Martin Diekmann

Aim: For improved predictions of the redistribution of species under climate change, there is a need to better understand how interactions among multiple abiotic drivers affect species distributions. We examined whether occupancy patterns of forest plant species change consistently along geographic and macroclimatic gradients. We further tested whether these changes co‐vary with species’ edaphic niche widths—specifically, whether niche widths contract as occupancy declines. Finally, we assessed ehether changes in occupancy and niche widths depend on species’ climatic and edaphic niche optima. Location: Sweden. Time Period: 2004–2013. Major Taxa Studied: Vascular plants. Methods: We used data from the National Forest Inventory on soil (pH and C:N‐ratio), and on occurrence of plant species from ca . 7000 inventory plots located along a climatic gradient (MAT −3 to +8°C). We modelled the probability of occurrence, as well as the edaphic pH and C:N‐ratio niche widths, along this gradient, using Generalized Additive Mixed Models. We then examined whether species’ occupancy and niche widths vary along the climatic gradient and whether this variation can be explained by climatic and edaphic niche optima quantified by ecological and climatic indicator values. Results: The probability of occurrence and niche widths of most species either increased or decreased along the temperature gradient. Generally, species that decreased in occurrence and edaphic niche widths towards colder climate had their niche optima in warmer climate and vice versa . Indicators related to the climatic niche of the species generally explained a much larger proportion of the variation in species occurrence and edaphic niche width than soil‐related indicator values. Main Conclusions: The results highlight that climate not only drives species’ redistributions directly, but also indirectly through modulating how plants interact with their soil environments, that is, their edaphic niches—an often‐overlooked mechanism important for understanding species responses to climate change.

Understanding genetic responses to forest dynamics is essential for predicting the long‐term viability of understory plant populations and for developing effective conservation strategies. We investigated genetic extinction debt and colonization credit in broad‐leaved enchanter's nightshade ( Circaea lutetiana ), a clonal forest understory species, across its European range. Using pooled genotype‐by‐sequencing data from 40 forest edge and core populations, we examined to what extent population size, latitude, and historical changes in forest configuration may predict genetic diversity. Historical forest configuration profoundly shaped present‐day genetic diversity. Long‐established forest edge populations exhibited significantly reduced allelic richness (−9%) compared with core populations, indicating the partial payoff of a genetic extinction debt. In contrast, populations from recently established forest edges maintained allelic richness similar to core populations, suggesting delayed population genetic responses to land‐use changes. Finally, populations established in areas that were afforested during the past 250 years exhibited lower allelic richness (−10%) than historical forest core populations, indicating a delay in genetic recovery and thus a potential genetic colonization credit. Our results highlight that C. lutetiana populations are not at equilibrium with the current forest configuration, underscoring the role of lagged genetic responses across very long periods. Population size, and to a lesser extent connectivity, further mediated genetic diversity, with smaller, isolated populations being particularly vulnerable to genetic erosion. Given the limited research on delayed evolution in forest understory species, our results improve understanding of extinction risk dynamics and underscore the need for history‐informed restoration efforts.

Aim: Widespread species encounter a range of variable climates that can lead to intraspecific trait clines. Such clines can be the result of phenotypic plasticity, genetic differences, or both. Although latitude often explains a large part of trait variation, it is crucial to investigate the underlying environmental variables to understand current and future trait responses. Cross‐continental comparisons of species that are native on multiple continents provide a rarely used approach that can help identify the environmental drivers of intraspecific trait clines. Location: Europe and North America. Time Period: 2021–2023. Major Taxa Studied: Milium effusum L. (Poaceae). Methods: To quantify the influence of environmental gradients on functional traits across geographical regions that vary in climate, we sampled M. effusum seeds from 23 European and 14 North American populations and transplanted them in a common garden. We measured 10 vegetative, reproductive, and phenological traits. We used 30‐year averages of 19 bioclimatic variables, while accounting for the latitudinal and elevational position of the population origins, to compare the trait‐environment relationships between continents. Results: Our results showed that European populations occupy a broader climatic range than North American populations. Differences between continents were found in most of the traits as well as in the multivariate trait space. The traits were affected more by bioclimatic variables than by latitude or elevation. While flowering, leaf thickness, specific leaf area, and reproductive height showed parallel clines to the environment between continents, vegetative height and biomass showed contrasting clines. Main Conclusions: Environmental influences from population origins revealed parallel clines between the continents for functional traits, suggesting shared selective pressures, while contrasting clines for plant size indicated different evolutionary trajectories, potential bottlenecks, or interactions with unknown ecological factors. This study highlights the complex interplay of genetic, environmental, and evolutionary factors in shaping phenotypic variation in native species across continents.

