Climate sensitivity of disturbance regimes and implications for forest management

Forest disturbance regimes have intensified distinctly across Europe in recent decades, and climate change is expected to further increase the frequency and severity of disturbance events. This intensification has distinct impacts on the sustainable provisioning of ecosystem services to society, with disturbances such as wind damage and bark beetle outbreaks increasingly challenging the continuous supply of renewable resources, protection against natural hazards and ecosystem carbon storage. While we are beginning to understand the responses of individual disturbance agents to changing environmental conditions, our knowledge and ability to predict realistic disturbance regimes (consisting of multiple agents interacting in space and time) is still limited. The development of adaptation strategies to climate-induced disturbance changes is further complicated by diverging appraisals of natural disturbances in the current literature: While disturbances pose a considerable risk for controlled forest management, they also foster the adaptive capacity and diversity of ecosystems, and are proposed as a blueprint for ecosystem-oriented management. In order to cope with the changing disturbance regimes of the future it is important to embrace both views on disturbance (risk vs. fundamental ecosystem process), yet such a comprehensive approach to disturbance management is currently still lacking. In this regard, the objectives of the proposed study are (i) to further understanding and prediction of climate- sensitive, multi-agent disturbance regimes, and (ii) assess impacts on ecosystem services as well as on biodiversity in order to (iii) deduce robust management strategies that minimize the risk for disturbance-related loss of ecosystem services while at the same time fostering ecosystem diversity and complexity. We will address the most detrimental disturbance regime in Europe, the windbark beetle complex, and exemplarily study the ecologically and socially diverse region of the northern Alps in Austria. Employing a combination of empirical and simulation approaches, we will investigate two contrasting forest landscapes, spanning a gradient from a conservation-oriented national park to a landscape managed for timber, protection against natural hazards and, increasingly, carbon. Drawing on 20 years of disturbance observations conducted by consortium members we will perform a detailed analysis of the spatio-temporal interactions between wind and bark beetle disturbances. These insights, in combination with previous efforts of the PI in disturbance modeling, will result in the development of a dynamic simulation model of disturbance interactions. Wer will subsequently use this model to study the climate sensitivity of the windbark beetle disturbance regime, and to investigate a variety of possible disturbance management strategies and their effects on ecosystem services as well as biodiversity. By addressing trade-offs among the latter explicitly, and accounting for dynamic interactions between disturbance agents, climate, and vegetation the project will improve the robustness of disturbance management and contribute to adapting sustainable forest management to changing climate and disturbance regimes.

Disturbances like windthrow and bark beetle outbreaks are natural processes in forest ecosystems. However, disturbances in Europes forests have increased over the past 40 years. Windthrow caused more than twice as much damage in 2001 2010 compared to 1971 1980, and bark beetle disturbances even increased seven-fold over the same period. In depth analyses at the local scale in Central Europe showed that the strong increase in bark beetle disturbances is related to a synchronous increase in susceptibility during drought events, as well as to a large and well-connected population of potential host trees for the beetles. Furthermore, amplifying interactions between windthrow and bark beetles made a strong contribution to recently observed large bark beetle outbreaks in Central Europe.Scenario analyses show that a further increase in disturbances has to be expected for the coming decades. All of the 13 investigated climate scenarios show a further increase in disturbances at the European scale. This increase significantly reduces the amount of carbon that is stored in forest ecosystems. As carbon (in the form of atmospheric CO2) is a major driver of climate change, increasing disturbances could further amplify global warming (positive feedback). Besides the climate regulating service of forests also other important ecosystem services are negatively affected by increasing disturbances. For a forest enterprise in Austrias Northern Front Range of the Alps, for instance, particularly the production and protection functions were reduced by changing climate and disturbance regimes. Consequently, forest management needs to adapt in order sustain forest ecosystem services provisioning also under changing climate and disturbance regimes. A particular focus should be to reduce risks and foster the resilience of forests, e.g. by promoting diverse forests that are well adapted to the climatic conditions expected for the future.Disturbances also have positive effects, however, as they create new niches for species and thus foster species diversity. A simulation study for Kalkalpen National Park showed that the diversity of a wide range of plant and animal species increases as a result of disturbances. Increasing disturbances could thus offset climate-induced biodiversity losses in some instances. Furthermore, disturbances catalyze the adaptation of forest ecosystems to changing climate conditions. Climate currently changes much more rapidly than forests are able to respond, causing a growing disequilibrium between trees and their environment. Disturbances can significantly speed up the natural adaptation of forests to new environmental conditions as they create opportunities for better adapted tree species to establish.