Sebastian Dittrich

The diversity of many forest-dwelling organisms depends on forest structure, but it remains unknown to which degree this is mediated by microclimatic temperature, and whether microclimatic buffering is universally beneficial for forest biodiversity. In 234 broad-leaved forest plots across nine climatically differing regions in Germany, we measured micro-and macroclimatic temperatures, canopy cover, and deadwood volume after experimental manipulations of forest structure through thinning operations and gap cuts. We surveyed 21 taxonomic groups, representing plants, bacteria, fungi, and animals, to assess how their diversity depended on the predictors. Multi-taxon diversity, and diversity of ten separate taxonomic groups, increased from closed to open canopies. Only necrophagous beetles were more diverse in closed stands. These effects of canopy cover on diversity were partly mediated by microclimatic temperatures. Thereby, diversity responses to microclimatic temperature depended on the macroclimatic context. In regions with colder summers, locally warmer microclimates in canopy gaps had strong positive effects on biodiversity, while these effects were not found in regions with warmer summers. Consequently, management strategies aimed at multi-taxon forest diversity conservation should be chosen based on the regional climate, favoring canopy gaps primarily in colder regions, and taking future climate warming into consideration.

Given increasing deer populations and rising canopy openness through tree mortality in European forests, understanding how these changes affect above‐belowground communities and ecosystem functioning is becoming more important. Earthworms comprise the largest animal biomass in European forest soils and play a central role in decomposition, carbon and nutrient cycling, and soil formation. However, their responses to canopy openness and deer browsing have been studied almost exclusively in northern North America, where interactions involve white‐tailed deer ( Odocoileus virginianus ) and mainly non‐native and invasive earthworms. Here, we examine how roe deer ( Capreolus capreolus ) and canopy openness interactively affect native earthworm communities in a temperate European forest across 75 experimental patches: 54 closed‐canopy and 21 experimentally created canopy‐gap patches. Five years after gap establishment, we sampled earthworms and associated environmental variables from two plots per patch: exclosure (fenced) and control (unfenced) plots. We found that roe deer and canopy gaps interactively influenced earthworm communities. Under closed canopy, roe deer reduced earthworm biomass and density. If roe deer were absent, canopy gaps had no effect on earthworms, but if roe deer were present, canopy gaps increased biomass and density through vegetation‐mediated pathways and enhanced soil water content. The magnitude of these effects differed among earthworm ecological groups (anecic species, i.e., earthworms that create deep, permanent burrows and connect surface litter with deeper soil layers, vs. endogeic species, i.e., earthworms that live and feed entirely within the soil) and life stages (juveniles vs. adults). Indirect effects of roe deer were particularly beneficial for juveniles. These results contrast with patterns reported for white‐tailed deer and non‐native earthworms in northern North American forests, where white‐tailed deer presence typically promotes earthworms. Our results on the direct and indirect effects of roe deer on belowground earthworms shed new light on the complex interactions of deer populations and canopy structure on forest soil biodiversity and ecosystem functioning.

Browsing by ungulates directly influences understorey plant communities through selective removal of certain species and indirectly by modifying plant competition, environmental conditions and successional dynamics. In Central Europe, roe deer ( Capreolus capreolus ) are the most widely distributed ungulate species forming locally dense populations. Understanding the effects of roe deer is essential for sustainable forest management, particularly in the context of changing environmental conditions such as increased tree mortality and expanding canopy gaps. To disentangle the direct effects of roe deer from indirect environmental modifications, we investigated the herb layer (vascular plants n = 54) or within experimental gaps ( n = 21) and were surveyed 5 years after their establishment. Roe deer increased herb layer species richness overall, but reduced functional diversity in closed forests. Canopy gaps decreased herb layer species richness but increased shrub layer cover. Soil pH was independently positively associated with herb layer species richness, but not with functional diversity and to a lesser extent with shrub layer cover. Structural equation models showed that canopy gaps increased light availability 1 m aboveground, but this effect was counteracted by already increased shrub layer cover. By reducing shrub layer cover, roe deer indirectly increased herb layer species richness. Roe deer presence directly reduced herb layer functional diversity, but also indirectly mitigated this negative effect by lowering shrub cover and thereby increasing light availability. Synthesis . Forest gaps primarily reshape herb layer composition by increasing light availability and shrub layer encroachment. Roe deer influence herb layer community assembly both directly and indirectly via modification of the shrub layer and light availability and thus shifting plant–plant competition. These findings emphasize the complex role of browsers in structuring herb layer communities and help to understand how changes in roe deer populations and canopy gaps will shape the future of temperate European forests.

