Improving the establishment submodel of a forest patch model to assess the long-term protective effect of mountain forests

Abstract

Simulation models such as forest patch models can be used to forecast the development of forest structural attributes over time. However, predictions of such models with respect to the impact of forest dynamics on the long-term protective effect of mountain forests may be of limited accuracy where tree regeneration is simulated with little detail. For this reason, we improved the establishment submodel of the ForClim forest patch model by implementing a more detailed representation of tree regeneration. Our refined submodel included canopy shading and ungulate browsing, two important constraints to sapling growth in mountain forests. To compare the old and the new establishment submodel of ForClim, we simulated the successional dynamics of the Stotzigwald protection forest in the Swiss Alps over a 60-year period. This forest provides protection for an important traffic route, but currently contains an alarmingly low density of tree regeneration. The comparison yielded a significantly longer regeneration period for the new model version, bringing the simulations into closer agreement with the known slow stand dynamics of mountain forests. In addition, the new model version was applied to forecast the future ability of the Stotzigwald forest to buffer the valley below from rockfall disturbance. Two scenarios were simulated: (1) canopy shading but no browsing impact, and (2) canopy shading and high browsing impact. The simulated stand structures were then compared to stand structure targets for rockfall protection, in order to assess their long-term protective effects. Under both scenarios, the initial sparse level of tree regeneration affected the long-term protective effect of the forest, which considerably declined during the first 40 years. In the complete absence of browsing, the density of small trees increased slightly after 60 years, raising hope for an eventual recovery of the protective effect. In the scenario that included browsing, however, the density of small trees remained at very low levels. With our improved establishment submodel, we provide an enhanced tool for studying the impacts of structural dynamics on the long-term protective effect of mountain forests. For certain purposes, it is important that predictive models of forest dynamics adequately represent critical processes for tree regeneration, such as sapling responses to low light levels and high browsing pressure.

Applying ecological model evaludation: Lessons learned with the forest dynamics model Samsara2

“…For time scales of a few decades, the model predicted stock dynamics with a quantitative bias less than 7% after 50 years in a forest managed with a selection system. This bias is within the lower range of the order of magnitude reported for comparable models (Wehrli et al, 2007;Pacala et al, 1996;Seidl et al, 2005;Didion et al, 2009). However, the number of small trees was overestimated in most stands.…”

“…For example, in forest dynamics models, it is common to compare the predictions of tree community composition to potential natural vegetation types (Bugmann, 1996;Lexer and Honninger, 2001;Botkin, 1993) or old growth forest plots (Pacala et al, 1996;Ruger et al, 2007), or to use historical records of forest inventories over several decades to compare the evolution of predicted and observed capital (e.g., basal area) (Wehrli et al, 2005;Wehrli et al, 2007) and/or distributions of trees in diameter classes (Seidl et al, 2005;Didion et al, 2009;Wehrli et al, 2005;.…”

Assessing potential impacts of climatic change on subalpine forests on the eastern Tibetan Plateau

“…As trees that grow very slowly are subject to enhanced mortality, they die within a short time. This severe limitation resulted in overestimating strongly mature tree mortality rates in dense stands (Wehrli et al 2005), especially in many mountain forests (Ott et al 1997;Wehrli et al 2007). Previous studies also emphasized the need for replacing the parabolic height-diameter function with an asymptotic one, because maximum height is only reached when tree diameter approaches infinity, thus avoiding large trees to become subject to an excessive growth-related mortality rate (Lindner et al 1997;Risch et al 2005;Wehrli et al 2007).…”

Modelling natural disturbances in forest ecosystems: a review

“…Wehrli et al (2007), for instance, introduced a lumped, site-specific ordinal factor for browsing intensity and occurrence in FORCLIM, which in combination with the respective susceptibility parameter results in browsing impact. For white-tailed deer browsing in an Eastern North American riparian hardwood forest, Seagle and Liang (2001) used a more detailed two-stage approach to modelling browsing probability, accounting for both density of tree regeneration (considering seedlings and saplings less than 2 cm diameter) and an ungulate density index.…”

“…small saplings are not as resistant to browsing as taller trees. Another important interaction for modelling browsing impacts exists with light availability and tree growth, as shading directly affects sapling growth and thus the time needed for trees to outgrow highly vulnerable development stages (Wehrli et al, 2007). In this regard Weisberg et al (2005: HUNGER) presented an approach that models the interaction of light availability and browsing impact.…”