Dynamic models are pivotal for projecting forest dynamics in a changing climate, from the local to the global scale. They encapsulate the processes of tree population dynamics with varying resolution. Yet, almost invariably, tree mortality is modeled based on simple, theoretical assumptions that lack a physiological and/or empirical basis. Although this has been widely criticized and a growing number of empirically derived alternatives are available, they have not been tested systematically in models of forest dynamics. We implemented an inventory-based and a tree-ring-based mortality routine in the forest gap model ForClim v3.0. We combined these routines with a stochastic and a deterministic approach for the determination of tree status (alive vs. dead). We tested the four new model versions for two Norway spruce forests in the Swiss Alps, one of which was managed (inventory time series spanning 72 years) and the other was unmanaged (41 years). Furthermore, we ran long-term simulations (-400 years) into the future under three climate scenarios to test model behavior under changing environmental conditions. The tests against inventory data showed an excellent match of simulated basal area and stem numbers at the managed site and a fair agreement at the unmanaged site for three of the four empirical mortality models, thus rendering the choice of one particular model difficult. However, long-term simulations under current climate revealed very different behavior of the mortality models in terms of simulated changes of basal area and stem numbers, both in timing and magnitude, thus indicating high sensitivity of simulated forest dynamics to assumptions on tree mortality. Our results underpin the potential of using empirical mortality routines in forest gap models. However, further tests are needed that span other climatic conditions and mixed forests. Short-term simulations to benchmark model behavior against empirical data are insufficient; long-term tests are needed that include both nonequilibrium and equilibrium conditions. Thus, there is the potential to greatly improve the robustness of future projections of forest dynamics via more reliable tree mortality submodels.

Dynamic vegetation models (DVMs) are important tools to understand and predict the functioning and dynamics of terrestrial ecosystems under changing environmental conditions. In these models, uncertainty in the description of demographic processes, in particular tree mortality, is a persistent problem. Current mortality formulations lack realism and are insufficiently constrained by empirical evidence. It has been suggested that empirically estimated mortality submodels would enhance DVM performance, but due to the many processes and interactions within a DVM, the claim has rarely been tested. Here, we compare the performance of three alternative growth‐dependent tree mortality submodels in the DVM ForClim: (1) a mortality function with theoretical foundation (ForClim v3.3); (2) a mortality function with parameters directly estimated based on forest inventory data; and (3) the same function, but with parameters estimated using an inverse approach through Bayesian calibration (BC). Time series of inventory data from 30 ecologically distinct Swiss natural forest reserves collected over 35+ yr, including the main tree species of Central Europe, were used for the calibration and subsequent validation of the mortality functions and the DVM. The recalibration resulted in mortality parameters that differed from the direct empirical estimates, particularly for the relationship between tree size and mortality. The calibrated parameters outperformed the direct estimates, and to a lesser extent the original mortality function, for predicting decadal‐scale forest dynamics at both calibration and validation sites. The same pattern was observed regarding the plausibility of their long‐term projections under contrasting environmental conditions. Our results demonstrate that inverse calibration may be useful even when direct empirical estimates of DVM parameters are available, as structural model deficiencies or data problems can result in discrepancies between direct and inverse estimates. Thus, we interpret the good performance of the inversely calibrated model for long‐term projections (which were not a calibration target) as evidence that the calibration did not compensate for model errors. Rather, we surmise that the discrepancy was mainly caused by a lack of representativeness of the mortality data. Our results underline the potential for learning more about elusive processes, such as tree mortality or recruitment, through data integration in DVMs.

Stand-scale climate change impacts on forests over large areas: transient responses and projection uncertainties. Ecological Applications 31(4)

Sensitivity of typical Swiss forest stands to climate change In Switzerland, first climate-induced changes of forest ecosystems can be observed. However, it is widely unknown how and to what extent the typical (widespread) forest stands will respond to future climate change. With the data of the third National forest inventory and the forest succession model ForClim we examined the development 71 typical stands under current and future climatic conditions (A2 emission scenario) with and without management, respectively. The simulations show a weak response until the middle of the century. In contrast, an increased sensitivity towards the end of the century becomes apparent, expressed by declines of basal area at lower elevations, respectively increases at higher elevations. The responses of forest stands are depending on site-specific characteristics. For example, Norway spruce is expected to decline up to higher elevations. Our results show an increase of deciduous tree species in higher elevation zones, particularly if management is applied. The impacts of climate change on important forest ecosystems services vary along a bioclimatic elevation gradient. Thereby, current forest management shows approaches how to at least partly counteract adverse effects of climate change. However, target- and site-specific strategies are needed and, particularly with regard to lower elevations, more knowledge on the potential of tree species to adapt is required.

“Close-to-nature” and multifunctional silviculture in times of climate change – a case study The available assessments of the impacts of the expected climate change on the dynamics of Swiss forests are prone to considerable uncertainties and are mostly of qualitative nature; recommendations on silvicultural measures are therefore typically quite generic. Using a quantitative method, we analyzed whether today's best-practice silviculture remains valid under changing climatic conditions. Based on a stratification of the data from the National Forest Inventory NFI3, 71 typical Swiss forest stands were identified. Thereof, we chose six illustrative examples and examined how timber production, protective function and tree diversity evolve under climate change, using the ForClim forest model. In cooperation with silviculture experts, we elaborated specific management schemes for the upcoming 100 to 150 years considering different silvicultural objectives. In order to reproduce these in detail, ForClim was extended, and thereby an important basis for plausible, practice-oriented modelling was laid. The results show a satisfying behaviour of the newly introduced management techniques “mountain plentering” and “Z-tree management”. In the latter, the modelling of the selection of Z-trees can potentially be improved. In the six stand types investigated here, no abrupt changes in forest dynamics became apparent under the considered climate change scenarios. The results indicate that today's silviculture may remain suitable in the coming decades.