Drivers of forest regeneration patterns in drought prone mixed-species forests in the Northern Calcareous Alps

In temperate mountain forests of Northern Calcareous Alps, shallow soils, in combination with steep slopes and high radiation input on sun-exposed slopes, imply severe conditions for tree regeneration. Considering the remarkable trend of temperature increase and related climatic extremes in the Greater Alpine Area, it is likely that conditions for successful regeneration further deteriorate at these sites. Hence, investigating the relation of regeneration patterns to site conditions is crucial for recent and future forest management. Spatial point process models are appropriate tools to test the effect of environmental covariates in plant pattern generation. By applying these methods, we aim at identifying driving factors of forest regeneration dynamics of Fagus sylvatica, Abies alba and Picea abies. We used tree regeneration data from two sample sites along transects from closed canopy to forest gaps of natural origin. A set of spatially continuous explanatory variables (light environment, rock outcrops, microrelief, distance to mature trees, estimated seed density and presence of already established saplings) were generated. We fitted nonstationary parametric intensity functions with Poisson point process models and Thomas cluster point process models. The first and second order processes were analysed with pair correlation summary statistics. Interactions between regeneration patterns of different species and height classes were assessed with cross-pair correlation summary statistics. Successful germination and growth, at least at some microsites, is possible for any of the investigated species when ungulate browsing is prevented by fencing. Light is the most important environmental factor for describing the regeneration patterns. As higher shading has a positive influence, it is apparently not a limiting resource but rather a proxy for other, latent unfavourable conditions like drying of seedbeds. We thus propose to limit canopy opening sizes to a maximum of 10-12 m. No indications of seed limitation as a cause for regeneration failure of any of the main tree species were detected. Conditions for A. alba seedlings and partly also for P. abies seedlings are more favourable under the canopy of F. sylvatica than under the canopies of conspecifics or the other conifer species. Therefore, we conclude that at our study sites with the current species mixture, at least for the two conifer species, safe microsites are more important than seed availability. With increasing height of P. abies, the spatial segregation of small A. alba and F. sylvatica seedlings became stronger, indicating that competition for resources becomes more dominant and determinant for species coexistence.