Overstorey control of understorey species composition in a near-natural temperate broadleaved forest in Denmark

Little is known about the importance of the forest overstorey relative to other factors in controlling the spatial variability in understorey species composition in near-natural temperate broadleaved forests. We addressed this question for the 19 ha ancient forest Suserup Skov (55 degrees 22' N, 11 degrees 34' E) in Denmark, one of the few old-growth temperate broadleaved forest remnants in north-western Europe, by inventorying understorey species composition and environmental conditions in 163 100 m(2) plots. We use unconstrained and constrained ordinations, variation partitioning, and Indicator Species Analysis to provide a quantitative assessment of the importance of the forest overstorey in controlling understorey species composition. Comparison of the gradients extracted by unconstrained and constrained ordinations showed that the main gradients in understorey species composition in our old-growth temperate broadleaved forest remnant are not caused by variability in the forest overstorey, but are related to topography and soil, edge effects, and unknown broad-scale factors. Nevertheless, overstorey-related variables uniquely accounted for 15% of the total explained variation in understorey species composition, with the pure overstorey-related (R-po), topography and soil (R-pt), edge and anthropogenic disturbance effects (R-pa), and spatial (R-ps) variation fractions being of equal magnitude. The forward variable selection showed that among the overstorey-related variables understorey light availability and to a lesser extent vertical forest structure were most important for understorey species composition. No unique influence of overstorey tree species identity could be documented. There were many indicator species for high understorey light levels and canopy gap centres, but none for medium or low light or closed canopy. Hence, no understorey species behaved as obligate shade plants. Our study shows that, the forest overstorey has a weak control of understorey species composition in near-natural broadleaved forest, in contrast to results from natural and managed forests comprising both conifer and broadleaved species. Nevertheless, > 20% of the understorey species found were indicators of high light conditions or canopy openings. Hence, variability in canopy structure and understorey light availability is important for maintaining understorey species diversity.