Replace me if you can: Abundance of advance regeneration under canopy trees in a primeval beech forest

Tree replacement patterns strongly shape species coexistence and dominance in forest ecosystems. In mixed forests subject to a small-scale disturbance regime, dead canopy trees are often replaced by advance regeneration. We studied the abundance of saplings (6–10 cm dbh) and poles (10–25 cm dbh) under canopy trees based on four inventories on a 10-ha permanent plot in a large primary forest dominated by Fagus sylvatica in Ukraine. Saplings and poles of Fagus sylvatica, Acer platanoides, A. pseudoplatanus and Ulmus glabra were spatially linked to canopy trees (‘hosts’, dbh > 25 cm) based on their crown radius, and the hosts’ neighborhood was classified according to the presence of dead trees. The number of saplings and poles under hosts was modelled with a Bayesian approach. There was a higher number of advance regeneration under hosts with increasing host diameter. The abundance of advance regeneration was the lowest under hosts in the canopy (i.e., under shaded conditions) and highest for hosts growing in gaps. 1) Under the canopy, only F. sylvatica poles can replace heterospecific hosts > 80 cm dbh, while other pole species are clearly below the replacement threshold. 2) Near dead lying trees, Acer pseudoplatanus rarely achieved an abundance of one pole at a host dbh > 80 cm. 3) In gaps, Acer spp. poles were able to replace F. sylvatica hosts. The proximity to dead lying trees and gaps is crucial for Acer spp. and U. glabra saplings for recruitment to the pole stage. The higher rate of recruitment under hosts adjacent to dead lying trees suggests that the total length of gap edges may be as important as gap size or disturbance frequency. In gaps > 550 m², advance regeneration without hosts (growing in gaps) of Acer spp. can recruit higher number of saplings than F. sylvatica, but not poles. We conclude that almost all canopy trees of Acer spp. and U. glabra are likely to be replaced by Fagus sylvatica if the current small-scale disturbance regime and single-tree mortality continue to prevail.