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Forest canopies maintain a high proportion of arthropod diversity. The drivers that structure these communities, however, are poorly understood. Therefore, integrative research connecting tree species identity and environmental stand properties with taxonomic and functional community composition of canopy arthropods is required. In this study, we investigated how the taxonomic, functional and trophic composition of arboreal spider communities is affected by tree species composition and associated differences in canopy structure and prey availability in temperate forests. We sampled canopy spiders as well as their potential prey using insecticidal fogging in monospecific and mixed stands of native European beech, native Norway spruce and non-native Douglas fir. Trophic metrics were obtained from stable isotope analysis and structural canopy properties were assessed with mobile laser scanning. Monospecific native spruce stands promoted local canopy spider abundance and diversity, but native beech and beech–conifer mixtures had the highest diversity at landscape scale. Spider community composition differed between monospecific stands, with broadleaf–conifer mixtures mitigating these differences. Irrespective of tree species identity, spider abundance, taxonomic diversity, functional richness and isotopic richness increased in structurally heterogeneous canopies with high prey abundances, but functional evenness and trophic divergence decreased. Our study shows that canopy spiders are differentially affected by tree species identity, canopy structure and prey availability. Broadleaf–conifer mixtures mitigated negative effects of (non-native) conifers, but positive mixture effects were only evident at the landscape scale. Structurally heterogeneous canopies promoted the dominance of only specific trait clusters. This indicates that intermediate heterogeneity might result in high stability of ecological communities. [ABSTRACT FROM AUTHOR]

Adaptive silvicultural approaches intend to develop forests that can cope with changing climatic conditions. Just recently, many parts of Germany experienced 3 years of summer drought in a row (2018–2020). This study analysed the effects of this event on beech (Fagus sylvatica L.) in two regions in northern Bavaria, Germany. For this purpose, 990 beech trees were studied on 240 plots in drought-stressed forests. We examined trees of different social position and different size. Their morphology (e.g. tree height, crown volume) was recorded by laser scanning, and drought stress was quantified by tree core sample analyses. In addition to increment analyses, the δ13C signal was determined by year. Results show that the dominant tree collective was particularly affected by the drought. They still managed to perform well in 2018, but the radial growth decreased significantly in 2019 and 2020, partly resembling the performance values of subordinate trees. Subordinate trees, on the other hand, provide some consistency in growth during drought years. The drought was so severe that the effects of competition on tree growth began to disappear. The difference in growth of two geographically distinct study areas equalized due to drought. With continuing drought, increasing levels of the δ13C signal were detected. Similar patterns at different δ13C levels were found across the social positions of the trees. The influence of tree morphological variables on tree resistance to drought showed no clear pattern. Some trends could be found only by focusing on a data subset. We conclude that the intensity of the 2018–2020 drought event was so severe that many rules and drivers of forest ecology and forest dynamics (social position, morphology and competition) were overruled. The influence of morphological differences was shown to be very limited. The weakening of dominant trees could potentially be no longer linear and drought events like the one experienced in 2018–2020 have the potential of acting as tipping points for beech forests. [ABSTRACT FROM AUTHOR]

Laser scanning has revolutionized the ability to quantify single-tree morphologies and stand structural variables. In this study, we address the issue of occlusion when scanning a spruce (Picea abies (L.) H.Karst.) and beech (Fagus sylvatica L.) forest with a mobile laser scanner by making use of a unique study site setup. We scanned forest stands (1) from the ground only and (2) from the ground and from above by using a crane. We also examined the occlusion effect by scanning in the summer (leaf-on) and in the winter (leaf-off). Especially at the canopy level of the forest stands, occlusion was very pronounced, and we were able to quantify its impact in more detail. Occlusion was not as noticeable as expected for crown-related variables but, on average, resulted in smaller values for tree height in particular. Between the species, the total tree height underestimation for spruce was more pronounced than that for beech. At the stand level, significant information was lost in the canopy area when scanning from the ground alone. This information shortage is reflected in the relative point counts, the Clark–Evans index and the box dimension. Increasing the voxel size can compensate for this loss of information but comes with the trade-off of losing details in the point clouds. From our analysis, we conclude that the voxelization of point clouds prior to the extraction of stand or tree measurements with a voxel size of at least 20 cm is appropriate to reduce occlusion effects while still providing a high level of detail. [ABSTRACT FROM AUTHOR]

