Aim of the study and hypotheses
The aim of this study was to determine (1) the relative importance of
light and nutrient soil drivers on affecting understory plant species
richness, (2) whether structural complexity of the tree canopy induces
heterogeneity in these abiotic factors at the patch scale, and (3)
whether such heterogeneity affects understory plant species richness. So
far, studies that have investigated the habitat-heterogeneity hypothesis
for forest understory plant species actually have rarely tested whether
species richness increases when resources are heterogeneously
distributed on such a small scale (Reich et al. 2012, Su et al.
2019).
With respect to the above- mentioned research questions, we hypothesize
that (H1) light is a more important driver of understory plant species
richness than soil nutrient factors, that (H2) an increase in stand
structural complexity results in an increase in light and soil resource
heterogeneity, and that (H3) understory plant diversity increases with
increasing resource heterogeneity. If hypotheses (H2) and (H3) are
verified, we will further test the hypothesis that (H4) one can use
measures of stand structural complexity to predict understory plant
diversity (Fig. 1).
To test these hypotheses, we determined understory plant species
richness and analyzed the spatio-temporal availability of light and soil
nutrient resources in temperate forest stands along a gradient of stand
structural complexity which had been created through different
management interventions in the past.
Material and methods