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).