Research on mixed forests has mostly focused on tree growth and productivity, or resistance and resilience in changing climate conditions, but only rarely on the effects of tree species mixing on timber quality. In particular, it is still unclear whether the numerous positive effects of mixed forests on productivity and stability come at the expense of timber quality. In this study, we used photographs of sawn boards from 90 European beech (Fagus sylvatica L.) trees of mixed and pure forest stands to analyze internal timber quality through the quality indicator knot surface that was quantitatively assessed using the software Datinf® Measure. We observed a decrease in knot surface with increasing distance from the pith as well as smaller values in the lower log sections. Regarding the influence of neighborhood species identity, we found only minor effects meaning that timber qualities in mixed stands of beech and Norway spruce (Picea abies (L.) H. Karst.) tended to be slightly worse compared to pure beech stands. [ABSTRACT FROM AUTHOR]

The unprecedented drought between 2018 and 2020 had a significant impact on European beech (Fagus sylvatica L.) forests in Central Europe. The role of different forest structures in mitigating drought stress remains controversial. This contentious debate prompted our study, in which we aimed to quantify the effect of forest structure on drought stress in beech forests in two ecoregions of northern Bavaria, Germany. Using a mobile laser scanner, we surveyed 240 plots in drought-stressed forests. We analyzed the responses of beech trees to the drought period through radial growth, wood-derived δ13C signal, and crown defoliation. Results revealed significant responses of beech forests in both regions to the drought event, including increased crown defoliation, reduced tree growth, and altered δ13C signatures compared to pre-drought conditions. Our results show a relationship between crown closure and crown defoliation in beech, suggesting an increased vulnerability of beech to drought in more open canopies. However, the potential for silvicultural intervention to mitigate drought stress, as measured by BAI and δ13C signal, appears limited. Neighboring trees and forest structure had little influence on average drought resistance. The δ13C signal showed minimal responsiveness to variations in canopy openness, as well as to distinctions between single and multi-layered forests. However, increased structural complexity within stands tended to increase resistance due to the compensatory effects of understory trees. Future forest management strategies could focus on promoting structural diversity, selecting resilient individuals but also actively enrich the forests with more drought-adapted species to increase the adaptive capacity of beech-dominated forests in the face of changing climate conditions. • We analyzed the response of 240 plots in European beech forests to the 2018-2020 drought period. • We found significant responses of beech forests to the drought event, but the responses were highly variable. • Sub-canopy trees were less affected by the drought event. • Silvicultural decisions can only mitigate drought events to a certain extent. • We emphasize the importance of promoting structural diversity but also tree species diversity through active enrichment with drought-tolerant species. [ABSTRACT FROM AUTHOR]

• Laser scanning allows evaluating structural effects of forest management concepts. • Management intensity affects spatial distribution of plant material in forests. • Management history influences tree lean, sweep, DBH and length of beech stems. • Silviculture can create old-growth-like structures in managed forests. For about half a century, attempts have been made to manage forests as close to nature as possible. This management targeted the creation of structures that resemble those found in primary forests. Laser scanning provides the opportunity to quantify such structural "naturalness" and allows to evaluate which management practices come closest to forest structures found in primary forests. In this paper, European beech (Fagus sylvatica L.) forests with different management intensity were compared to primary forests in terms of their structural complexity and the shape of tree stems. For this purpose, data from mobile and terrestrial laser scans (MLS and TLS) of managed forests, of forests whose management has been abandoned, and of primary forests was used. We found that management intensity influenced the distribution of plant material in the forest stand and thus the structural complexity on stand level. Also, management affected the shape of the stems. Here, it is important to consider the management history of the forest stands and the forest development phase in which management was abandoned. The stem shapes of trees in primary forests were significantly different (larger stem diameters and longer branch free boles) from those of the other investigated forests. Nevertheless, our results showed that it is possible to achieve old-growth-like structures such as high standing volumes, a multi layered canopy, high number of large trees and high variation in tree size and age through targeted silvicultural treatments in order to accelerate the close-to-nature development of forests. The present study illustrates the possibility of using mobile and terrestrial laser scanning to objectively compare and evaluate the structural effects of different silvicultural concepts. [ABSTRACT FROM AUTHOR]

