Table 1.

The relationship between old-growth forest species richness at the site-level and drought severity, edge exposure and their interaction in model 1. Drought intensity is defined as the absolute precipitation (low precipitation is high drought intensity), in this model over the whole summer of 2018, 1st May - August 31st. Negative coefficients thus denote a negative relationship between drought and species richness. Species richness was analyzed in total, and categorized into organism group and substrate association. The table shows the standardized parameter estimates and significance is indicated as follows: *** p < 0.001 ** p < 0.01, * p < 0.05, . p < 0.1. Interaction effects are best evaluated by inspecting the graphs in Fig. 3

	Drought intensity	Edge exposure	Interaction	Background climate		Pseudo R2 main variables	Total num of species
	Summer 2018	Edge exposure (%)	Summer drought *Edge exposure	Average summer precipitation (mm)	Growing degree days
Overalla, P	− 0.09	− 0.32 ***	− 0.19 **	− 0.09	− 0.33***	0.002	75
Organism group
Lichensa, QP	− 0.07	− 0.55**	− 0.31 **	− 0.15	− 0.61***	0.05	39
Cyano lichensa, P	− 0.18	− 0.70***	− 0.33*	− 0.20	− 0.51**	0.13	13
Chloro lichensa, QP	− 0.02	− 0.45	− 0.31*	− 0.11	− 0.70**	0	26
Bryophytesa, P	− 0.25*	− 0.09	− 0.06	− 0.08	− 0.08	0.08	29
Vascular plantsb, QP	0.11	− 0.18	− 0.11	− 0.07	0.10	0	7
Substrate association
Epiphytica, P	− 0.04	− 0.47***	− 0.25**	− 0.18.	− 0.50***	0	31
Epiphytic low-pH barkb,P	0.15	− 0.08	− 0.04	− 0.18	− 0.50 ***	0	12
Epiphytic high-pH barka,P	− 0.13	− 0.54**	− 0.30**	− 0.23	− 0.42**	0.10	19
Epilitic + epixylicb, QP	− 0.11	− 0.20	− 0.22**	− 0.14	− 0.26	0.20	42
Epigeicb, QP	− 0.07	− 0.24*	− 0.06	− 0.09	− 0.07	0	14

Drought intensity is defined as the absolute precipitation (low precipitation is high drought intensity), in this model over the whole summer of 2018, 1st May–August 31st. Negative coefficients thus denote a negative relationship between drought and species richness. Species richness was analyzed in total, and categorized into organism group and substrate association. The table shows the standardized parameter estimates and significance is indicated as follows: ***p < 0.001, **p < 0.01, *p < 0.05, p < 0.1. Interaction effects are best evaluated by inspecting the graphs in Fig. 3

P Poisson distribution, QP quasi-Poisson distribution

^an = 35

^bn = 60