Growth Decline in Large Regions of Forest Distribution across Europe

By Laura ThomsonApr 6 2022

Ecosystems in various parts of the world are impacted by global environmental change. Forests are important terrestrial ecosystems where evidence is mounting of cascading effects from anthropogenic-induced climate change, with far implications for the water and carbon (C) cycles, as well as societal services.

Image Credit: Lillac/Shutterstock.com

Large-scale assessments including the whole distribution range of major species can be used to answer evolving questions about those consequences.

This article looks at recent research published in Communications Biology, which looks at the climate-change-driven growth decline of European beech forests.

Forest cover prediction research has a long history of focusing on the relationship between climate change and forest presence/abundance. Ecologically based projections of species growth performance are less well understood.

Local data is often presented in dendroecological analyses, which have provided a useful regional understanding of growth responses to local habitat conditions and climate change. Despite current breakthroughs in tree-ring research, spatio-temporal investigations of actual and expected growth, particularly at scales that include species’ geographic and climatic distributions, are rare.

International dendrochronological databanks have been produced by the tree-ring community, however, they are often biased or limited in species, biomes, and trailing-edge populations, limiting their utility for ecologically focused applications.

However, extensive networks encompassing the whole variety of bioclimatic and ecological conditions of the subject species are required for the successful upscaling of tree-ring data.

To make this possible, researchers built a rich and species-specific network that supports comparative ecological analysis, with over 780,000 ring width measurements from 5800 trees and 324 sample sites across Europe, covering the complete biological spectrum of Fagus sylvatica L. (hereinafter beech) (Figure 1).

Figure 1. Spatial and climatic range of beech sites. A. Geographical distribution of the 324 study sites (black dots) in the natural distribution range of European beech (green area based on the EUFORGEN map. B. Climatic envelope of European beech sampling sites, considering annual temperature and precipitation. Sites are labeled according to the environmental zones detailed in Metzger et al. Image Credit: Caudullo, et al., 2017; Metzger, et al., 2005

From an ecological and socioeconomic standpoint, beech is among Europe’s most important forest species. Beech played a significant part in postglacial reforestation, spreading fast from its Mediterranean refuges to the continent’s central and northern areas. Beech is presently endangered in its geographical and ecological range due to fast climate change.

The findings show that this species’ development patterns changed dramatically over time, particularly in the southern regions. The simulations revealed that recent expansion has been constrained by climate and modified by site-specific variables.

In this regard, predicted decreases in precipitation and temperature increases would result in a fall in total tree productivity, especially if both occur at the same time. Even in the worst-case climate change situation, tree growth could be greatly accelerated at high latitudes in the future.

Results and Discussion

According to the Akaike Information Criterion (AIC) values, the entire model encompassing all examined and significant variables and interactions was the most accurate in predicting the species’ development across Europe (Table 1).

Table 1. Models’ validation. Each row represents a single model and each colored column represents the inclusion of each group of predictor variables (red, moisture conditions (AI), blue, seasonal climatic temperature and precipitation variables (Cli), yellow, geographical variables (Geo), and green, random effects (RE)). For each model, Akaike Information Criterion value and difference, χ2 test value and degrees of freedom (Df) of the χ2 test and p (Chisq, Pr > Chisq)) are shown. Source: Martinez del Castillo, et al., 2022

The GLMM was used to show that the interaction of geographical variables like latitude and altitude with the aridity index (AI) was important in explaining beech growth variability.

Variable interactions have a significant impact on model beech development. Regardless of the relative importance and weight of each seasonal climatic variable, the findings reveal that mean aridity, as well as the geographic components altitude and latitude, regulate the majority of sensitivity to a yearly shifting climate.

Mean growth rates of two successive 31-year periods were calculated for a population-wide typical beech tree with a stable basal area of 86059.03 to evaluate beech forest performance in the past decades (Figure 2).

Figure 2. Spatial patterns of beech growth during the last decades. Mean estimates of BAI (in mm²) from 1955–1985 (a) and 1986–2016 (b), were calculated for a theoretical tree derived from a 324-site chronology network. Image Credit: Martinez del Castillo, et al., 2022

The findings demonstrate significant spatial and temporal disparities in beech development across Europe over the last six decades (Figure 3). When compared to the southern distribution limit of beech, the tree growth rate was substantially greater at low altitudes in NW and central Europe, especially coastal sites in Belgium, the Netherlands, Denmark, and the British Isles. At higher altitudes in central Europe, the Alps, and along the Carpathians, lower tree growth is also modeled.