To examine effects of local environmental conditions on the vegetation, measurements on the microhabitat level are highly relevant. Unlike most soil chemical variables, soil moisture is rarely considered in studies investigating the impact of local factors on plant species composition. Here, we conducted soil moisture measurements with TMS4 sensors over a two-month period in 25 deciduous forest plots in northern Germany, performed site-specific moisture calibrations and studied the quantitative effect of soil properties on the relationship between the TMS4 signal and gravimetric soil moisture. Moreover, we investigated the effect of measured microhabitat conditions (light availability, soil variables including soil moisture) on plant species richness and composition. We found that soil carbon significantly influenced the relationship between the TMS4 raw signal and gravimetric soil moisture. The number of herb layer species and ancient woodland indicators (AWI) increased with increasing soil moisture. For the herb layer, soil moisture and soil C/N ratio had an interactive effect: species richness increased more steeply with increasing soil moisture in plots with a low C/N ratio. AWI species richness was significantly higher in plots with a low C/N ratio. Herb layer species composition was shaped by two main gradients, reflecting (1) variation in C/N ratio and pH and (2) variation in soil moisture, other soil variables and light availability. Our findings indicate that soil carbon, a frequently measured variable in vegetation studies, could, with sufficiently good calibrations, be directly used to translate sensor raw signals into reliable soil moisture values for predicting plant species occurrence.

The microclimate temperature within forests can substantially deviate from measurements outside forests. Yet, the amount and direction of this buffering strongly depend on meteorological conditions outside forests, such as free‐air temperature, wind speed, soil moisture, precipitation and solar radiation. Most studies on microclimate buffering focus on long‐term averages at coarse temporal resolution (e.g., monthly to seasonal), potentially overlooking the impact of daily fluctuations, which are nonetheless critical for many forest species and processes. Here, we linked in situ measured air and soil temperatures, at the daily resolution over a period of more than 5 years across 45 European deciduous forests spanning a 2300‐km latitudinal gradient, to daily variation in weather conditions (i.e., free‐air temperature, precipitation, wind speed, soil moisture, solar radiation, day length and snow cover). We assessed the interactive effects of weather conditions and forest structural complexity (plant area index) and distance to the forest edge on air and soil temperature offsets. Temperature offsets are the difference between understory air or soil temperature and macroclimate reference conditions measured at weather stations in the open field. We show that relative effect sizes of free‐air temperature prevailed over the effects of other weather variables to determine air and soil temperature buffering. In general, higher free‐air temperatures, soil moisture, wind speed and snow cover led to more negative air and soil temperature offsets, with variations depending on forest structural complexity and season. Conversely, slightly increased air and soil temperature offsets were found as precipitation and solar radiation increased. Our study highlights the dominant role of daily free‐air temperature fluctuations outside forests in shaping microclimatic buffering. We emphasize complex interaction paths between daily weather conditions, and forest structure and edge effects, offering valuable insights for next‐generation microclimate models and enhancing our understanding of forest ecosystem responses to environmental change that account for microclimate.

