Light analysis
We determined light environment at the subplot level with hemispheric
photography taken with a Nikon D90 (Nikon, Tokyo, Japan) camera equipped
with a Sigma 4,5 mm F2,8 EX DC HSM circular fisheye-lens (Sigma,
Kawasaki, Japan). The camera was placed in the centre of the subplots 1
m above the forest floor with the camera lens facing north and levelled
horizontally. The hemispheric photographs were taken in 2018, one year
after the major plant survey. Missing values are caused by tree removal
through logging on the plot or the absence of plot markings. The photos
were prepared with an image processing software (Darktable version
2.4.2, GPL 3.0 ©2017, Darktable team) to adjust lighting. We calculated
the diffuse light index (DLI) with the software
Hemisfer
(WSL, Schleppi et
al.
2007). As a measure of spatial light heterogeneity, we calculated the
standard deviation of DLI from the six subplots.