Aboveground tree architecture is neither fully deterministic nor random. It is likely the result of mechanisms that balance static requirements and light‐capturing efficiency. Here, we used terrestrial laser scanning data to investigate the relationship between tree architecture, here addressed using the box‐dimension (Db), and the architectural benefit‐to‐cost ratio, the light availability, and the growth of trees. We detected a clear relationship between Db and the benefit‐to‐cost ratio for the tested three temperate forest tree species (Fagus sylvatica L., Fraxinus excelsior L., and Acer pseudoplatanus L.). In addition, we could also show that Db is positively related to the growth performance of several tropical tree species. Finally, we observed a negative relationship between the strength of competition enforced on red oak (Quercus rubra L.) trees and their Db. We therefore argue that Db is a meaningful and integrative measure that describes the structural complexity of the aboveground compartments of a plant as well as its relation to structural efficiency (benefit‐to‐cost ratio), productivity, and growing conditions (competition or availability of light). [ABSTRACT FROM AUTHOR]

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Silvicultural success in achieving, among other management goals, maximum productivity strongly depends on knowledge of the relationship between stand density and the resulting growth response of a stand. However, there are still controversial discussions whether wood production can be enhanced by silvicultural thinning or reaches its maximum in unmanaged forest stands if time plays no role. Moreover there is no universal answer whether structural diversity promotes or reduces productivity. In the present study we applied terrestrial laser scanning (TLS) to investigate the relationship between three-dimensional space filling, forest management intensity, productivity and conventional measures of structural diversity. We examined 35 beech-dominated forest plots along a gradient of management intensity in three regions of Germany. We found that space filling in leaf-on condition increased with management intensity, particularly in the shaded crown. Increased space filling in the shaded crown due to tree removals also resulted in higher stand productivity. We conclude that an increased space filling in the shaded canopy of managed European beech stands is responsible for the compensation of production losses in the upper canopy due to thinning activities. Conventional measures of structural diversity were not sensitive to the applied silvicultural activities. We also found no relationship between structural diversity described by conventional measures and stand productivity. [ABSTRACT FROM AUTHOR]

Measurements of physical tree crown dimensions were of subjective character in the past, even though they can be considered important for the management of many silvicultural operations, such as timing of thinning operations. In our study we investigated if and how measures of physical crown dimensions of trees differed when quantified conventionally versus based on 3D-terrestrial laser scanning and how they are related to basal area increment. Some 24 randomly selected predominant or dominant beech trees between 90 and 110 yrs of age and of varying height were used as study trees. We hypothesized that tree crown dimensions obtained from scans are more closely related to tree radial growth than those obtained from conventional field measurements. It was found that from a variety of compared crown size characteristics the scan-based tree attributes mean crown radius, maximum area of the crown and crown projection area were most closely related to individual tree growth. We conclude that the horizontal extension of a tree crown in general is to be considered one of the most important drivers of tree growth. We also conclude that terrestrial laser scanning is a powerful tool to reliably measure physical crown dimensions and TLS-based measurements are more reliable than conventional ones. [ABSTRACT FROM AUTHOR]

