Ecosystem Diversity: A Key Defense Against Climate Risks in European Forests
Giovanni Forzieri
University of Florence, Italy
Winning article: Ecosystem heterogeneity is key to limiting the increasing climate-driven risks to European forests (One Earth, 2024)
“Quantifying the impact of forest disturbances and disentangling the underlying ecological processes is key to identifying effective adaptation measures and preserving the long-term stability of forests.”
The rapid increase in climate driven disturbances worldwide – forest fires in Australia, bark beetle outbreaks in North America and Europe, and drought related forest-mortality in the humid tropics and temperate biomes – are increasingly threatening global forests, endangering their role in carbon sequestration, and therefore their contribution to national pledges which rely on nature-based climate solutions. Our study deals with the urgent need to identify effective and sustainable adaptation strategies to reduce the increasing climate-driven risks to European forests and proves the key importance of increasing forest diversity and heterogeneity to foster ecosystems’ resilience.
Figure 1. Widespread mortality in low heterogeneity patterns of Norway spruce (Picea abies Karst.) caused by bark beetle (Ips typographus L.) at the edge of a recent windthrow in the Italian Alps. In the background, the Marmolada glacier, the highest peak in the Dolomites (3343 m), is retreating rapidly due to global warming. Photo credit: Alessandro Cescatti.
European forests provide essential ecosystem services that support human well-being and climate mitigation. Unfortunately, natural forest disturbances, such as fires, strong winds, or insect outbreaks, represent a serious peril for maintaining productive forests. Such disturbances are intensifying both in Europe and globally, a trend expected to worsen with climate change. As a result, key forest ecosystem services such as biomass production and carbon sequestration could be seriously affected in the near future, potentially undermining the expansion of the bioeconomy and the land-based mitigation efforts currently being implemented. Additional weakening of these carbon sinks would further destabilize the climate system and push the Earth system across critical planetary boundaries, ultimately leading to dramatic consequences for the development of human societies. Despite decades of initiatives to strengthen our carbon sinks, signs of saturation are already emerging worldwide, closely linked to rising natural disturbances.
It became apparent to us that, despite the rising relevance and urgency of these issues, the influencing factors of forest disturbances and their spatiotemporal patterns on the continental scale were largely unknown, and a comprehensive analysis of the resulting climate risks to European forests was lacking. Due to this knowledge gap, the development and implementation of adaptation strategies in the European forest sector have remained critical. Increasing ecosystem heterogeneity is widely proposed to enhance forest resilience, but its effectiveness across multiple disturbances remains uncertain. Large-scale empirical evidence is scarce, and it is unclear whether findings from localized silvicultural practices can be applied more broadly.
Quantifying the impact of forest disturbances and elucidating the underlying ecological processes is of paramount importance in identifying effective adaptation measures and preserving the long-term vitality and stability of European forests. However, knowledge and methodological gaps in observing, understanding, and predicting natural disturbances have hampered substantial progress on these issues so far.
Our contribution therefore: provides the first observation-based, comprehensive assessment of the impact of natural disturbances to European forests occurring over the 1979-2018 period, elucidates their key drivers, and estimates the benefits associated with an enhanced forest heterogeneity. The proposed methodology is fully consistent with the IPCC (Intergovernmental Panel on Climate Change) risk assessment framework and is based on a purely data-driven machine learning approach (Earth observations, climate drivers, database of forest disturbances) and therefore reproducible, applicable at large scales, and in line with the measurement/reporting/verification process of UNFCCC (United Nations Framework for Combating Climate Change).
Results show an average overall annual biomass loss of 41.6 ± 5.3 Mt at European level, subject to a significant rise of 2.3 ± 0.3 Mt year−1, largely influenced by climate change (72%–98%). The contribution of insect outbreaks appears prominent (79%) compared to windthrows (20%) and fires (1%), and is linked to their upsurge after the year 2000. However, impacts vary greatly across Europe depending on local environmental conditions. We estimate that enhancing ecosystem heterogeneity could reduce biomass loss by about 18%, yet local environmental conditions play an important role in modulating these potential benefits.
Figure 2. Risk assessment framework adopted in this study. Source: Forzieri et al., One Earth, 2024