Forest disturbance in a changing world

Anthropogenic climate change has profound impacts on ecological processes and is rapidly changing the composition, structure, and functioning of ecosystems. For long-lived ecosystems such as forests the unprecedented pace of these changes will strongly limit the ability for adaptation through genetic and evolutionary processes. As systems are increasingly ill-adapted to their environment, abrupt and disruptive changes are getting increasingly likely. Over the past decades, disturbance events such as windthrow, bark beetle outbreaks, and wildfires have already increased markedly in forests around the globe. Yet, our understanding of the long-term changes in forest disturbance regimes and their impacts is still limited, hampering our ability to address these changes in ecosystem management. Using an innovative multi-method approach the research proposed here will (i) improve our systems understanding of the drivers of forest disturbance regimes, (ii) lead to a comprehensive appraisal of the impacts of current and future disturbance regimes on biological diversity and ecosystem services, and, based on these insights, (iii) develop strategies on how ecosystem management and society at large can cope with changing forest disturbance regimes. To address these questions we will investigate paired study landscapes replicated over Central Europe, contrasting unmanaged forests (e.g., national parks) with managed forest landscapes in similar biogeographical settings. For this network of study landscapes we will use dendroecology to reconstruct the disturbance regimes of past centuries, apply remote sensing to estimate current disturbance severity and extent, and harness simulation modeling to project future trajectories under an ensemble of climate change scenarios. Based on this multi-century analysis we will for the first time in Europe be able to determine whether and how current and future disturbance regimes transgress the historical range of variability, and whether no-analog disturbance regimes are likely to emerge in the future. We will furthermore assess how changing disturbance regimes will affect biodiversity, and quantify their impacts on provisioning (e.g., timber production), regulating (e.g., carbon storage, water retention), cultural (e.g., the recreational value of a landscape), and supporting (e.g., primary productivity) ecosystem services of forest ecosystems. Using simulation modeling embedded in a stakeholder process we will develop strategies of how to deal with such disturbance impacts, and how to optimally manage landscapes under different management objectives (conservation of biological diversity, provisioning of different ecosystem services). Novel simulation approaches and high performance computing will furthermore be used to scale results up to the regional scale (i.e., a forest area of >10 Mill. ha), and provide policy makers and disaster risk managers with detailed information on future forest-related risks. The project will thus not only strongly advance the frontiers in disturbance ecology both with regard to process understanding and science-based ecosystem management, but will also deliver novel solutions for mitigating risks and fostering resilience to abrupt and disruptive changes in the environment in general.

As a result of ongoing climate change, forest ecosystems around the world are changing rapidly. Forest disturbances, i.e., an abrupt loss of live biomass because of wildfire, wind, or insect outbreaks, are among the most climate-sensitive processes in forest ecosystems. They are of particular concern for forest managers, because they can have considerable negative impacts on the ecosystem services forests provide to society. The project "Forest disturbance in a changing world" aimed to (1) quantify the ongoing changes in forest disturbance regimes, (2) assess the impacts of disturbances on forest ecosystem services, and (3) identify strategies to cope with changing disturbance regimes in ecosystem management. To address these questions we employed a combination of methodological approaches, from remote sensing to simulation modeling and the analysis of observational data. We found that between 1986 and 2016, 39 million hectares of Europe's forests (17% of forest area) was disturbed by human or natural causes. Disturbance rates have increased steadily since the mid 1980s (+1.50% per year). While episodes of intensive disturbances occurred also in the past it is likely that a continuing increase in disturbances will push the forests of Central Europe outside of their historic range of variability. We showed that episodes of high disturbance activity are consistently linked to warmer and drier than average conditions in temperate and boreal forests throughout the world. Simulations under future climate scenarios suggest that disturbance activity in Central Europe will intensify further in the coming decades. These ongoing changes in forest disturbance regimes can have considerable impacts on ecosystem services and forest biodiversity. We showed that it takes Central European mountain forests approximately 30 years to recover the carbon lost in a major disturbance event. The introduction of alien pest species (i.e., tree-killing species that are not native to an area) could further reduce the carbon storage potential of forest ecosystems. In addition, also the provisioning of timber and the protection against natural hazards are negatively affected by disturbances. In contrast, forest disturbances are increasing the structural diversity of forest landscapes in Central Europe. Furthermore, we found that disturbed areas provide habitat for a large number of species, equaling old forests in their species richness. To address changing forest disturbance regimes in management we identified a number of management measures, such as consequent salvage logging and the reduction of the rotation age. In depth analyses showed, however, that these risk reduction measures can have unintended consequences for forest carbon and biodiversity. Another promising approach to address changing disturbance regimes is to foster tree species diversity both within and between stands. In conclusion, the project highlights that changing forest disturbance regimes should be a priority for future forest policy and management.