Central European Forests Research Articles

Synthesis of palaeoecological data from the Polish Lowlands suggests heterogeneous patterns of old-growth forest loss after the Migration Period

Human impact on Central European forests dates back thousands of years. In this study we reanalyzed 36 published pollen data sets with robust chronologies from Polish Lowlands to determine the patterns of large-scale forest decline after the Migration Period (fourth to sixth century CE). The study revealed substantial heterogeneity in the old-growth forest decline patterns. Using new high-resolution studies, we could better understand the timing of this transition related to increasing economic development. After the Migration Period, forest expansion continued until the seventh to ninth centuries cal. CE, when the dawn of Slavic culture resulted in large-scale forest decline, especially in north-western and north-central Poland. Later, forest decline was recorded mainly in north-eastern Poland and was related to Prussian settlements, including activities associated with the Teutonic Order, as well as with new settlements from the fourteenth century. The composite picture shows a varied spatio-temporal forest loss and transition towards the present-day, human activity dominated landscapes. However, some sites, such as in north-eastern Poland, are characterized by a less abrupt critical transition. The pristine nature of the oak-hornbeam forest had already been destroyed in Early Medieval times (eighth to ninth centuries cal. CE) and the potential for recovery was largely lost. Our study has confirmed previous assumptions that the decline of hornbeam across the Polish Lowlands may be an early indicator of local settlement processes, preceding severe forest loss, and establishment of permanent agriculture.

Central European forest floor bryophytes: Richness, species composition, coexistence and diagnostic significance across environmental gradients of forest habitats

Bryophytes are good indicators of habitat conditions and show associations with different types of forests. In this study we assessed the diversity and distribution of forest floor bryophytes across a wide gradient of Central European forests using vegetation records from the Polish Vegetation Database (PVD). We identified forest types using the Habitat Classification of the European Nature Information System (EUNIS) – the main comprehensive pan-European hierarchical classification of habitats. Based on 9258 plots containing bryophyte species and representing 20 forest types, we assessed the species number and cover, species composition of the bryophyte layer, species tendency for coexistence, and bryophyte diagnostic value for forest types. We explained the observed trends by environmental variables and bryophyte species traits to understand how bryophytes function in forest habitats.Bryophyte species number and cover were high in forest types on moist sites, both with deciduous and coniferous tree stands. The richness and abundance of bryophyte species, as well as the species composition of the bryophyte layer depended mainly on substrate moisture, fertility and pH, as well as on the percentage cover of coniferous and deciduous tree species in the stand. Species traits which responded to environmental factors were, first of all, requirements for substrate moisture, fertility and pH. Bryophytes of closed forests and short-lived shuttle species preferred deciduous stands, while bryophytes occurring in forests as well as on open land and perennial species were associated with coniferous stands.Bryophyte species showed a clear preference for forest types, but their fidelity was usually not high. Based on the fidelity of species, we also identified 10 groups of coexisting species which were also indicators of forest types and were dependent on environmental variables.Forest floor bryophytes respond to environmental gradients by species number, cover and composition, and they play a diagnostic role in forest types. Our research shows the importance of databases in learning about bryophyte ecology. However, the use of vegetation data has some limitations.

Tree Size Drives Diversity and Community Structure of Microbial Communities on the Bark of Beech (Fagus sylvatica)

Tree bark constitutes an ideal habitat for microbial communities, because it is a stable substrate, rich in micro-niches. Bacteria, fungi, and terrestrial microalgae together form microbial communities, which in turn support more bark-associated organisms, such as mosses, lichens, and invertebrates, thus contributing to forest biodiversity. We have a limited understanding of the diversity and biotic interactions of the bark-associated microbiome, as investigations have mainly focused on agriculturally relevant systems and on single taxonomic groups. Here we implemented a multi-kingdom metabarcoding approach to analyze diversity and community structure of the green algal, bacterial, and fungal components of the bark-associated microbial communities of beech, the most common broadleaved tree of Central European forests. We identified the most abundant taxa, hub taxa, and co-occurring taxa. We found that tree size (as a proxy for age) is an important driver of community assembly, suggesting that environmental filtering leads to less diverse fungal and algal communities over time. Conversely, forest management intensity had negligible effects on microbial communities on bark. Our study suggests the presence of undescribed, yet ecologically meaningful taxa, especially in the fungi, and highlights the importance of bark surfaces as a reservoir of microbial diversity. Our results constitute a first, essential step toward an integrated framework for understanding microbial community assembly processes on bark surfaces, an understudied habitat and neglected component of terrestrial biodiversity. Finally, we propose a cost-effective sampling strategy to study bark-associated microbial communities across large spatial or environmental scales.

