Analyses of tree regeneration after wind throw in a central Swiss forest: testing a forest succession model’s abilities

Abstract

Understorey vegetation plays an important role in forest succession. After stand replacing events (e.g., wind storm, forest fire), understorey vegetation can expand very quickly and compete with tree regeneration. This can severely reduce and retard tree regeneration due to competition for resources (e.g., light, water, nutrients). However, understorey vegetation is often not explicitly taken into account in forest succession models. The forest succession model ForClim was developed to study forest succession dynamics in Central Europe, but it has never been tested with regard to the effects of understorey vegetation on tree regeneration. This bachelor’s thesis investigates whether ForClim is able to reproduce the dynamics of a secondary forest succession with competition from understorey vegetation after a stand replacing disturbance event. For the evaluation of the model performance, inventory data from the Habsburg Forest 20 years after the wind storm Lothar were used. ForClim has eight model variants with two different establishment routines for tree ingrowth. For each model variant, a simulation was initialized, and the results were compared to observed stand characteristics. For both establishment routines, the model variant with the smallest deviation from the observed stem number and basal area was selected for the further analyses. The old establishment routine E0 reproduced the observed stand characteristics better than the new routine. Within the two establishment routines, model variants 02 and 12 had the lowest deviation from the observed stand characteristics. By adjusting the model parameter for the establishment probability of young trees, the simulation performance of model variants 02 and 12 could be improved by 2 % and 208 %, respectively. Despite the large improvement of model variant 12, model variant 02 still reproduced the observed data more accurately by 12 %. Subsequently, the simulated succession dynamics were compared with the expected forest development using long-term simulations. Model variant 02 and 12 simulated the expected successional phases of the long-term succession well, but they both overestimated the time span for a full succession cycle and the maximum basal area. Overall, model variant 12 reproduced the expected long-term forest succession better. Additionally, the impact of implementing a plant functional type for understorey vegetation into ForClim’s species list was tested. It could be shown that it is technically possible to implement a plant functional type to qualitatively emulate competition from understorey vegetation. The results of this bachelor’s thesis show that ForClim can be adapted to the observed forest succession, with competition from herbaceous and shrubby understorey vegetation. Additionally, the results indicate that the implementation of understorey vegetation could be a promising approach to improve ForClim’s succession dynamics.