species density]
Bryophyte species density
Bryophyte species density
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
Bryophyte species density
R2adj.
36.1%
1.88*
5.28*
18.1%
2.13*
3.78†
11.71***
27.5%
1.75†
1.10
11.7%
1.65†
20.53***
26.5%
1.78†
1.71
13.7%
1.95*
0.92
8.00**
21.0%
1.74†
0.04
11.6%
1.76†
5.50*
16.8%
1.59†
3.65†
12.7%
1.51
0.00
1.75
8.9%
1.51
0.08
8.0%
JEC_613.fm Page 924 Wednesday, November 21, 2001 1:11 PM
924
A. Bergamini et al.
Fig. 1 Relationships between species density, biomass and number of shoots for two plant groups (bryophytes and vascular
plants). Dependent variables are adjusted for the effects of site (see Methods). Regression lines are only drawn for significant
relationships (P ≤ 0.05).
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
since biomass was more variable at high densities than
low densities. Omitting the three plots with the lowest
biomass did not influence the outcome of the regression
for bryophytes, which showed a much closer relationship than vascular plants (R2adj . = 85.9% vs. 25.1%).
The relationship between bryomass and the vascular
plant layer depended on the variable considered (Table 2a,
Fig. 2a). Bryomass was negatively related to vascular
plant biomass and vascular plant shoot density, but
weakly unimodally related to LAI and unrelated to litter
mass.
Bryophyte shoot density was related only to vascular
plant biomass (linear decrease) and LAI (unimodal,
although this must be interpreted with caution, since
shoot density was more variable at high and low light
availability than at intermediate light availability
(Table 2b, Fig. 2b)).
Bryophyte species density also declined with vascular
plant biomass (Table 2c, Fig. 2c). Highest and lowest species densities were reached at intermediate light availability (Fig. 2C). Unlike for bryomass and bryophyte shoot
density this relationship was therefore not significant.
JEC_613.fm Page 925 Wednesday, November 21, 2001 1:11 PM
Fig. 2 Relationships between bryophyte variables (a, biomass; b, shoot density; and c, bryophyte species density) and properties of the vascular plant layer (vascular plant biomass, vascular plant shoot density,
LAI, and litter mass). Dependent variables are adjusted for the effects of sites (see Methods). Regression lines are only drawn for significant (P ≤ 0.05) relationships.
JEC_613.fm Page 926 Wednesday, November 21, 2001 1:11 PM
926
A. Bergamini et al.
Table 3 Best multiple regressions for the relationships between properties of the vascular plant layer and bryophyte biomass or
shoot density, respectively. For bryophyte species density the most parsimonious regression was on site and log [vascular plant
biomass]. Results of this regression are already presented in Table 2c. Percentage variance accounted for by each regression is the
adjusted R 2 statistic. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001
Dependent variable
Source of variation
d.f.
(a) Bryophyte biomass
Site
Log [Vascular plant biomass]
LAI
LAI2
Residuals
16
1
1
1
64
318.82
209.07
0.17
75.55
469.67
2.72**
28.49***
0.02
10.29**
Site
LAI
LAI2
Log [Vascular plant biomass]
Residuals
16
1
1
1
64
20859232
7284222
11996153
7960968
41106841
2.03*
11.34***
18.68***
12.39***
(b) Bryophyte shoot density
For both bryomass and bryophyte species density,
stepwise multiple regression showed that vascular
plant biomass and light availability (LAI + LAI2 ) were
the best explanatory variables, although in the opposite
order (Table 3). Both regressions explained only about
40% of the variation of the dependent variables, indicating that there are other major sources of variation. Biomass was the only vascular plant variable significantly
contributing to the explanation of bryophyte species
density (Table 2c).
Discussion
   
