Tree growth at gap edges. Insights from long term research plots in mixed mountain forests

Abstract

The structure and dynamics of Central Europe’s forests are increasingly characterized by canopy gaps which either result from disturbances or from planned silvicultural actions. It remains, however, unclear, whether the neighborhood to a gap has effects on tree growth which cannot be sufficiently covered by existing standard models so far. In order to test for such effects, we used data from a series of long term experiments in Southern German mixed mountain forests. In parallel, we developed a method for an automatized detection of canopy gaps and gap edge trees given such data. We found that the basal area growth of such trees amounted to about 15–30% more compared to what could be covered by a classic spatial competition index, with a plausible ranking of the main species. Our results suggest, in addition, that an exposition to a gap has a longer lasting effect on tree growth, even after the gap has closed again. With regard to such long-term effects, we found that tree size at the first exposition matters, with strong species-specific differentiation. We concluded that gap exposition effects on tree growth, given their order of magnitude, require being included in tree growth models which are used for planning purposes.