Gap size and position influence variable response of Fagus sylvatica L. and Abies alba Mill.

•We compared silver fir and beech in managed Dinaric karst forests during three years.•Quantum yield was studied in same four light microsites and three different gaps.•Response of species within same microsites was different between gaps.•With gap size quantum yield increased for beech and decreased for silver fir.•Gaps with low direct light or extended towards the southern gap edge favour fir.

Montane forests of silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.) in the Dinaric region are the largest continuous forest area in Central Europe, where silver fir’s regression is one of the region’s major concerns. Designing appropriate sized canopy gap spatio-temporal dynamics is the main silvicultural tool for indirect promotion of silver fir. Within gap microsites, categories based on shares of direct and diffuse radiation proved to be an interesting conceptual model for explaining microsite variation and seedling success induced by light and general climate variability.In order to evaluate the response of beech and fir in different gap microsites, different light regimes were evaluated based on the proportion between direct and diffuse radiation in three different sizes of forest gap in managed forests. The photosynthesis response to different light conditions (max. photosynthetic rate – Amax and quantum yield – Φ) in beech and silver fir was compared on all microsites during three consecutive g periods (2009, 2010, and 2011) under controlled environmental conditions.A different response between species in microsite light categories was shown between different gap sizes. In all microsite categories of both species, quantum yield appeared at lower light intensities with increasing gap size, with the exception of beech with a predominant direct light component, in which yield significantly increased. In order to favour fir, gaps with low direct light levels should be created, while existing gaps should be extended towards the southern gap edge. Our conclusions emphasize the importance of gaps, relatively smaller in size (0.03–0.11 ha) but significant in contributing to increasing the competitive advantage of fir in mountainous, mixed-species Dinaric forests.