Coarse woody debris facilitates oak recruitment in Bialowieza Primeval Forest, Poland

Abstract

While oaks contribute to the ecology and economy of temperate lowland forests, recruitment into larger size classes is generally scarce. Ungulate herbivory is a limiting factor for tree recruitment, thus natural structures protecting against ungulates may be important for recruitment success. We studied oak recruitment in relation to coarse woody debris (CWD) in the Białowieża National Park, Poland, one of the last remaining reference sites for undisturbed temperate lowland forests in Europe that contains the full natural assemblage of large ungulate species and their predators. We compared two contrasting management types: one that excludes all forestry activities (the strict reserve) and one where sanitary cutting and removal of bark-beetle infested Picea abies occurs to prevent further spread. We sampled 4 m wide transects (19 ha in total, of which 15.8 ha in the strict reserve) and recorded sapling height and sapling browsing, distance from, height of and CWD encirclement, distance to and diameter of mature oaks, tree composition, distance to and diameter of nearest trees, canopy openness and cover of herbs, mosses, bare soil and litter in an equal number of plots with and without saplings. We found 161 oak saplings (8.5/ha). CWD was predominantly formed by P. abies. Sapling plots contained more often CWD than plots without saplings. Also, the distance to CWD was smaller and the CWD encirclement was higher in sapling plots, while canopy cover did not differ. Sapling height increased and sapling browsing decreased with increasing presence of CWD. Both CWD and recruitment beyond 50 cm were reduced in the managed zone. Our study shows that CWD plays an essential role for the recruitment of oaks in old-growth temperate forest systems, predominantly via protection against ungulate browsing. Sanitary cutting of infested trees, a common management practice in temperate forests, prevents the natural accumulation of CWD and so can be counter-productive for the recruitment of appreciated species