Changes in the Concentrations of Trace Elements and Supply of Nutrients to Silver Fir (Abies alba Mill.) Needles as a Bioindicator of Industrial Pressure over the Past 30 Years in Świętokrzyski National Park (Southern Poland)

The aim of this study was to identify changes in the concentrations of elements in fir assimilation organs over the past 30 years in order to assess fir reactions as a bioindicator of changes in the functioning of Central European fir forest ecosystems under conditions of reduced anthropogenic emissions. In particular, we selected the example of the Świętokrzyskie Mountains (Świętokrzyski National Park located in the northern range of Abies alba Mill.). The research was carried out in the “Łysica-Święty Krzyż” area under strict protection, including multi-species and uneven-aged tree stands with a complex structure, dominance of beech and fir, and numerous admixtures of other tree species. A decrease in the concentrations of pollutants in fir needles indicates a significant reduction in pressure on the environment and an improvement in the conditions of ecosystems in 2018 compared with those in 1986. In the period of more than 30 years between the sets of research, the concentrations of lead in fir needles decreased threefold and those of sulfur decreased twofold. A significant increase in the concentrations of cadmium, copper, manganese, and zinc in the three-year-old needles showed that they are a good indicator of environmental pollution with trace elements and may be used in biomonitoring.

Classifying surface fuel types based on forest stand photographs and satellite time series using deep learning

With the increasing threat of wildfires globally, improving the availability of accurate, spatially explicit fuel type information is critical for fire behavior predictions that can support management decisions to mitigate fire hazards. Since mapping surface fuel types using airborne or spaceborne sensors relies on ground truth data from laborious field assessments, here we propose a novel proximate sensing-based approach for classifying surface fuel types from in-forest RGB photographs using convolutional neural networks (CNNs). We test different configurations of deep learning models that integrate photographs of the forest stand and the forest floor as well as time series of multispectral satellite data from Sentinel-2 using long short-term memory (LSTM), and compare their performance in classifying understory and litter fuel types of Central European forests. We also investigate how ensemble approaches based on majority voting can help to improve classification results. We found that understory fuel types were classified with highest accuracy after cross-validation (0.78) using a combination of horizontal stand photos and forest floor photos. This accuracy was further improved by post-classification decision fusion of model predictions on multiple photographs of a forest stand and by considering the model’s confidence in its predictions (0.85). Litter fuel type classification based on forest photographs resulted in lower overall accuracy (0.60), but using model ensemble predictions on both photographs and Sentinel-2 time series significantly improved the results (0.72). We found that the accuracy of our models was mostly limited by naturally smooth transitions between the defined fuel type classes and the co-occurrence of multiple fuel types in a photograph. This study shows that deep learning methods can provide an efficient means to assess fuel types from GNSS-located photos of forest stands as a basis for generating and validating fuel type and finally fire risk maps. The necessary data can be readily collected by forest managers or citizen scientists.

Annual shoot length of temperate broadleaf species responses to drought

In recent years, Central European forests and trees have been severely suffered from drought events in the growing season. To examine the tree growth response to drought stress, measurements of annual shoot lengths (ASL) of various tree species with distinct ages and habitats were taken in southeast Germany. The index of relative ASL increase (RAI), which is a standardized ASL increase, was calculated to indicate the growth rate of various tree species between the non-drought (2015–2017) and the drought period (2018–2020). The results showed the ASLs of most early-young trees (< 10 years) significantly showed a decreasing tendency in the drought period, whereas the middle-young (11–15 years) and late-young trees (> 16 years) presented a relatively stable trend. The ring-porous species with anisohydric behavior were likely to have higher RAIs associated with higher drought tolerance than diffuse-porous species with isohydric behavior. However, tree growth rate did not correlate with drought tolerance (resistance) of tree species indicated by leaf turgor loss point (Ψtlp, MPa). Therefore, this study suggested parameters, such as growth rate, age phases, and microhabitats, should be considered to predict tree species’ drought tolerance other than Ψtlp. Finally, we emphasized tree shoot growth responses to drought were affected by multiple internal and external factors, and strongly depended on species- and site-specific characteristics, such as age phases, xylem structure, hydraulic strategy, microhabitat, grafting effect, and competition. ASL measurement can be recommended to be an easy, fast, and effective method to detect tree growth rate and response to environmental stress.