According to recent climate change scenarios, temperate forests will be increasingly exposed to droughts in the 21st century which are thought to affect productivity. Although decreasing timber yield with reduced precipitation has frequently been reported from temperate forests, the dependence of forest net primary production (NPP) on precipitation is little understood. In a 3-year transect study (2009–2011) carried out in 12 mature beech forests ( Fagus sylvatica) along a precipitation gradient (820–540 mm y −1) in Northern Germany, we measured all aboveground NPP components (NPP a; stem wood, leaf mass, flower and fruit production) and analyzed relationships with monthly weather data. Because we measured NPP a under a broad range of precipitation levels, drought lengths and mast fruiting intensities, the climatic controls of aboveground productivity and carbon allocation could be analyzed in detail. Despite a significant decrease in annual (and growing season) precipitation sums along the transect, NPP a remained largely invariant in each of the years, but varied remarkably between the years (means of 981, 702, 955 g DM m −2 y −1, respectively). Variation in NPP a was most closely related to current year’s early summer weather conditions (June–July), whereas the patterns of biomass allocation to wood, leaf, and fruit production responded to the previous summer’s weather. Wood production cannot predict NPP a in beech due to alternative allocation priorities of vegetative and reproductive growth. Our results show that apparent drought-induced reductions in beech timber yield often are the result of allocation shifts toward fruit production triggered by warm and dry weather in the previous summer. [ABSTRACT FROM AUTHOR]

One of the main goals of modern silviculture is to emulate the structural complexity of old-growth forests. In this context, it is of advantage to identify a target state of structural complexity at the stand level and to analyze the spatial characteristics that led to the desired complexity of forest structures in primary forest references. In this study, we used 3D forest scenes captured by terrestrial laser scanning (TLS) to identify spatial patterns of structural complexity of differently managed and unmanaged European forests dominated by beech (Fagus sylvatica L.). We scanned in managed even-aged and uneven-aged stands, as well as in formerly managed forests (National Parks) and primary forests. For three different forest strata, representing the understory, the midstory, and the overstory of a forest stand, we determined the structural complexity mathematically using fractal analysis. Beyond that, we analyzed the density, as well as the horizontal and vertical distribution of plant material. For all three forest strata, we observed differences in structural complexity between the different forest types. Within the lower and middle strata, the investigated primary forests showed a random to regular distribution of plant material, as well as a complex understory structure as a result of pronounced natural decay. Compared to the primary forests, the managed uneven-aged stands showed quite similar spatial patterns of distribution of plant material, but on average a higher space occupation in the lower and middle forest stratum. Our results suggest that single tree or group selection cutting is a useful management tool to imitate old-growth structures of undisturbed beech-dominated forests. [ABSTRACT FROM AUTHOR]

Individual tree architecture and the composition of tree species play a vital role for many ecosystem functions and services provided by a forest, such as timber value, habitat diversity, and ecosystem resilience. However, knowledge is limited when it comes to understanding how tree architecture changes in response to competition. Using 3D-laser scanning data from the German Biodiversity Exploratories, we investigated the detailed three-dimensional architecture of 24 beech (Fagus sylvatica L.) trees that grew under different levels of competition pressure. We created detailed quantitative structure models (QSMs) for all study trees to describe their branching architecture. Furthermore, structural complexity and architectural self-similarity were measured using the box-dimension approach from fractal analysis. Relating these measures to the strength of competition, the trees are exposed to reveal strong responses for a wide range of tree architectural measures indicating that competition strongly changes the branching architecture of trees. The strongest response to competition (rho = −0.78) was observed for a new measure introduced here, the intercept of the regression used to determine the box-dimension. This measure was discovered as an integrating descriptor of the size of the complexity-bearing part of the tree, namely the crown, and proven to be even more sensitive to competition than the box-dimension itself. Future studies may use fractal analysis to investigate and quantify the response of tree individuals to competition. [ABSTRACT FROM AUTHOR]

