Nothing else matters? Tree diameter and living status have more effects than biogeoclimatic context on microhabitat number and occurrence: An analysis in French forest reserves

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.

The Effects of the Selective Removal of Adjacent Trees on the Diversity of Oak-Hosted Epiphytes and Tree-Related Microhabitats

Restoration efforts to maintain oak-dominated habitats and enhance biodiversity often employ selective thinning, but its long-term effect on multiple taxa remains unclear. This study examines the effects of halo thinning around pedunculate oak on epiphytic and tree-related microhabitat (TreM) diversity in the boreal-nemoral forest zone. We revisited nine sites in Latvia with 150-331-year-old oaks, where thinning was conducted in 2003-2004. Epiphyte species composition and cover were assessed, and TreMs were evaluated using standardised methods. Diversity indices (Hill numbers, q = 0; 1; 2; 3) and statistical models (LMMs, Poisson GLMMs) were used to analyse the effects of thinning on species richness and TreM occurrence. Halo thinning over the past 20 years has not caused significant differences in epiphyte or TreM diversity, though managed trees exhibited a higher occurrence of the most common microhabitats. These findings suggest halo thinning may enhance specific TreM features but do not substantially promote epiphyte and TreM diversity. Future research should implement systematic monitoring, deriving the relationships between the conditions after the thinning and their effects, thus serving as the basis for adaptive habitat management strategies. Expanding the scope of such studies is essential for developing evidence-based forest management strategies.

Profile of tree-related microhabitats in the primeval Białowieża Forest: A benchmark for temperate woodlands

Tree-related Microhabitats (TreMs) are a key structural element having a significant impact on the biodiversity and functioning of forest ecosystems. Although forests enjoying long-term protection host richer and more abundant TreMs compared to managed stands, the quantity and quality of such microstructures in primeval temperate forests are unknown. This study investigates for the first time the assemblage of TreMs in the Białowieża Forest (BF), which is regarded as the last surviving fragment of pristine lowland forests in the temperate zone of Europe. Relatively undisturbed by human activity since the last glacial period, the BF ecosystem has remained remarkably intact, which may have given rise to its unique TreM assemblage. Here, we show that a primeval forest is characterized by an exceptionally high richness and density of TreMs compared to previously studied natural forests, and that the richness, density and diversity of TreMs are spatially heterogeneous at the micro-scale but homogeneous at the macro-scale. This indicates that adjacent small fragments of habitat (0.05 ha) may have different TreM profiles, but large patches of forest (several ha) host similar assemblages of TreMs. Our profile of TreMs depends on the basal area and density of living trees, the basal area of dead standing trees and the dominance of specific TreM-hosting tree species in a stand. Our study suggests that both the ecological continuity and complexity of a forest supporting many different tree species and the diversity of TreM-forming biota that typically occurs in primeval temperate forests are factors that appear to contribute to the observed profile of TreMs. The results of our study set a benchmark for the quantity and quality of TreMs in broadleaved temperate forests and indicate that the long-term spontaneous natural processes occurring in primeval forests lead to the emergence of ultra-rich, complex assemblages of TreMs.

Pioneer tree species accelerate restoration of tree-related microhabitats in 50-year-old reserves of Białowieża Forest, Poland

Retention of structural elements such as deadwood and habitat trees at the level of forest stands has been promoted to integrate biodiversity conservation into multiple-use forest management. The conservation value of habitat trees is largely determined by the presence, richness, and abundance of tree-related microhabitats (TreMs). Since TreMs are often lacking in intensively managed forests, an important question of forest conservation is how the abundance and richness of TreMs may be effectively restored. Here, we investigated whether the strict protection of forest through cessation of timber harvesting influenced TreM occurrence at tree and stand levels. For that purpose, we compared four managed and four set-aside stands (0.25 ha each) in the Białowieża Forest, with identical origin following clear-cuts approximately 100 years ago. We found that the abundance and richness of TreMs on living trees were not significantly different between stands that were either conventionally managed or where active forest management ceased 52 years ago. Yet, our analysis of TreMs on tree species with contrasting life-history traits revealed that short-lived, fast-growing species (pioneers) developed TreMs quicker than longer-lived, slower-growing species. Hence, tree species such as Populus or Betula, which supply abundant and diverse TreMs, can play an important role in accelerating habitat restoration.

Importance of conserving large and old trees to continuity of tree-related microhabitats

Protecting structural features, such as tree-related microhabitats (TreMs), is a cost-effective tool crucial for biodiversity conservation applicable to large forested landscapes. Although the development of TreMs is influenced by tree diameter, species, and vitality, the relationships between tree age and TreM profile remain poorly understood. Using a tree-ring-based approach and a large data set of 8038 trees, we modeled the effects of tree age, diameter, and site characteristics on TreM richness and occurrence across some of the most intact primary temperate forests in Europe, including mixed beech and spruce forests. We observed an overall increase in TreM richness on old and large trees in both forest types. The occurrence of specific TreM groups was variably related to tree age and diameter, but some TreM groups (e.g., epiphytes) had a stronger positive relationship with tree species and elevation. Although many TreM groups were positively associated with tree age and diameter, only two TreM groups in spruce stands reacted exclusively to tree age (insect galleries and exposed sapwood) without responding to diameter. Thus, the retention of trees for conservation purposes based on tree diameter appears to be a generally feasible approach with a rather low risk of underrepresentation of TreMs. Because greater tree age and diameter positively affected TreM development, placing a greater emphasis on conserving large trees and allowing them to reach older ages, for example, through the establishment of conservation reserves, would better maintain the continuity of TreM resource and associated biodiversity. However, this approach may be difficult due to the widespread intensification of forest management and global climate change.