Context: Volatile organic compounds (VOCs) create invisible chemical landscapes that influence ecosystem processes. Yet whether VOC β-diversity (i.e., variability in VOC composition between patches) responds to structural heterogeneity and reflects silvicultural habitat management remains unclear. Objectives: In a large field experiment, we quantified how enhanced structural beta complexity (ESBC) affects VOC β-diversity patterns and investigated potential drivers and ecological effects in temperate production forests. Methods: We sampled VOCs in ambient forest air using Tenax/Carboxen adsorbent traps at forest floor and 1 m heights across 234 treatment and control forest patches in six German regions. We analyzed VOCs via thermal desorption-gas chromatography mass-spectrometry (TD-GCMS) and examined environmental drivers including deadwood characteristics, canopy cover, tree species dissimilarity, and herb layer dissimilarity. We tested potential ecological relevance by analyzing saproxylic beetle community responses. Results: VOC β-diversity increased significantly at 1 m height in heterogeneous forests compared to homogeneous forests, but we found no significant change at the forest floor. Deadwood volume and deadwood structural diversity, rather than canopy openness, were identified as the main drivers of increasing VOC β-diversity. Dissimilarity in beetle community composition was associated with VOC β-diversity, but only for forest floor VOCs, suggesting these chemical patterns may correlate with variables beetles respond to. Conclusions: Our findings suggest that volatile β-diversity represents an overlooked dimension of habitat heterogeneity, one that creates invisible chemical heterogeneity influencing inter- and intra-species interactions and ecosystem processes. We demonstrate that enhancing forest heterogeneity through deadwood retention increases both structural heterogeneity and volatile β-diversity.

Forest microclimates, their dependence on forest structure, and their impact on biodiversity are crucial for future forest management under climate change. However, standard approaches for measuring forest microclimates do not capture within-plot heterogeneity, which, according to the habitat heterogeneity hypothesis, is a key driver of local biodiversity. We quantified horizontal and vertical microclimatic heterogeneity within 30 broad-leaved forest plots in Central Europe using a three-dimensional design with high spatial resolution. Moreover, we examined whether microclimatic heterogeneity differs among silvicultural treatments and whether it can be predicted using forest structure indices derived from laser scanning. Additionally, we explored the relationship between microclimatic heterogeneity and biodiversity. In the understory of canopy gaps, warm and cold habitats co-existed in close proximity, leading to a high horizontal microclimatic heterogeneity. In closed stands with high structural complexity, we found steep gradients of increasing temperature and vapor pressure deficit from the ground to the canopy during mid-day. Canopy cover and forest structural complexity were strong indicators of microclimatic heterogeneity. We found positive relationships between herb layer temperature heterogeneity and the diversity of plants, Hymenoptera, and Diptera. Our results demonstrate that single-point measurements fail to capture the substantial microclimatic heterogeneity within plots, potentially misrepresenting the conditions experienced by forest species. However, laser scanning provides reliable indicators for within- plot microclimatic heterogeneity. With canopy gaps featuring high horizontal microclimatic heterogeneity and promoting the biodiversity of several taxonomic groups, we argue that managing forests for maximized temperature buffering should not be the only strategy to conserve forest biodiversity. Highlights: High small-scale horizontal microclimatic heterogeneity in canopy gaps Steep vertical microclimatic gradients in closed-canopy forests Canopy cover and structural complexity: indicators for microclimatic heterogeneity Positive relationship between herb layer temperature heterogeneity and biodiversity

