Identifying drivers of non-stationary climate-growth relationships of European beech

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species’ ecological amplitude. Our results show that drought sensitivity is highly variable and non-stationary, though consis- tently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees’ rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech’s drought sensitivity, which needs to be taken into consideration when assessing this species’ response to climate change.Additional authors: Isabel Dorado-Liñán, Choimaa Dulamsuren, Balázs Garamszegi, Michael Grabner, Andrew Hacket-Pain, Jon Kehlet Hansen s, Claudia Hartl, Weiwei Huang, Pavel Janda, Marko Kazimirović, Florian Knutzen, Jürgen Kreyling, Alexander Land, Nicolas Latte, François Lebourgeois, Christoph Leuschner, Luis A. Longares, Edurne Martinez del Castillo, Annette Menzel, Renzo Motta, Lena Muffler-Weigel, Paola Nola, Momchil Panayatov, Any Mary Petritan, Ion Catalin Petritan, Ionel Popa, Cǎtǎlin-Constantin Roibu , Álvaro Rubio-Cuadrado, Miloš Rydval, Tobias Scharnweber, J. Julio Camarero, Miroslav Svoboda, Elvin Toromani, Volodymyr Trotsiuk, Marieke van der Maaten-Theunissen, Ernst van der Maaten, Robert Weigel, Martin Wilmking, Tzvetan Zlatanov, Anja Rammig, Christian S. Zan

Analysis spatial distribution trees in forest stands

The analyses of the horizontal distribution of trees have not been used in the research of stand structure in Serbia’s forestry to date. Taking into account the high economic and ecological applicability of such information, this paper deals with the numerical description of the form of spatial distribution of trees. The applied methods are the so-called methods of statistical distributions and dispersion index. The study results in the all-aged mixed stand of fir and beech and in the mature pure stand of fir on the mountain Goč show that all trees together, but also the trees per species, are predominantly randomly distributed in space, with the tendencies towards a weaker or stronger aggregation or uniformity. Based on the identified spatial distribution of trees, it can be concluded that in the greater part of the study stands the environment is homogeneous and the basic conditions of tree growth are uniform. This points to the fact that real assessment of growth development and the competition of fir and beech, due to the identified absence of a marked competition for the basic growth factors, emphasizes, primarily, the quantification of the competition for growth space.

Spatial distribution of spruce trees in the nature reserve 'Jankove bare' on mt. Kopaonik

The methodology and applicability of the study of spatial distribution of trees in Serbian forestry have been insufficiently reported and presented. This paper, based on mathematical­statistical principles, analyses the method of spatial distribution of spruce trees in the Nature Reserve 'Jankove Bare' in the National Park 'Kopaonik'. The following methods in the group of distance methods were applied: Kotar's method (1993), T2­method, and the index of distance dispersion (Johnson, Zimmer, 1985). The study results in a pure uneven-aged spruce stand based on all three methods showed that spruce trees are randomly distributed over the stand area. Therefore, environmental conditions in the study stand can be regarded as homogeneous and equally suitable for tree growth, and there are no significant interactions between the trees, which could cause a higher competition between the trees for nutrients, water, etc

Identifying drivers of non-stationary climate-growth relationships of European beech

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species’ ecological amplitude. Our results show that drought sensitivity is highly variable and non-stationary, though consis- tently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees’ rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech’s drought sensitivity, which needs to be taken into consideration when assessing this species’ response to climate change

Еmpirical and process-based models predict enhanced beech growth in European mountains under climate change scenarios: a multimodel approach

Process-based models and empirical modelling techniques are frequently used to (i) explore the sensitivity of tree growth to environmental variables, and (ii) predict the future growth of trees and forest stands under climate change scenarios. However, modelling approaches substantially influence predictions of the sensitivity of trees to environmental factors. Here, we used tree-ring width (TRW) data from 1630 beech trees from a network of 70 plots established across European mountains to build empirical predictive growth models using various modelling approaches. In addition, we used 3-PG and Biome-BGCMuSo process-based models to compare growth predictions with derived empirical models. Results revealed similar prediction errors (RMSE) across models ranging between 3.71 and 7.54 cm2 of basal area increment (BAI). The models explained most of the variability in BAI ranging from 54 % to 87 %. Selected explanatory variables (despite being statistically highly significant) and the pattern of the growth sensitivity differed between models substantially. We identified only five factors with the same effect and the same sensitivity pattern in all empirical models: tree DBH, competition index, elevation, Gini index of DBH, and soil silt content. However, the sensitivity to most of the climate variables was low and inconsistent among the empirical models. Both empirical and process based models suggest that beech in European mountains will, on average, likely experience better growth conditions under both 4.5 and 8.5 RCP scenarios. The process-based models indicated that beech may grow better across European mountains by 1.05 to 1.4 times in warmer conditions. The empirical models identified several drivers of tree growth that are not included in the current process-based models (e.g., different nutrients) but may have a substantial effect on final results, particularly if they are limiting factors. Hence, future development of process-based models may build upon our findings to increase their ability to correctly capture ecosystem dynamics

No systematic effects of sampling direction on climate-growth relationships in a large-scale, multi-species tree-ring data set

Ring-width series are important for diverse fields of research such as the study of past climate, forest ecology, forest genetics, and the determination of origin (dendro-provenancing) or dating of archaeological objects. Recent research suggests diverging climate-growth relationships in tree-rings due to the cardinal direction of extracting the tree cores (i.e. direction-specific effect). This presents an understudied source of bias that po- tentially affects many data sets in tree-ring research. In this study, we investigated possible direction-specific growth variability based on an international (10 countries), multi-species (8 species) tree-ring width network encompassing 22 sites. To estimate the effect of direction-specific growth variability on climate-growth relationships, we applied a combination of three methods: An analysis of signal strength differences, a Principal Component Gradient Analysis and a test on the direction-specific differences in correlations between indexed ring-widths series and climate variables. We found no evidence for systematic direction-specific effects on tree radial growth variability in high-pass filtered ring-width series. In addition, direction-specific growth showed only marginal effects on climate-growth correlations. These findings therefore indicate that there is no consistent bias caused by coring direction in data sets used for diverse dendrochronological applications on relatively mesic sites within forests in flat terrain, as were studied here. However, in extremely dry, warm or cold environments, or on steep slopes, and for different life-forms such as shrubs, further research is advisable