Summary: We face increasing concerns about how the local diversity of native plant communities responds to various drivers of global change, yet often lack comprehensive studies that integrate several components of diversity and the effects of both local and regional drivers of change. We analyzed changes in taxonomic, functional, and phylogenetic diversity across 2681 (semi‐)permanent temperate forest understory plots surveyed and resurveyed for all vascular plants over intervals of 15–78 yr, spanning 72 regions distributed across Europe. We quantified temporal changes in these diversity indices and assessed their responses to changes in both local drivers (plot‐level overstory cover, indicator values for soil nutrients) and regional shifts in macroclimate and nitrogen deposition. Overall, local changes in taxonomic, functional, and phylogenetic diversity were centered around zero, reflecting – on average – little net change in forest diversity. Observed diversity changes mostly reflected local conditions such as overstory cover change and baseline soil nutrients rather than regional drivers of large‐scale change. Changes in phylogenetic diversity correlated positively with changes in taxonomic diversity but negatively with changes in functional diversity. Our findings underscore the importance of local habitat management and multifaceted diversity monitoring for effective biodiversity conservation in temperate forests.

Urban areas can support diverse communities of plants and animals. Yet, urbanisation can affect functionally important species traits, potentially impacting population dynamics. The saproxylic beetle Elater ferrugineus L. is associated with large trees and is often used as an indicator of species‐rich saproxylic communities. It is an important target for conservation and it is listed as a near‐threatened species on the European Red List. Few studies have quantified the impact of urbanisation on the ecology and intraspecific variation in functional traits of arthropods, other than pollinators. We studied how the local abundance of E. ferrugineus and functionally important response traits (e.g., width of the pronotum, length of the elytron and wing, wing area, body mass and wingload) changed along urbanisation gradients in eight European cities using pheromone traps installed on large solitary trees. We analysed the effects of the surrounding built‐up area and tree cover on our response variables while accounting for potential confounding effects due to tree size and the availability of microhabitats. Urbanisation had a strong negative effect on the local abundance of E. ferrugineus , while the amount of tree cover had a positive influence. We found no significant impact of urbanisation on the functional traits of this species, except for a significantly higher wingload in city centres. Our results provide a better understanding of the ecological processes impacting this saproxylic beetle and underpin the importance of future research on urbanisation's impact on arthropods.

Questions: How do local forest conditions and characteristics at the forest patch ‐ scale and landscape ‐ scale affect plot‐scale plant diversity in Europe? Do these patterns vary between forest specialists and generalists? Do community saturation patterns differ between forests varying in their surrounding landscape type?Location: Deciduous forests sampled along a European gradient from southwest to northeast comprising eight regions in five countries (France, Belgium, Germany, Sweden, Estonia).Methods: We examined the effects of local conditions assessed by means of Ellenberg indicator values (soil moisture, soil nitrogen, soil pH, light availability), patch‐scale characteristics (patch‐scale plant diversity, forest patch age, forest patch size) and a landscape‐scale variable (representing low and high connectivity of forest patches) on plot‐scale plant diversity, separately for forest specialist and generalist species. Additionally, we ran regression models to examine community saturation patterns.Results: We found patterns of niche partitioning among forest specialists and generalists. Low light availability and medium soil moisture favored forest specialists, while generalists were mostly present at higher light availability and medium and high soil moisture. In general, we found the highest plot‐scale diversity at medium soil pH. Patch‐scale diversity showed a positive impact on plot‐scale diversity and plots in the high‐connectivity landscape had a higher diversity than plots in the low‐connectivity landscape. Further, we observed a high degree of community saturation in both landscape types.Conclusion: The positive impact of a high connectivity of forest patches on local plant diversity emphasizes the importance of small semi‐natural habitats like tree lines, unused field margins and hedgerows to enhance the potential dispersal of forest plants across agricultural landscapes. Community saturation patterns revealed the increasing relevance of local conditions and processes for plot‐scale diversity when patch‐scale diversity increases.