Species-specific growth and transpiration response to changing environmental conditions in floodplain forest

Recent changes in the floodplain forests of Central Europe, caused mainly by changes in hydrological management and the increased frequency of droughts due to climate change, have led to severe degradation of floodplain ecosystems. Our main objective was to determine the sensitivity of trees to drought by observing the response of the tree phenology, stem radial growth, and physiology (sap flow) of three predominant tree species, namely English oak, narrow-leaved ash, and common hornbeam, to the environmental variables (climate). Stem radial growth began before bud break in ring-porous oak and ash, whereas in diffuse-porous hornbeam, growth onset occurred after leaf formation. The early onset with intense growth during favorable months (April–May) observed in ring-porous species was a major prerequisite for the successful growth of oak and ash at this site. Tree water deficit (TWD), an indicator of stem hydration, was triggered by decreasing soil moisture in all species, and was most prominent in ash, followed by oak. Intriguingly, sap flow was decoupled from TWD in all species and was driven primarily by evaporative demand from the atmosphere. Oak was the least conservative in regulating sap flow under atmospheric drought followed by hornbeam, whereas ash was most restricted and reduced its transpiration during dry periods. In contrast, ash was characterized by the highest radial growth and growth-based water-use efficiency. The lower water storage capacity of oak and ash is likely compensated by deep rooting and drought avoidance strategies, respectively. Tree species that tend to use surface soil water could be severely limited by more extractive species such as hornbeam. Despite the contrasting leaf and wood phenology, stomatal control, and rooting depth among the studied floodplain tree species, they exhibited analogous sap flow and water storage dynamics responses to drier conditions that enabled them to co-exist in the South Moravian Region. Nevertheless, our results suggest that the severe droughts and human-induced alterations in groundwater pose serious threats to floodplain forests in Central Europe, with certain tree species being unable to adapt to these altered conditions.

Central European forest–steppe: An ecosystem shaped by climate, topography and disturbances

AbstractThe occurrence and origin of dry grasslands and their rich biota in the moderately humid Central‐European climate have fascinated scientists for over a century. Modern palaeoecological and phylogeographical data support earlier hypotheses that these grasslands are late Pleistocene relicts and can therefore be considered part of the Eurasian forest–steppe biome. However, it is still unclear which factors fostered the maintenance of steppe patches in Central Europe throughout the Holocene. Here, we provide an overview of the main hypotheses, which stress, respectively, the effects of climate, edaphic conditions and disturbances. We then develop a general conceptual framework on how these three factors interact to form forest–steppe mosaics. We thereby emphasize the role of topography as a crucial control on forest–steppe patterns at the landscape scale. Topography is related to several mechanistic drivers that influence vegetation processes, such as near‐surface microclimate and soil formation. Consequently, topographic variation allows both forest and steppe patches to occur beyond their macroclimatic niche, favouring the development of forest–steppe mosaics. To illustrate our framework, we demonstrate the interactive effect of macroclimate and topography on the occurrence of steppe patches at 108 selected Central European forest–steppe sites. Although we developed our framework focusing on the current distribution of Central European forest–steppe, we suggest that it contributes to the understanding of similar transitions between temperate forest and steppe biomes in the past as well as elsewhere in the world.

