Modeling the interacting effects of browsing and shading on mountain forest tree regeneration (Picea abies)

Although ungulate herbivory influences forest dynamics over a variety of spatial and temporal scales, relatively few models have been developed for investigating browsing effects on tree regeneration processes. We describe a new, mechanistic model (HUNGER) that modifies a well-established mathematical formulation for plant nutrient transport and conversion processes to simulate sapling response to ungulate browsing and light availability. The HUNGER model simulates primary production, height and diameter growth, dry matter allocation, and population dynamics of tree saplings at the scale of small (e.g. 0.001 ha) regeneration patches. The model was applied to Picea abies saplings in mountain forests of eastern Switzerland after calibration based on data for sapling height growth, radial growth, and biomass components under varying light conditions. Independent data were used to test the ability of the model to represent sapling responses to browsing. The model slightly over-predicted sapling height, while no significant differences were found between simulated and observed basal diameter, total biomass or leaf biomass. Model experiments were conducted to explore the interactive effects of winter browsing intensity and relative light availability upon sapling net growth. Simulated shading effects were gradual until approximately 40% relative light availability (if browsing pressure was low) or 60% relative light availability (if browsing pressure was high). Below these values, the model simulates sharp declines in net growth rate. Model results suggest nonlinear responses to browsing and light availability, and the existence of light intensity thresholds for forest regeneration that should be relevant for management activities affecting deer density and crown cover.