Przepiora, F. | bioRxiv Science

bioRxiv2025

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. HighlightsO_LITreMs in coniferous forests are abundant but less diverse than in deciduous ones C_LIO_LICanopy gaps have a distinct TreM profile compared to intact old-growth forest C_LIO_LICanopy gaps are hot-spots of certain TreMs in coniferous forests C_LIO_LISaproxylic TreMs are 3-9 times more abundant in canopy gaps than in intact forests C_LIO_LITree mortality triggered by bark beetles increases habitat heterogeneity C_LI

ecology↗

bioRxiv2024

Spatial distribution of Tree-related Microhabitats in a primeval mountain forest: from natural patterns to landscape planning and forest management recommendations

Tree-related Microhabitats (TreMs) are essential for sustaining forest biodiversity. Although TreMs represent ephemeral resources that are spread across the landscape, their spatial distribution within temperate forests remains poorly understood. To address this knowledge gap, we conducted a study on 90 sample plots (0.05 ha each) located in a primeval mountain European beech Fagus sylvatica-dominated forest (Bieszczady Mountains, Carpathians). We explored the TreM profile with its link to habitat characteristics and described the spatial distribution of TreM indices. We identified 61 TreM types, with a mean richness of 19.7 {+/-}4.9 SD TreM types per plot, a mean density of 740.7 {+/-}292.5 SD TreM-bearing trees ha-1 and a mean TreM diversity of 1.2 {+/-}0.1 SD. The diameter and living status of trees (living vs dead standing tree) were correlated with TreM richness on an individual tree. The stand structure, i.e. density and/or basal area of living and/or dead standing trees, and topographic conditions, i.e. slope exposure, were correlated with the TreM richness, density and diversity recorded on a study plot. We found no relationship between TreM richness, density and diversity and the presence of canopy gaps, which indicates that the influence of small-scale disturbances on the TreM profile is limited. However, our analysis revealed a clustered spatial pattern of TreM indices, with TreM-rich habitat patches (hot-spots) covering [~]20% of the forest. A moderate TreM richness, density and diversity dominated [~]60% of the forest, while TreM-poor habitat patches (cold-spots) covered [~]20%. Based on our findings, we advise the transfer of knowledge on the spatial distribution of TreMs from primeval to managed forests and advocate the 2:6:2 triad rule: to allocate 20% of forests as strictly protected areas, to dedicate 60% to low-intensity forest management with the retention of large living trees and all dead standing trees, and to use the remaining 20% for intensive timber production. To ensure the continuance of the majority of TreM types, [≥] 55 living trees ha-1 > 60 cm in diameter should be retained. Such an approach will maintain a rich and diverse TreM assemblage across a broad spatial scale, which in turn will support biodiversity conservation and ecosystem restoration in secondary or managed forests. HighlightsO_LIPrimeval mountain beech forest hosts rich and abundant TreM assemblage C_LIO_LIStand structure, slope exposure, but not canopy gaps mould TreM profiles C_LIO_LIHabitat patches with high/low TreM indices are spatially clustered C_LIO_LIHot- and cold-spots each make up 20% of TreM richness, density and diversity C_LIO_LI2:6:2 - the protected: managed forest ratio for maintaining the TreM assemblage C_LI

ecology↗