Ecological behaviour of deciduous hardwood trees in Boreo-nemoral Sweden in relation to light and soil conditions

The study is concerned with the ecological responses of deciduous hardwood trees to light, moisture, pH and nitrogen, in forests located in the Boreo-nemoral zone of Sweden. Analyses were carried out for three data sets comprising releves from different areas: (1) Oland, (2) Uppland/Sodermanland, and (3) Halland/Bohuslan/Vastergotland. The values of environmental factors in the releves were assessed by calculating weighted averages of Ellenberg's species' indicator values. For each factor, the tree species' responses were determined by fitting regression curves to the log-transformed cover-abundance values of the tree species at the sites, as a function of the weighted averages at the corresponding sites. Regressions were carried out separately for the tree and field layers. In the majority of cases, significant linear or quadratic response curves were obtained. Fraxinus excelsior and Ulmus glabra represent two species with similar ecological behaviour. Both are very demanding with respect to soil moisture, pH and nitrogen, and in the field layer, both are highly tolerant of shade. Regarding the edaphic factors, Acer platanoides has its optima at lower values than Fraxinus and Ulmus, but is at least as shade-tolerant as these two species. Tilia cordata generally holds an intermediate position along all gradients in question. Quercus robur and Quercus petraea have their optima at the comparatively lowest values for the edaphic factors, but are the most light-demanding among the deciduous hardwood trees. The results are also discussed in view of the ecological literature from central and western Europe. Some species show geographically differing behaviour with respect to their responses to various environmental factors.