Light quantity determines understory plant layer diversity (Hypothesis 1)
We found that light explained most of the variation in understory species richness, and that species richness asymptotically increases with light availability. Thus, we support the generally accepted view of light quantity being the most important driver of composition and diversity of understory plant species (e.g. Márialigeti et al. 2016). In forest understories, light is a limited resource for many plant species as species richness increases with increasing light. Nevertheless, this relationship was humpbacked as species richness reached a plateau and started to decline above 40% to 50% light availability. However, there were few data above 50% light so we have to be careful about making strong inferences regarding the decline in understory plant species richness above 50% light. Of course, plant species richness in the understory is not only controlled by light. Among the other resources tested, pH also showed a hump-shaped relationship with species richness. An increase in species richness with increasing pH is widely accepted (Grime and others 1973, Ewald 2003, Hofmeister et al. 2009, Leuschner et al. 2017). Surprisingly, species richness decreased at moderately high pH values (~ pH 6) in our study. Plant diversity slightly decreased with increasing soil C:N–ratio, while neither C nor N concentrations in soil were correlated with species richness. We suggest that this decrease is related to the prevailing forms of stand humus, with mull-type (low soil C:N ratios, rather high pH values) forest communities harbouring generally more understory species than raw humus or moder type communities with high C:N ratios and relatively low pH (Leuschner et al. 2017).