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