Patterns, drivers and detectability of infestation symptoms following attacks by the European spruce bark beetle

Recent outbreaks of the European spruce bark beetle (Ips typographus) in Norway spruce (Picea abies) forests in Central Europe highlight the importance of timely detection and sanitation of infested trees for pest management efficacy. This study provides novel quantitative evidence on the manifestation of infestation symptoms and their visual detectability, to guide accelerated, optimized terrestrial bark beetle monitoring, as well as establishing benchmarks for potential alternative (e.g. sensor-based) monitoring approaches. We employed bi-weekly, individual tree-level assessments on 85 hectares of spruce-dominated unmanaged forest over a 2-year period in south-western Germany (detecting a total of 1,176 infested trees). By applying decision tree-type models, we quantified the predictive power of observed symptoms and their correlation with environmental factors and time. Terrestrial detection accuracy and timeliness were high, suggestive of being sufficient to suppress I. typographus outbreak propagation by subsequent sanitation felling. Among the six studied symptoms, boring dust occurred most frequently (in 82% of correctly detected infestations) and is most suitable for timely detection. Total symptom abundance was best explained by two site parameters (slope, Standardized Precipitation-Evapotranspiration-Index) and I. typographus population density, while it was widely independent of tree parameters and time. Though individual symptoms varied over time and among trees, patterns were clearly identified. For instance, infestations in spring were most critical to be timely detected, while increasing crown discoloration and defoliation facilitated detection in late summer and autumn. Findings further imply that hibernation trees would optimally be detected already in late summer with sanitation felling applied before November.

Fungal Community Development in Decomposing Fine Deadwood Is Largely Affected by Microclimate.

Fine woody debris (FWD) represents the majority of the deadwood stock in managed forests and serves as an important biodiversity hotspot and refuge for many organisms, including deadwood fungi. Wood decomposition in forests, representing an important input of nutrients into forest soils, is mainly driven by fungal communities that undergo continuous changes during deadwood decomposition. However, while the assembly processes of fungal communities in long-lasting coarse woody debris have been repeatedly explored, similar information for the more ephemeral habitat of fine deadwood is missing. Here, we followed the fate of FWD of Fagus sylvatica and Abies alba in a Central European forest to describe the assembly and diversity patterns of fungal communities over 6 years. Importantly, the effect of microclimate on deadwood properties and fungal communities was addressed by comparing FWD decomposition in closed forests and under open canopies because the large surface-to-volume ratio of FWD makes it highly sensitive to temperature and moisture fluctuations. Indeed, fungal biomass increases and pH decreases were significantly higher in FWD under closed canopy in the initial stages of decomposition indicating higher fungal activity and hence decay processes. The assembly patterns of the fungal community were strongly affected by both tree species and microclimatic conditions. The communities in the open/closed canopies and in each tree species were different throughout the whole succession with only limited convergence in time in terms of both species and ecological guild composition. Decomposition under the open canopy was characterized by high sample-to-sample variability, showing the diversification of fungal resources. Tree species-specific fungi were detected among the abundant species mostly during the initial decomposition, whereas fungi associated with certain canopy cover treatments were present evenly during decomposition. The species diversity of forest stands and the variability in microclimatic conditions both promote the diversity of fine woody debris fungi in a forest.

Impacts of Forest Management on Forest Bird Occurrence Patterns—A Case Study in Central Europe

The global increase in demand for wood products, calls for a more sustainable management of forests to optimize both the production of wood and the conservation of forest biodiversity. In this paper, we evaluate the status and future trends of forest birds in Central European forests, assuming different forest management scenarios that to a varying degree respond to the demand for wood production. To this end, we use niche models (Boosted Regression Trees and Generalized Linear Models) to model the responses of 15 forest bird species to predictors related to forest stand (e.g., stand volume of specific tree species) and landscape structure (e.g., percentage cover), and to climate (bioclimatic variables). We then define five distinct forest management scenarios, ranging from set-aside to productivity-driven scenarios, project them 100 years into the future, and apply our niche models into these scenarios to assess the birds’ responses to different forest management alternatives. Our models show that the species’ responses to management vary reflecting differences in their ecological niches, and consequently, no single management practice can benefit all species if applied across the whole landscape. Thus, we conclude that in order to promote the overall forest bird species richness in the study region, it is necessary to manage the forests in a multi-functional way, e.g., by spatially optimizing the management practices in the landscape.