The use of natural regeneration techniques is one of the key elements of modern (close-to-nature) forestry. In natural forests, changes in canopy cover, such as the emergence and successive re-closure of canopy gaps are particularly important, as they influence the light availability on the forest floor. Creating canopy gaps of different size is a promising silvicultural tool allowing the regulation of the light availability in managed forests in order to control regeneration composition and development. In this study, we used terrestrial laser scanning data to investigate the relationship between canopy-gap dimensions and emerging natural regeneration along a gradient of management in forests dominated by European beech (Fagus sylvatica L.). We analyzed the spatial distribution and height of regeneration patches in dependence of gap characteristics. Mean regeneration height decreases progressively from the gap polygon over a transition zone towards the area under the canopy, while the tallest regeneration plants were placed in positions midway between center and gap edge, and not directly in the gap center as we initially assumed. The centers of regeneration patches were not displaced when compared to the associated canopy gap centers, as has been reported in other studies conducted on the northern hemisphere for various tree species. The observed patterns did not depend on management strategies, indicating that regeneration responded equally to naturally created gaps and gaps that were caused by logging. We conclude that establishment and development of shade-tolerant European beech regeneration in forest stands is driven by gap openings, but not necessarily direct radiation. If at all, pronounced direct radiation mainly occurs at the northern edge of large gaps. Neither regeneration patch center, nor regeneration tree height pointed in that direction. Our study suggests that in the investigated beech-dominated forests the effect of increased light availability at the northern edge of a gap is overruled by other factors increasing towards the gap edge, such as increased belowground competition of the overstory trees. [ABSTRACT FROM AUTHOR]

Understory vegetation influences several ecosystem services and functions of European beech (Fagus sylvatica L.) forests. Despite this knowledge on the importance of understory vegetation, it is still difficult to measure its three-dimensional characteristics in a quantitative manner. With the recent advancements in terrestrial laser scanning (TLS), we now have the means to analyze detailed spatial patterns of forests. Here, we present a new measure to quantify understory complexity. We tested the approach for different management types, ranging from traditionally and alternatively managed forests and national parks in Germany to primary forests of Eastern Europe and the Ukraine, as well as on an inventory site with more detailed understory reference data. The understory complexity index (UCI) was derived from point clouds from single scans and tested for its relationship with forest management and conventional inventory data. Our results show that advanced tree regeneration is a strong driver of the UCI. Furthermore, the newly developed index successfully measured understory complexity of differently managed beech stands and was able to distinguish scanning positions located on and away from skid-trails in managed stands. The approach enables a deeper understanding of the complexity of understory structures of forests and their drivers and dependents. [ABSTRACT FROM AUTHOR]

• Canopy habitats require more attention in times of global biodiversity loss. • Using LiDAR and canopy fogging, we link canopy structure and arthropod communities. • We use machine learning regression to select variables driving arthropod communities. • Canopy structure drives arthropod communities stronger than tree species identity. • Structural complexity and variability of inter and intra-canopy gaps are key drivers. Forest canopies host an abundant but often neglected diversity of arthropods, which requires careful attention in times of ongoing biodiversity loss. Yet, how tree species composition interacts with canopy structure in shaping arthropod communities remains largely unknown. Here, by combining mobile laser scanning and insecticidal fogging with a machine learning algorithm, we studied which canopy architectural properties affect canopy arthropod communities in monospecific and mixed stands of broadleaved European beech and the coniferous Norway spruce and non-native Douglas fir in Germany. Evaluating the abundances and ecological guild diversity of ∼ 90,000 arthropods and 27 partly novel high-resolution structural variables, we identified vegetation volume and tree species identity as weak predictors of arthropod abundance and ecological guild diversity. In contrast, structural heterogeneity, i.e. structural complexity, vertical layering and variability of canopy gaps—which were highest in coniferous stands—were strong positive drivers. Despite this, arthropod ecological guild diversity was lower in non-native Douglas fir. Mixed stands had intermediate arthropod abundance and ecological guild diversity. Our study shows that habitat heterogeneity and tree species-identity are closely interlinked in shaping associated canopy arthropod communities. Positive effects of habitat heterogeneity on arthropod ecological guild diversity were often uncoupled from resource availability, and the key role of our novel intra-canopy gap indices suggests that they should be considered as indicators in future research on forest heterogeneity-diversity relationships. Broadleaf-conifer mixtures may be suitable to mediate negative tree-species identity effects when adapting forests to global change. [ABSTRACT FROM AUTHOR]