Abstract
Models are pivotal for assessing future forest dynamics under the impacts of changing climate and management practices, incorporating representations of tree growth, mortality, and regeneration. Quantitative studies on the importance of mortality submodels are scarce. We evaluated 15 dynamic vegetation models (DVMs) regarding their sensitivity to different formulations of tree mortality under different degrees of climate change. The set of models comprised eight DVMs at the stand scale, three at the landscape scale, and four typically applied at the continental to global scale. Some incorporate empirically derived mortality models, and others are based on experimental data, whereas still others are based on theoretical reasoning. Each DVM was run with at least two alternative mortality submodels. Model behavior was evaluated against empirical time series data, and then, the models were subjected to different scenarios of climate change. Most DVMs matched empirical data quite well, irrespective of the mortality submodel that was used. However, mortality submodels that performed in a very similar manner against past data often led to sharply different trajectories of forest dynamics under future climate change. Most DVMs featured high sensitivity to the mortality submodel, with deviations of basal area and stem numbers on the order of 10–40% per century under current climate and 20–170% under climate change. The sensitivity of a given DVM to scenarios of climate change, however, was typically lower by a factor of two to three. We conclude that (1) mortality is one of the most uncertain processes when it comes to assessing forest response to climate change, and (2) more data and a better process understanding of tree mortality are needed to improve the robustness of simulated future forest dynamics. Our study highlights that comparing several alternative mortality formulations in DVMs provides valuable insights into the effects of process uncertainties on simulated future forest dynamics.
Highlights
Forests have a pivotal role in providing ecosystem functions and services at multiple scales, from the global across the regional to the local scale
We evaluated 15 dynamic vegetation models (DVMs) regarding their sensitivity to different formulations of tree mortality under different degrees of climate change
Model behavior was evaluated against empirical time series data, and the models were subjected to different scenarios of climate change
Summary
Forests have a pivotal role in providing ecosystem functions and services at multiple scales, from the global (carbon and water cycling, energy balance, biodiversity conservation) across the regional (e.g., timber production, recreation) to the local scale (e.g., protection from natural hazards such as flooding or rockfall). A wide range of models of the dynamics of forest structure and composition at decadal to centennial time scales were developed over the past decades (cf reviews by Shugart 1984, Liu and Ashton 1995, Bugmann 2001, Keane et al 2015). Many of these dynamic vegetation models (DVMs) have been used to project the impacts of climate change on future forest trajectories (Solomon 1986, Elkin et al 2013). It is becoming increasingly evident that the impacts of climate, CO2, and other driving forces on growth alone would lead to fairly smooth future changes in stand structure, species composition, and other forest properties (Lloret et al 2012, Rasche et al 2013)
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