Forest management impact on soil organic carbon: A paired-plot study in primeval and managed European beech forests

Forest soils have been recognized as important reservoirs of older stable carbon (C) in the biosphere and thus play a key role in the global C cycle. While much research has addressed the consequences of forest conversion and forest use on soil organic carbon (SOC) stocks, some controversies remain as to whether primary forest soils store significantly more SOC than soils in managed forests. Here, we explore the effect of forest management on SOC stocks in European beech (Fagus sylvatica) forests through a paired-plot study in nearby primeval and managed forests in the western Carpathians, testing the hypotheses that primeval forests store significantly more SOC especially in the topsoil and that larger biomass and deadwood amounts are important drivers. The three primeval forests stored on average 15 % more SOC (∼1.8 kg C m−2) in the organic layers and mineral soil to 50 cm depth than the managed forests prior to harvest (difference marginally significant at p = 0.09). Contrary to expectation, the difference was significant in the subsoil (30–50 cm depth) but not in the topsoil. Consequently, the primeval forests had a greater proportion of their SOC pool located in the subsoil than the managed forests. Despite considerably larger wood biomass and deadwood amounts in the primeval forests, SOC pools were neither related to biomass nor deadwood mass. PCA and correlation analyses revealed a prominent negative effect of the availability of Ca, Mg, K and P on SOC stocks, suggesting that clear-cutting has slightly (but significantly) increased nutrient supply in the managed forests, likely stimulating soil biological activity and reducing SOC storage in comparison to the primeval forests. We conclude that the primeval forest reference is important for assessing forest management effects on SOC storage, and that a long history of management likely has significantly reduced the SOC stocks of Central European beech forests.

Forest structure determines nest box use by Central European boreal owls

Nest boxes represent a popular tool to support secondary cavity-nesting species. Surprisingly, the benefits and limitations of nest boxes for target species in different environments are poorly understood. We performed a 3-years experimental study in two different Central European forests to evaluate nest box use and breeding performance of boreal owl (Aegolius funereus) — a species well known for its readiness to occupy nest boxes. Based on territorial vocalisation, two boreal owl populations 200 km apart were similarly abundant in their environments. However, only the boreal owl population in young restored Norway (Picea abies) and blue (Picea pungens) spruce-dominated forests on mountain plateaus readily occupied nest boxes with the occupancy reaching 8–15%. Nest boxes lost their supporting function for the boreal owl in mature Scots pine (Pinus sylvestris)-dominated forests in the lowland, where the nest box occupancy reached 0–1%. As a result, the population of boreal owls that used nest boxes in the young restored forests produced 10 times more fledglings than the population inhabiting mature Scots pine forests. We explain the differences by the contrasting availability of natural tree cavities between the two study areas being much higher in mature Scots pine forests. For the first time, this study documents differences in nest box use despite similar food availability and population size of the target species. The study provides the findings-related recommendations for deploying nest boxes for boreal owls and points out a general lack of practical guides.

Towards ecosystem service assessment: Developing biophysical indicators for forest ecosystem services

Forests are specifically relevant for biodiversity and provide a wide range of ecosystem services. However, Europe’s forests and their services are threatened. Temperate, boreal and Mediterranean forests are facing deforestation, fragmentation and conversion due to human impact and recent developments in climatic conditions.Currently, not all forest ecosystem services are comprehensively accounted for and often, primarily timber production is considered. The aim of the present article is to further develop forest ecosystem service assessment using a systematic approach to assess biophysical indicators for ecosystem services. In this way, we aim to promote the use of forest function mapping, which is an important planning tool in the Central European forest sector for mapping forest areas of particular relevance for protection and recreation. Building on forest function mapping and the Common International Classification of Ecosystem Services (CICES 5.1), we selected six ecosystem services and propose indicators for their biophysical quantification. The indicator selection process followed specific selection criteria.We propose indicators for the potential, supply, demand and flow of the following forest ecosystem services: (i) groundwater for drinking purposes, (ii) timber provision, and (iii) global climate regulation. For the ecosystem service (iv) local air pollution reduction, indicators for potential, demand and flow are proposed, for (v) recreation, as well as (vi) education and training, indicators for the ecosystem services’ supply, demand and flow are proposed. In addition, possible measuring methods and units, and necessary data sources for applying the indicators are provided.

