Gap regeneration patterns in relationship to light heterogeneity in two old-growth beech-fir forest reserves in South East Europe

Gap regeneration in two old-growth forest reserves in Slovenia (Rajhenavski Rog) and Croatia (Corkova Uvala) was analysed in relation to within gap light heterogeneity. Both reserves were located in the Dinaric mountain range in south-central Europe and were dominated by beech ( Fagus sylvatica L.) -silver fir ( Abies alba Mill.) forest communities with similar growing conditions. In total, the two largest gaps (700-2000 m(2)) in each reserve were included in the study ( n = 4), plus a further four gaps in Rajhenavski Rog and three in Corkova Uvala ( 200-500 m(2)). All the gaps were similar to 10 years old and originated from one or two successive events, mostly due to a combination of fungi attack and windthrow. Consequently, all gaps had complex geometry and were covered by a well-developed regeneration layer. Each gap was mapped, followed by establishment of a N-S oriented 5 x 5 m grid within and around the area of the canopy opening. At the grid intersections 773, 1.5 x 1.5 m plots were established. On each plot, the coverage of tree regeneration and ground vegetation, seedling density in different height classes and browsing damage were recorded. In addition, the total stretched length and last growing season increment of the leading stem was measured on selected seedlings in each plot. Relative diffuse and direct radiation were estimated using digital hemispherical photographs. All plots were classified into four microsites according to direct and diffuse radiation levels, and microsites were tested for differences in regeneration height and density. While there was more woody regeneration and an almost fivefold higher (6.2 vs 1.3 m -2) average total regeneration density in Rajhenavski Rog, mainly due to a high (5.5 vs 0.6 m(-2)) density of beech, there was more ground vegetation and a higher density of silver fir seedlings in Corkova Uvala. The within-gap regeneration patterns proved to be similar in both forest reserves and showed that 1-year-old seedlings of beech and silver fir and small beech seedlings preferably recruit on microsites under closed canopy or close to gap edges with lower levels of direct and diffuse radiation. There was no significant difference in density of large-beech seedlings among the microsites, yet height and height increment were higher on microsites receiving the highest levels of direct and diffuse radiation. Within-gap heterogeneity in light conditions appears to significantly influence general patterns of beech and silver fir regeneration, while the differences in total average density, regeneration and ground vegetation abundance and spatial distribution cannot solely be explained by light. Since climate, parent material, soil, exposition, relief and stand conditions were very similar between both reserves, it appears that these differences are a result of different densities of large herbivores, which are reported to be much higher in Rajhenavski Rog. This was also confirmed in our research, as much more browsing damage was found in Rajhenavski Rog compared with Corkova Uvala. Implications for silvicultural practice are outlined.