,    
    
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
The two taxonomic groups showed different biomassspecies density relationships: there was a positive linear
relationship for bryophytes but no significant correlation in vascular plants. Neither showed the expected
unimodal relationship between biomass and species
density as predicted by Grime (1973), Tilman & Pacala
(1993) and Huston (1994).
No comparable data are available for wetland bryophytes. Virtanen et al. (2001), studying the relationship
between biomass and species density in rheophytic
bryophyte assemblages, found three different patterns:
a positive linear relationship in two streams (out of 14),
a unimodal relationship in five streams, and no relation
at all in seven streams. Unimodal relationships occurred
only in streams with largely different microhabitats
that allowed for small-scale community diversification.
Our results for vascular plants were consistent with
the findings of some previous wetland studies (e.g.
Vermeer & Verhoeven 1987; Moore & Keddy 1989),
although others have detected a humped relationship
in such habitats (e.g. Wheeler & Giller 1982; Moore et al.
1989; Garcia et al. 1993).
A comprehensive review by Waide et al. (1999) showed
that only 30% of the studies investigating the biomassspecies density relationship revealed a unimodal pattern
SS
F-ratio
R 2adj.
43.2%
40.2%
and there is growing evidence that this depends upon
the spatial scale that is studied (Moore & Keddy 1989;
Waide et al. 1999; Weiher 1999; Gross et al. 2000;
Virtanen et al. 2001). When studies within plant communities are considered, only 24% reveal a unimodal
pattern, with 42% showing no relationship and 22% a
positive one (Waide et al. 1999). The limited range of
biomass values within a community may be too narrow
to demonstrate an underlying unimodal relationship
(Rosenzweig & Abramsky 1993; Grace 1999), consistent
here with the two plots with the highest vascular plant
biomass showing low species density but not with the
bryophyte data (highest biomass in plots with highest
number of species, Fig. 1). The influence of other variables
(e.g. successional age or edaphic conditions of sites) may
also affect productivity and thus the species densitybiomass relationship (Gough et al. 2000; Loreau 2000).
Favourability gradients
The denser bryophytes grow, the more biomass they
produce in total and the more species live together in
small plots (see Fig. 1). Økland (1994) was the first to
talk about such bryophyte ‘favourability gradients’
whereby the water content of the shoots rises as their
density increases (Proctor 1982) and bryophytes are
therefore able to remain photosynthetically active for a
greater part of the growing season and show greater
biomass production (Bates 1988). This positive densitydependent relationship appears to be a common phenomenon in bryophyte assemblages (e.g. Økland &
Økland 1996; Økland 2000) and, in combination with
high clonal fragmentation and the absence of competitive
hierarchies (at least in comparable bryophyte communities in chalk grasslands, During & van Tooren 1988),
this is probably responsible for the parallel increases in
biomass, shoot density and species density.
In contrast, vascular plants showed no such favourability gradient. Although denser growing shoots produced more biomass, species density was not related to
biomass or shoot density. Tilman & Pacala (1993) have
explained decreases in vascular plant species density at
JEC_613.fm Page 927 Wednesday, November 21, 2001 1:11 PM
927
Productivity, shoot
number, and species
number
high levels of productivity as a response to light limitation. Under these conditions, slow-growing species are
outcompeted by a few highly competitive fast-growing
species: according to an extension of the self-thinning
law to thinning in mixed communities, larger species
replace smaller species under more productive conditions
(Bazzaz & Harper 1976; Schmid 1991; but see Stevens
& Carson 1999b). Although this may be true for some
vascular plant communities, it seems unlikely for bryophytes, whose shoot size is much more restricted by
morphological and physiological constraints (cf. the
tall herb Angelica sylvestris vs. the small grass Festuca
rubra). Thus, under more productive conditions, the
increase of bryomass per plot is likely to be mainly an
effect of increased shoot density, leading to the very close
relationship between these variables in bryophytes but
not in vascular plants.
Effects of clonality
The domination of the vegetation in these wetlands by
clonal plants (personal observation) may have important consequences for the relationship between species
density and biomass. Specifically, the probability of
two neighbouring shoots belonging to the same species
is much higher than in communities dominated by nonclonal species, where species density and shoot density
are often found to be closely related (Condit et al. 1996;
Stevens & Carson 1999a). The lack of a relationship for
vascular plants was therefore as expected.
Most bryophytes exhibit clonal growth patterns
(During 1990) and experimental evidence is growing
that physiological integration of ramets in ectohydric
bryophytes may reach levels comparable with those of
clonal vascular plants (Alpert 1989; Økland et al. 1997;
Eckstein & Karlsson 1999). There was, nevertheless, a
close relationship between bryophyte species density
and shoot density, suggesting dominance of non-clonal
plants (cf. Stevens & Carson 1999a). Light limitation in
dense stands and therefore browning of shoots in the
lower strata of the bryophyte canopy (van der Hoeven
& During 1997), may lead to earlier physical disintegration of connections between ramets and parent plants in
bryophytes than in many clonal vascular plants, such
as tussock-forming grasses and sedges, so that physiological integration is unlikely to regulate bryophyte
shoot density. Even in vascular plants, whether regulation of shoot density operates via physiological integration or via external factors, is still a matter of debate
(Suzuki & Hutchings 1997; Meyer & Schmid 1999).
   
   
 
© 2001 British
Ecological Society,
Journal of Ecology,
89, 920 – 929
Of all the measured variables of the vascular plant
layer, biomass was the best single predictor of each of
the three, intercorrelated bryophyte variables, with bryophyte favourability decreasing with increasing vascular
plant biomass. There was a unimodal relationship between
bryophyte favourability and light availability, most
likely caused by a combination of optimal radiation
and moisture at intermediate light levels, which leads to
longer periods of photosynthetic activity for the ectoand poikilohydric bryophytes and, thus, to their higher
growth rates (Callaghan et al. 1978; Bates 1988; Økland
& Økland 1996; Økland 2000). However, scatter in our
data was high, indicating that important sources of
variation were not included in our restricted set of
explanatory variables. Marrs et al. (1996) proposed
that such variable data sets should be characterized by
boundary conditions rather than by regression analyses
that obscure valuable information. This would include
the observation that values of bryophyte biomass, shoot
density and species density were particularly unpredictable
at intermediate light levels, although favourability was
consistently low at both ends of the gradient.
Litter mass of vascular plants accounted for only a
small part of the bryophyte variation, possibly due to
the low levels maintained by yearly mowing.
Although biomass is a rather crude descriptor of the
structure of the vascular plant layer (Spehn et al. 2000)
and thus of the environment experienced by bryophytes
(Watson 1960; Sveinbjörnsson & Oechel 1992), the negative relationship is important for bryophyte conservation: stands of low vascular plant production are richer
in bryophytes. Since vascular plant growth in fens is
mainly controlled by the availability of phosphorus
and nitrogen (Verhoeven & Schmitz 1991; Pauli 1998),
it is crucial to curb supply of these nutrients in order to
maintain bryophyte diversity.
Acknowledgements
We are grateful to the nature conservancy authorities
of the cantons of Schwyz, Glarus, St Gallen and
Appenzell-Ausserrhoden and to the land owners of the
investigated sites who allowed us to conduct this study.
We thank E. Urmi, P. Alpert, R. Økland and one anonymous referee for various comments which improved
earlier versions of the manuscript considerably,
M. Naegeli for helping with the very time-consuming
separation of vascular plant litter and bryophytes, and
M. Matthews for linguistic aid.
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Received 2 October 2000
revision accepted 19 March 2001