Bark beetles as ecosystem engineers: triggered tree mortality rearranges the assemblage of Tree-related Microhabitats in old-growth coniferous forest

Although species with ecosystem engineering capabilities like woodpeckers, ungulates or saproxylic insects may initiate the formation of Tree-related Microhabitats (TreMs), their overall impact on TreM assemblages remains unexplored. Bark beetle activity can increase tree mortality, accelerating the emergence of canopy gaps with altered tree species composition, stand structure and deadwood resources, all of which can influence TreM profiles and cascade on biodiversity. We investigated the TreM assemblage in a strictly-protected old-growth Norway spruce Picea abies forest in the Tatra Mountains (Carpathians, S Poland), comparing bark beetle-induced canopy gaps with undisturbed closed-canopy reference sites. Although TreM richness, density and diversity did not differ between the canopy gaps and reference sites, the frequency and density of woodpecker cavities, splintered stems, cracks, bark pockets, bark shelters, fruiting bodies of polypore fungi and epiphytic ferns was higher in the canopy gap TreM assemblage. In contrast, the frequency and density of exposed sapwood, shaded dead branches, dead tree tops, coarse bark, root buttress cavities, cankers, resin runs and microsoil in the tree crown or bark were higher in the reference sites. TreM richness, density and diversity were correlated with the density of living or dead standing trees and tree species richness, all related to previous bark beetle activity. Bark beetle-induced canopy gaps did not increase the total TreM abundance or diversity, but by hosting more saproxylic TreMs, locally altered the TreM composition. Our study underlines the importance of ecosystem engineering by bark beetles as drivers of the heterogeneity of temperate coniferous montane forests.