Natural Disturbances are Essential Determinants of Tree-Related Microhabitat Availability in Temperate Forests

Assessing the impacts of natural disturbance on the functioning of complex forest systems are imperative in the context of global change. The unprecedented rate of contemporary species extirpations, coupled with widely held expectations that future disturbance intensity will increase with warming, highlights a need to better understand how natural processes structure habitat availability in forest ecosystems. Standardised typologies of tree-related microhabitats (TreMs) have been developed to facilitate assessments of resource availability for multiple taxa. However, natural disturbance effects on TreM diversity have never been assessed. We amassed a comprehensive dataset of TreM occurrences and a concomitant 300-year disturbance history reconstruction that spanned large environmental gradients in temperate primary forests. We used nonlinear analyses to quantify relations between past disturbance parameters and contemporary patterns of TreM occurrence. Our results reveal that natural forest dynamics, characterised by fluctuating disturbance intervals and variable severity levels, maintained structurally complex landscapes rich in TreMs. Different microhabitat types developed over time in response to divergent disturbance histories. The relative abundance of alternate TreMs was maximised by unique interactions between past disturbance severity and elapsed time. Despite an unequal distribution of individual TreMs, total microhabitat diversity was maintained at constant levels, suggesting that spatially heterogeneous disturbances maintained a shifting mosaic of habitat types over the region as a whole. Our findings underscore the fundamental role of natural processes in promoting conditions that maximise biodiversity potential. Strict conservation and management systems that preserve natural disturbance outcomes, including associated biological legacies, may therefore safeguard biodiversity at large scales.

Tree microhabitats in natural temperate riparian forests: An ultra-rich biological complex in a globally vanishing habitat

Tree-related microhabitats (TreMs) are among the most important structural components of a forest, and have a significant impact on biodiversity and influence ecosystem functioning. Although forests that depend on natural lowland water regimes are severely endangered worldwide, and floodplain forests are considered to be the most complex and biologically rich habitats in the temperate zone, the TreMs in them have yet to be identified. This study investigates the assemblage of TreMs in natural Willow-Poplar riparian forests and analyses the environmental factors that influence their qualitative and quantitative compositions. A total of 90 sample plots (0.05 ha each) were selected at random in old-growth riparian forests that occur along a large unregulated river, the Vistula (Poland). A total of 62 TreM types were identified with a mean number of 16.0 ± 4.6 SD TreM types per plot and a mean density of 829.4 ± 360.1 SD TreM-bearing trees ha^-1. The number of TreMs found on an individual tree depends on its diameter, the number of trunks, its living status (living vs. dead tree) and the species it belongs to. The richness, density and diversity of TreMs found on a plot depends on the density of living trees, the basal area of living or dead trees, the number of tree species, and the percentage of Willows Salix sp. or of multi-trunk trees. Our study records for the first time the assemblage of TreMs in natural Willow-Poplar riparian forests and provides a reference for floodplain habitats. The results indicate that multi-species forests influenced by natural waterflow-related disturbances are hot-spots of TreM richness and abundance, and highlight the urgent need for the protection or restoration of these vanishing habitats.

Tree-Related Microhabitats Follow Similar Patterns but are More Diverse in Primary Compared to Managed Temperate Mountain Forests

The impact of forest management on biodiversity is difficult to scrutinize along gradients of management. A step towards analyzing the impact of forest management on biodiversity is comparisons between managed and primary forests. The standardized typology of tree-related microhabitats (TreMs) is a multi-taxon indicator used to quantify forest biodiversity. We aim to analyze the influence of environmental factors on the occurrence of groups of TreMs by comparing primary and managed forests. We collected data for the managed forests in the Black Forest (Germany) and for the primary forests in the Western (Slovakia) and Southern Carpathians (Romania). To model the richness and the different groups of TreMs per tree, we used generalized linear mixed models with diameter at breast height (DBH), altitude, slope and aspect as predictors for European beech (Fagus sylvatica (L.)), Norway spruce (Picea abies (L.)) and silver fir (Abies alba (Mill.)) in primary and managed temperate mountain forests. We found congruent results for overall richness and the vast majority of TreM groups. Trees in primary forests hosted a greater richness of all and specific types of TreMs than individuals in managed forests. The main drivers of TreMs are DBH and altitude, while slope and aspect play a minor role. We recommend forest and nature conservation managers to focus: 1) on the conservation of remaining primary forests and 2) approaches of biodiversity-oriented forest management on the selection of high-quality habitat trees that already provide a high number of TreMs in managed forests based on the comparison with primary forests.

Machine Learning Algorithms to Predict Tree-Related Microhabitats using Airborne Laser Scanning

In the last few years, the occurrence and abundance of tree-related microhabitats and habitat trees have gained great attention across Europe as indicators of forest biodiversity. Nevertheless, observing microhabitats in the field requires time and well-trained staff. For this reason, new efficient semiautomatic systems for their identification and mapping on a large scale are necessary. This study aims at predicting microhabitats in a mixed and multi-layered Mediterranean forest using Airborne Laser Scanning data through the implementation of a Machine Learning algorithm. The study focuses on the identification of LiDAR metrics useful for detecting microhabitats according to the recent hierarchical classification system for Tree-related Microhabitats, from single microhabitats to the habitat trees. The results demonstrate that Airborne Laser Scanning point clouds support the prediction of microhabitat abundance. Better prediction capabilities were obtained at a higher hierarchical level and for some of the single microhabitats, such as epiphytic bryophytes, root buttress cavities, and branch holes. Metrics concerned with tree height distribution and crown density are the most important predictors of microhabitats in a multi-layered forest.