Predicting Wildfire Fuels and Hazard in a Central European Temperate Forest Using Active and Passive Remote Sensing

Climate change causes more extreme droughts and heat waves in Central Europe, affecting vegetative fuels and altering the local fire regime. Wildfire is projected to expand into the temperate zone, a region traditionally not concerned by fire. To mitigate this new threat, local forest management will require spatial fire hazard information. We present a holistic and comprehensible workflow for quantifying fuels and wildfire hazard through fire spread simulations. Surface and canopy fuels characteristics were sampled in a small managed temperate forest in Northern Germany. Custom fuel models were created for each dominant species (Pinus sylvestris, Fagus sylvatica, and Quercus rubra). Canopy cover, canopy height, and crown base height were directly derived from airborne LiDAR point clouds. Surface fuel types and crown bulk density (CBD) were predicted using random forest and ridge regression, respectively. Modeling was supported by 119 predictors extracted from LiDAR, Sentinel-1, and Sentinel-2 data. We simulated fire spread from random ignitions, considering eight environmental scenarios to calculate fire behavior and hazard. Fuel type classification scored an overall accuracy of 0.971 (Kappa = 0.967), whereas CBD regression performed notably weaker (RMSE = 0.069; R2 = 0.73). Higher fire hazard was identified for strong winds, low fuel moisture, and on slopes. Fires burned fastest and most frequently on slopes in large homogeneous pine stands. These should be the focus of preventive management actions.

Drought impacts in forest canopy and deciduous tree saplings in Central European forests

Forests worldwide are increasingly exposed to extreme weather events. Drought deteriorates the health, structure, and functioning of forests, which can lead to reduced diversity, decreased productivity, and increased tree mortality. Therefore, it is an urgent need to assess the impact of drought on tree species. Due to differences in tree physiology, saplings and mature trees are likely to respond specifically to drought conditions. In contrast to mature trees, little is known about the response of saplings to drought. Here, we combine in-situ field measurements for saplings of deciduous tree species with remote sensing for forest canopy to assess drought damage, recovery, and sapling mortality patterns during a centennial drought (2018, 2019) and beyond (2020). We measured 2051 saplings out of 214 plots in Central Germany. Forest canopy health was assessed using 10 × 10 m resolution satellite observations for the same locations. We (1) demonstrate that forest canopy exhibits long-lasting drought-induced effects, (2) show that saplings have a remarkable capacity to recover from drought and survive a subsequent drought, (3) demonstrate that reduced sapling recovery leads to their mortality, (4) reveal that drought damage on saplings increases from pioneer to non-pioneer species, and mortality is ranking from Sorbus aucuparia > Sambucus nigra > Fraxinus excelsior, Acer campestre, Frangula alnus > Ulmus glabra > Carpinus betulus > Betula pendula, Fagus sylvatica > Acer pseudoplatanus > Quercus petraea > Corylus avellana, Crataegus spp., >Prunus avium, Quercus robur; and (5) link drought response to site conditions, indicating that species diversity and winter precipitation as relevant indicators of tree health. If periods of drought become more frequent, as expected, this could negatively impact mid-term forest recovery, alter long-term tree species assemblages and reduce biodiversity and functional resilience of forest ecosystems. We suggest that models of forest response to drought should differentiate between the forest canopy and understory and also consider species-specific responses as we found a broad spectrum of responses within the same plant functional type of deciduous tree species in terms of drought damage and recovery.

Tensile strength grading of beech (Fagus sylvatica L.) lamellas from multiple origins, cross sections and qualities

Abstract The market share of European beech (Fagus sylvatica L.) wood in the construction sector is low despite an increase in beech stock in Central European Forests in recent years. More efficient sawing techniques, higher lamella grading yields and solving of adhesion challenges may increase the competitiveness of beech glulam and promote its use. The aim of this paper is to revise the lamella grading system in the current German technical approval for beech glulam Z-9.1-679:2019 (DIBt (2019). BS-Holz aus Buche und BS-Holz Buche Hybridträger und zugehörige Bauarten. Allgemeine bauaufsichtliche Zulassung Z-9.1-679:2019. Deutsches Institut für Bautechnik) and to suggest modifications in the lamella grading rules for glulam production allowing higher yields and reliable tensile strength values at the same time. The unique dataset in this study combined different origins of lamellas and covered a wide range of visual, physical and mechanical wood characteristics including a high amount of low quality material. Indicating properties (IPs) for tensile strength, such as knot parameters and dynamic modulus of elasticity, were contrasted with tensile strength and static modulus of elasticity. Beech lamellas, graded by means of Z-9.1-679:2019 (DIBt (2019). BS-Holz aus Buche und BS-Holz Buche Hybridträger und zugehörige Bauarten. Allgemeine bauaufsichtliche Zulassung Z-9.1-679:2019. Deutsches Institut für Bautechnik), did not achieve the tensile strengths required for glulam production in many grading classes and the yield was low. A machine grading approach with dynamic modulus of elasticity as a single grading criterion gave higher yields than the current grading procedure and high reliability for tensile strength prediction with a prediction accuracy of R 2 = 0.67.

