Combined retention of large living and dead trees can improve provision of tree-related microhabitats in Central European montane forests

Abstract

Retention of habitat trees is a common biodiversity conservation practice in continuous cover forests of temperate Europe. Commonly, living habitat trees are selected on the basis of their tree-related microhabitats (TreMs) such as cavities or crown deadwood. Owing to the increasing frequency and intensity of climate change-related disturbances, habitat trees in particular are expected to experience increased mortality rates. This may impact the long-term provisioning of TreMs. Here, we compared the TreM occurrence on living and dead trees to investigate whether dead trees support more and other TreMs than living trees. We also hypothesized that a combination of living and dead trees results in the most diverse stand-level TreM composition. We surveyed the TreM composition of living and dead habitat trees in 133 one-hectare plots in the Black Forest region managed according to a continuous cover approach. We fitted generalized linear mixed models to identify the main predictors of TreM occurrence to predict their abundance and richness. Tree identity (as a combination of species and vitality status) and diameter were the main drivers of TreM abundance and richness, which were highest on dead Abies alba. Even though dead A. alba and Picea abies supported TreM numbers similar to those provided by large living trees, their TreM composition was significantly different. This suggests that dead trees cannot substitute the habitat functions of living habitat trees, but they can complement them to increase the overall stand-level TreM diversity, in particular through decayed, large snags.

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

“…North-facing slopes, with their more humid microclimate, favour tree cavity formation (Remm and Lõhmus, 2011) and epiphyte growth (Esseen and Ekström, 2023), potentially leading to an increased abundance of TreMs. However, the relationship between TreM numbers and northfacing slopes was not evident in mixed beech-spruce-fir forests (Asbeck et al, 2022). Hence, our findings suggest that the influence of slope exposure on TreMs depends on the local landscape context that determines the climatic conditions prevailing in the studied region.…”

“…Our study failed to reveal any relationship between TreM indices and the presence of canopy gaps and their characteristics. A previous study of mixed Norway spruce-European beech-silver fir forests showed that the TreM density increased with the percentage of canopy gaps (Frey et al, 2020), although this research was conducted in a managed forest, where certain ecological processes, including TreM formation, may be disrupted as a result of former tree removal (Asbeck et al, 2022). In natural European beech forests, canopy gaps, originating mostly from small-scale disturbances caused by strong winds or snowfall, lead to the accumulation of TreM-rich dead, wounded and damaged living trees (Lewandowski et al, 2021).…”

“…Secondary or previously managed forests have very much smaller numbers of snags and large living trees, resulting in the absence of certain TreM types Paillet et al, 2017). As a result, managed forests typically have fewer TreMs and do not have TreM profiles typical for ancient stands (Asbeck et al, 2022).…”

Enhancing structural complexity: An experiment conducted in the Black Forest National Park, Germany

“…These structures have been reported to increase with tree diameter at breast height (DBH) and are often more abundant in broadleaved trees than in conifers and differ between dead and live trees (Paillet et al, 2017;Spinu et al, 2022) and in primary compared to managed forests (Asbeck, Kozák, et al, 2021). To our knowledge, this is the first structural complexity enhancement (SCE) experiment done in secondary spruce forests that also addressed the dynamics of TreMs.…”

“…These structures provide important habitats for forest‐dwelling species and are defined as “distinct, well delineated structures occurring on living or standing dead trees, that constitute a particular and essential substrate or life site for species or species communities during at least a part of their life cycle to develop, feed, shelter or breed” (Larrieu et al, 2018 ). These structures have been reported to increase with tree diameter at breast height (DBH) and are often more abundant in broadleaved trees than in conifers and differ between dead and live trees (Paillet et al, 2017 ; Spinu et al, 2022 ) and in primary compared to managed forests (Asbeck, Kozák, et al, 2021 ). To our knowledge, this is the first structural complexity enhancement (SCE) experiment done in secondary spruce forests that also addressed the dynamics of TreMs.…”