Although habitat heterogeneity is known to enhance local species diversity, the effects of management-driven structural heterogeneity on understorey plant communities across spatial scales remain poorly understood, despite their crucial role for forest biodiversity and ecosystem functioning. To analyse how forest understorey plant communities respond to an enhancement of structural heterogeneity in managed forests, we established 11 experimental sites consisting of two paired forest landscapes, an untreated homogenous control and a treatment district (ESBC). In treatment districts, structural heterogeneity was enhanced through different combinations of local patch-scale manipulations of light and deadwood features, leading to greater between-patch heterogeneity at the landscape scale. We performed a meta-analysis across these 11 sites using a Hill-Chao number and sample coverage standardisation framework. Gamma diversity increased across taxonomic, functional and phylogenetic facets in structurally heterogeneous forests (ESBC districts) via higher alpha diversity. This effect was positively associated with heterogeneity in light availability between forest patches, but not with their mean light availability. In contrast, we found no support that species turnover among patches (i.e., beta diversity) significantly contributes on average to the observed increase in gamma diversity. However, both the direction and magnitude of beta diversity responses varied substantially among landscapes. On average, structurally heterogeneous forests supported higher species richness for both open and closed forest habitat species. Synthesis and applications: Our findings highlight the benefits of enhancing structural heterogeneity for understorey plant diversity in managed forest landscapes. Specifically, management strategies that create a spatial mosaic of interventions, such as combining single-tree removal with group felling, can increase the variety of light niches among forest patches, thereby supporting the conservation of a wide range of understorey plant species, including forest specialists.

Many moss species are associated with nitrogen (N)‐fixing bacteria (diazotrophs) that support the N supply of mosses. Our knowledge relates primarily to pristine ecosystems with low atmospheric N input, but knowledge of biological N fixation (BNF) and diazotrophic communities in mosses in temperate forests with high N deposition is limited. We measured BNF rates using the direct stable isotope method and studied the total and potentially active diazotrophic communities in two abundant mosses, Brachythecium rutabulum and Hypnum cupressiforme , both growing on lying deadwood trunks in 25 temperate forest sites. BNF rates in both mosses were similar to those observed in moss species of pristine ecosystems. H. cupressiforme fixed three times more N ₂ and exhibited lower diazotrophic richness than B. rutabulum . Frankia was the most prominent diazotroph followed by cyanobacteria Nostoc. Manganese, iron, and molybdenum contents in mosses were positively correlated with BNF and diazotrophic communities. Frankia maintained high BNF rates in H. cupressiforme and B. rutabulum even under high chronic N deposition in Central European forests. Moss N concentration and ¹⁵ N abundance indicate a rather minor contribution of BNF to the N nutrition of these mosses.

Dittrich, S., Lang, R., Albrecht, M., Stetzka, K. & von Oheimb, G. 2022. Vertikalverbreitung kryptogamischer Epiphyten an Bäumen in mitteldeutschen Hartholz-Auwäldern: Relevanz für Bioindikation und Naturschutz. – Herzogia 35: 443 – 461. Epiphytenerfassungen beschränken sich häufig auf die unteren Stammabschnitte der Trägerbäume. Hierbei werden Arten, die vorzugsweise in Baumkronen vorkommen, weitgehend übersehen. Unter Berücksichtigung der vertikalen Verteilungsmuster epiphytischer Moose und Flechten in Hartholzauenwäldern wären solche Erhebungen unvollständig und ihre Ergebnisse unterschieden sich von Erfassungen ganzer Bäume. Das wäre besonders kritisch im Hinblick auf die Erfassungen der Gesamtartendiversität solcher Waldbestände, die Einschätzung ihrer Schutzwürdigkeit sowie für die Bioindikation (z. B. den Einfluss des Klimawandels). Im größten Komplex geschützter Auenwälder in Mitteldeutschland untersuchten wir 20 bei Stürmen geworfene und gebrochene Bäume. Hierbei wurden die Epiphyten abschnittsweise von der Stammbasis bis in die äußere Krone erfasst. 75 % der Flechtenarten waren auf die Krone und den oberen Stamm beschränkt, während 30 % der Moosarten nur im Kronenbereich gefunden wurden, und weitere 30 % nur im unteren Stammbereich sowie der Stammbasis vorkamen. Sowohl bei Moosen als auch bei Flechten wurden schutzbedürftige Arten nur in den Kronen gefunden, wie auch die meisten Indikatorarten für Klima-Erwärmung. Unter Verwendung von Ellenberg-Zeigerwerten fanden wir weiterhin signifikante Unterschiede in den Feuchteund Lichtverhältnissen, Substratreaktion, Kontinentalität und Nährstoffversorgung über die verschiedenen Baum-Abschnitte. Die vollständige Erfassung epiphytischer Flechten und Moose auf Auenwaldbäumen führt daher zu weitaus differenzierteren Ergebnissen als konventionelle Erhebungen in den unteren Stammabschnitten von Bäumen.