Resource heterogeneity increases plant diversity (Hypothesis 3)
The heterogeneity-diversity relationship is one of the central
hypotheses explaining the diversity of plant species. It states that
environmental heterogeneity should prevent competitive equilibrium,
increasing the available niche space and thus allowing more species to
coexist
(Huston
1979, Tokeshi 2009). Although it is often tested at the macro-ecological
scale
(Stein
et al. 2014), it should in principle be applicable at smaller scales as
well. In terms of light conditions, this hypothesis predicts coexistence
of species that are adapted to different levels of light. Our findings
generally support this view. Species richness was found to increase with
light heterogeneity. To our knowledge, only one other study(Reich et al.
2012) explicitly tested the heterogeneity-diversity hypothesis for light
with direct measurements of light heterogeneity. In that study,
understory species richness was also found to increase with increasing
light heterogeneity.
Light heterogeneity increased species richness significantly, while the
heterogeneity of other resources such as soil pH, C:N ratio, P and K did
not. This finding can be explained in two ways: either the scale was not
suitable (see above, i.e. if grain size of heterogeneity in soil
resources does not match the sampling size of the plant community), or
the level of heterogeneity was simply not large enough to provide
different resource niches for understory plants. Moreover, among tested
resources, light was the most important factor for increased species
richness. Possibly there is a link between the importance of a resource
and the effect of its heterogeneity on species richness.
Canopy
complexity begets plant diversity (Hypothesis 4)
We have shown that the structural complexity of the forest canopy
results in spatio-temporal heterogeneity of light conditions at the
forest floor. We have further shown that this heterogeneity in light
conditions promotes plant species richness. These findings suggest that
the determination of canopy complexity by remote sensing techniques
might have considerable potential for predicting environmental
conditions and understory plant diversity of forests. Former studies
have shown that the structural complexity index SSCI, which was also
used here, reliably predicts important microclimatic parameters such as
diurnal temperature and vapour pressure differences
(Ehbrecht
et al. 2019).
Getzin
et al. (2012) found a significant relationship between gap size and
species richness, which points to the importance of light availability
for species richness in forests.
Nevertheless, for both a
mechanistic understanding of ecosystem functioning and nature
conservation, not only the number of species present, but also species
composition, including the occurrence of dominant or rare species, are
important aspects of local biodiversity. Thus, methods for indirectly
predicting species richness will never replace field surveys, but may be
helpful under certain circumstances, as in remote areas or inaccessible
terrains.
This study indicates that both light quantity and heterogeneity at the
microscale positively affect understory plant species diversity in
mountainous temperate forests of Germany. By confirming the
heterogeneity-diversity hypothesis, we were able to answer a fundamental
research question. However, these results also have important practical
implications for how best to manage these forests to maintain understory
plant diversity. As plants are primary producers and are important for
many other species at higher trophic levels, it is essential to preserve
or enhance plant diversity in the understory. In this study the
relationship found between canopy crown complexity and light
heterogeneity suggests that understory plant diversity could be
increased in forests managed by single tree harvesting by spatially
varying the quantities of trees to be logged to create more heterogenous
understory light environment.
Acknowledgements – We are grateful to the foresters of the state
of Baden-Württemberg for the provided facilities in the forests. We
thank Ilse Storch and Johannes Penner for the coordination of ConFoBi.
Further, we thank Lana Ruddick for language revision.
Funding – This study was funded by the German Research
Foundation (DFG) within the Research Training Group ConFoBi (grant
number GRK 2123/1 TPX) and a scholarship from the Internationale
Graduiertenakademie (IGA) at University of Freiburg to JH.
Data accessibility statement – The data that support the
findings of this study are openly available in Dryad Digital Repository.
(Data will be uploaded by acceptance of the manuscript)
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