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.