The Past Matters: Previous Management Strategies Modulate Current Growth and Drought Responses of Norway Spruce (Picea abies H. Karst.)

Due to high productivity and past management approaches, the forests of Central Europe are heavily dominated by conifers, even on sites where they do not occur naturally at all. One prominent example is Norway spruce (Picea abies H. Karst.), a species considered particularly vulnerable to severe droughts, especially outside of its ecological niche where it has been widely planted over the past centuries. In the face of global change, it is a major task for foresters to increase these forests’ ability to cope with the impacts of increasing climatic extremes. Therefore, gaining more knowledge about how different management strategies affect the drought responses of trees is crucial. However, we still know little about the influence of the individual treatment history of a tree on its growth. We used a dendroecological approach to address this issue and to assess how initial spacing, structural diversity, tree size, and density regulation approaches modulate annual growth, especially in drought years. We hypothesized that stand establishment and past silvicultural treatment codetermine tree growth and drought resilience. Our study took place at the combined spacing-thinning trial Fürstenfeldbruck 612 (FFB 612) in Southern Germany, since it delivered precise long-term data covering a broad range of treatments. Based on linear mixed effect models, we showed that the individual treatment history of a tree affects its annual growth and drought responses considerably. In more detail, we found that (i) high structural diversity in the vicinity of each tree favored growth and improved a tree’s performance under drought; (ii) larger trees were more climate-sensitive; (iii) previous high variations in thinning intensity, and consequently strong fluctuations in growth, affected current growth negatively and reduced recovery from droughts. Furthermore, we sought to understand the underlying mechanisms and to draw potential implications for refining silvicultural guidelines.

Molecular Probing of DOM Indicates a Key Role of Spruce-Derived Lignin in the DOM and Metal Cycles of a Headwater Catchment: Can Spruce Forest Dieback Exacerbate Future Trends in the Browning of Central European Surface Waters?

Peatlands of the Northern Hemisphere and Central European coniferous forests experience significant environmental change. The resultant browning of surface waters, that is, elevated concentrations of dissolved organic matter (DOM) and metals, is of interest in the context of the global C cycle, peatland and forest management, and water treatment. In an attempt to identify the causes of this process in the Harz Mountains (Central Germany), we studied the spatiotemporal variations in DOM molecular composition (thermally assisted hydrolysis and methylation combined with GC-MS) and metal concentrations in headwater stream samples. We found strong relationships between DOM and metals and seasonal variations in the DOM quality and tentatively DOM-metal binding mode: during summer base flow, DOM and metal concentrations are low, and all elements other than the alkali and alkaline earth metals (Ca, Mg, Sr, K, and Na) are positively correlated to DOM, whereas during spring and autumn (high discharge), only metals with strong affinity for DOM (Fe, As, Cu, Cr, Pb, and Ti), but not weakly binding ones (Al, Cd, La, Mn, Ni, Zn, and Zr), are correlated to DOM, indicative of selectivity in DOM-metal interactions. The products of polyphenols are the key ingredients of the DOM-metal complexes. We argue the importance of spruce lignin-derived vanillic acid moieties, which are involved in weak (all seasons) and strong, multidentate and/or colloidal, binding (spring and autumn) of metals. Considering the ongoing spruce forest dieback and climate change acceleration, it is tempting to conclude that spruce necromass and forest soils may release vast amounts of lignin-derived DOM and associated metals to headwater streams. This would have significant implications for forest soil C stocks and the management of connected drinking water reservoirs.

Soil phosphorus status and P nutrition strategies of European beech forests on carbonate compared to silicate parent material

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry,https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIP) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition.