• Energy Research

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 consistently 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.

Dans la forêt ardennaise, les chênes sont en nette diminution. Plusieurs facteurs sont responsables de cette diminution, voici quelques exemples : régénération plus difficile que le hêtre, abroutissement par le gibier, mauvaise réputation de la qualité du chêne ardennais dû aux anciens traitements, conditions de marché du bois défavorable aux essences à grandes révolution. Pourtant, ces dernières années, on prend conscience de leur importance. Les apports du chêne dans la hêtraie ardennaise sont nombreux, notamment dans le contexte du changement climatique : le chêne favorise le développement d’un cortège d’organismes diversifiés, favorise la décomposition de la litière et la fertilité des sols, permet l’exploitation des stations impropres au hêtre, favorise la stabilité des peuplements et permet la production de bois de qualité. Favoriser la régénération du chêne requiert une sylviculture interventionniste, les éléments principaux sont l’apport de lumière et la protection contre le gibier. Même s’il est possible de donner des pistes pour favoriser le chêne, de nombreuses questions se posent encore. Dans le contexte de l’Accord-cadre de recherche et vulgarisation forestières, l’ULg (GxABT) et l’UCL ont mis en place des dispositifs qui devraient bientôt permettre de mettre au point des scénarios sylvicoles.

ABSTRACTWith ongoing global warming, increasing water deficits promote physiological stress on forest ecosystems with negative impacts on tree growth, vitality, and survival. How individual tree species will react to increased drought stress is therefore a key research question to address for carbon accounting and the development of climate change mitigation strategies. Recent tree‐ring studies have shown that trees at higher latitudes will benefit from warmer temperatures, yet this is likely highly species‐dependent and less well‐known for more temperate tree species. Using a unique pan‐European tree‐ring network of 26,430 European beech (Fagus sylvatica L.) trees from 2118 sites, we applied a linear mixed‐effects modeling framework to (i) explain variation in climate‐dependent growth and (ii) project growth for the near future (2021–2050) across the entire distribution of beech. We modeled the spatial pattern of radial growth responses to annually varying climate as a function of mean climate conditions (mean annual temperature, mean annual climatic water balance, and continentality). Over the calibration period (1952–2011), the model yielded high regional explanatory power (R2 = 0.38–0.72). Considering a moderate climate change scenario (CMIP6 SSP2‐4.5), beech growth is projected to decrease in the future across most of its distribution range. In particular, projected growth decreases by 12%–18% (interquartile range) in northwestern Central Europe and by 11%–21% in the Mediterranean region. In contrast, climate‐driven growth increases are limited to around 13% of the current occurrence, where the historical mean annual temperature was below ~6°C. More specifically, the model predicts a 3%–24% growth increase in the high‐elevation clusters of the Alps and Carpathian Arc. Notably, we find little potential for future growth increases (−10 to +2%) at the poleward leading edge in southern Scandinavia. Because in this region beech growth is found to be primarily water‐limited, a northward shift in its distributional range will be constrained by water availability.

Tilia cordata Mill. is a minor European broadleaved species with a wide but scattered distribution. Given its scarcity and low value in the wood market, it has received little attention from researchers and forest managers. This review summarizes the main aspects of T. cordata ecology and growth. Its main limiting factor is its need for warm summer temperatures to ensure successful seed production. It has a height growth pattern relatively similar to that of Acer pseudoplatanus L., with a slight delay in the early stages. Yield tables report great productivity, especially in eastern Europe. T. cordata used to be a major species in Europe, in contrast to its present distribution, but it is very likely to receive renewed interest in the future. Indeed, with the potential change of competition between species in some regions and the need for important diversification in others, T. cordata may play an important role in forest adaptation to climate change, especially owing to its wide ecological tolerance and its numerous ecosystem services. It is necessary to increase our knowledge about its regeneration and its responses to environmental and silvicultural factors, to establish clear management recommendations.

Abstract To increase forest resilience to global change, forest managers are often directing forest stands towards a broader diversity of tree species. The small-leaved lime (Tilia cordata Mill.), a rare and scattered species in northwestern Europe, is a promising candidate for this purpose. Its life traits suggest a high resilience to climate change and a favourable impact on forest ecosystem services. This study used a dendroecological approach to assess how lime tree radial growth had responded to the past climatic change. First, 120 lime trees from nine sites were selected in southern Belgium based on criteria adapted to the rareness of the species. Chronology quality was assessed and resulting tree-ring series were validated at site and region levels. Second, a range of dendrochronological methods was used to analyze the changes over time in the variability and long-term trends of lime tree growth and their relation to climate during the period 1955–2016. Last, behaviour of lime trees was compared with that of beech from the same region and time period. For this purpose, the same methodology was applied to an additional beech tree-ring dataset (149 trees from 13 sites). Beech is the climax tree species of the region, but is known to be drought-sensitive and has shown weaknesses in the current climate. The quality of our tree-ring series attests that dendroecological investigation using rare and scattered species is possible, opening the way to further analysis on other such lesser-known forest tree species. The analysis showed that the small-leaved lime had been resilient to the past climatic change in multiple ways. Lime growth increased during the preceding decades despite an increased frequency and intensity of stressful climatic events. Lime growth quickly recovered in the years following the stresses. The growth–climate relationships were either stable over time or had a positive evolution. The behaviour of lime contrasted strongly with that of beech. Lime performed better than beech in every analysis. Small-leaved lime is thus a serious candidate for addressing climate change challenges in the region. It should be considered by forest managers planning to improve the sustainability and resilience of their forests, in particular in vulnerable beech stands.

AbstractThe growth of past, present, and future forests was, is and will be affected by climate variability. This multifaceted relationship has been assessed in several regional studies, but spatially resolved, large-scale analyses are largely missing so far. Here we estimate recent changes in growth of 5800 beech trees (Fagus sylvatica L.) from 324 sites, representing the full geographic and climatic range of species. Future growth trends were predicted considering state-of-the-art climate scenarios. The validated models indicate growth declines across large region of the distribution in recent decades, and project severe future growth declines ranging from −20% to more than −50% by 2090, depending on the region and climate change scenario (i.e. CMIP6 SSP1-2.6 and SSP5-8.5). Forecasted forest productivity losses are most striking towards the southern distribution limit of Fagus sylvatica, in regions where persisting atmospheric high-pressure systems are expected to increase drought severity. The projected 21st century growth changes across Europe indicate serious ecological and economic consequences that require immediate forest adaptation.