Figure 3. The spatial pattern of beech growth changes across Europe. Tree growth changes are expressed in percent BAI change from 1986 to 2016 relative to the 1955–1985 period mean. Image Credit: Martinez del Castillo, et al., 2022

The spatial representation of growth variations from 1955 to 1985 and from 1986 to 2016 demonstrates a significant drop in growth across the majority of the European beech range (Figure 3). The strength of this reduction varies across Europe, with lesser strength at low latitudes and increasing strength at higher latitudes, suggesting a distinctive latitudinal gradient of forest growth decline.

Under various climate change scenarios, the GLMM produces variable BAI patterns (Figure 4). Even in the more optimistic SSP1-2.6 scenario, growth differences across geographic gradients are larger than the observed growth changes between the two historical periods. Southern Europe’s growth is expected to slow by up to 30% between 2020 and 2050, relative to a 1986–2016 baseline (Figure 4a).

Apart from more pronounced growth declines in southern Europe, such as the Balkans, and more polarized patterns towards the end of the 21^st century, the patterns from 2040–2070 and 2060–2090 (Figure 4b, c) are comparable.

Figure 4. Relative changes in tree growth. Changes are projected under SSP1-2.6 (a–c) and SSP5-8.5 (d–f) CMIP6 climate scenarios for different periods: 2020–2050 (a, d), 2040–2070 (b, e), and 2060–2090 (c, f) In this panel, BAI changes were expressed in percentage of change compared to the 1986–2016 period. Image Credit: Martinez del Castillo, et al., 2022

Figure 4d–f shows how the more likely SSP5-8.5 scenario results in substantial declines in beech output throughout wide swaths of Europe.

The research reveals dramatic changes in the growth patterns of a significant European tree species during the last 60 years and the next 80 years. Growth decreases have been particularly severe in recent decades towards Europe’s southern distribution boundaries, and these general patterns will continue as the climate warms and becomes drier.

GLMM models show that variations in temperature and water availability, which are all controlled by site-specific climate variables, account for the majority of growth disparities.

Moreover, the findings corroborate recent claims of growth slowdowns in Central Europe. In this way, this research bridges gaps between studies and offers a comprehensive analysis, indicating a steady decline in beech tree productivity and carbon sequestration during the 1980s in the most northern parts of the species’ range.

Adaptive management is critical for ensuring forest viability and limiting climate change’s negative consequences. Researchers must determine species-specific climate effects at various spatiotemporal scales to implement such management measures. In this way, empirical modeling has aided forest managers in anticipating climate influences on future forest growth, allowing for species selection in tree plantations or helping migration planning, for example.

Conclusion

Researchers find a prevalent growth rate drop from 1955 to 2016 after analyzing the causes of growth across an unprecedented network of beech sites across Europe. This reduction is widespread across Europe, except in northern distribution range locations in Denmark, Norway, and Sweden, as well as higher elevations in mountainous areas.

Since falling growth has been observed as a prelude to tree die-off, these major growth changes suggest greater forest mortality. These findings cast doubt on recent projections of rising terrestrial carbon stocks because of climate change, given the strength of beech forests as a carbon sink is expected to diminish.

Journal Reference

Martinez del Castillo, E., Zang, C. S., Buras, A., Hacket-Pain, A., Esper, J., Serrano-Notivoli, R., Hartl, C., Weigel, R., Klesse, S., Resco de Dios, V., Scharnweber, T., Dorado-Liñán, I., van der Maaten-Theunissen, M., van der Maaten, E., Jump, A., Mikac, S., Banzragch, B-E., Beck, W., Cavin, L., Claessens, H., Čada, V., Čufar, K., Dulamsuren, C., Gričar, J., Gil-Pelegrín, E., Janda, P., Kazimirovic, M., Kreyling, J., Latte, N., Leuschner, C., Longares, L. A., Menzel, A., Merela, M., Motta, R., Muffler, L., Nola, P., Petritan, A. M., Petritan, I. C., Prislan, P., Rubio-Cuadrado, Á., Rydval, M., Stajić, B., Svoboda, M., Toromani, E., Trotsiuk, V., Wilmking, M., Zlatanov, T., de Luis, M. (2022) Climate-change-driven growth decline of European beech forests. Communications Biology, 5(1), p. 1-9. Available Online: https://www.nature.com/articles/s42003-022-03107-3#citeas.

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