Effects of climate, stand structure, soil characteristics and topography on leaf unfolding of trees in Switzerland

Leaf unfolding of trees is influenced by a wide range of environmental factors, such as climate, stand structure, soil characteristics, and topography. However, few studies have investigated the compound effects of those environmental factors. Furthermore, several factors, such as stand structure and soil characteristics, have not been sufficiently studied. In particular, the influence of water-holding capacity and the resulting soil-water balance on the phenological behaviour of adult trees is largely unexplored.

This study aims to understand how interactions between multiple environmental factors, including climate, stand structure, soil characteristics, and topography, influence the phenological timing of Fagus sylvatica, Picea abies, Larix decidua and Tilia cordata. To understand the effects of these factors, field data on soil characteristics (depth, texture, bulk density, and organic carbon), topography (elevation, relief type, slope aspect and slope inclination) and stand structure (basal area, tree height and stand height) were collected at 52 long-term monitoring sites from a phenological network across all biogeographic regions of Switzerland. Data is currently being analysed using linear mixed-effects models.

Preliminary results indicate that growing degree days, drought and elevation are the most important factors determining the timing of leaf unfolding across species. Among those factors, drought consistently seems to have the highest impact on advancing leaf unfolding. Other factors such as relief type, stand height and basal area appear to be relevant as well, but seem more species specific. For example, relief type, which can affect heat accumulation irrespective of weather conditions, seems important for Larix decidua, but not Fagus sylvatica. In contrast, there is reasonable evidence that competition parameters such as stand height and basal area affect Fagus sylvatica, but not Larix decidua.

This study considerably improves our understanding about the compound effect of established as well as rarely investigated environmental factors on the timing of leaf unfolding across species throughout a wide range of environmental conditions.