Shifts in ecological behaviour of herbaceous forest species along a transect from northern Central to North Europe

We investigated the ecological behaviour (the response to environmental fac tors in the field, synonymous to the term realized niche) of four closely r elated species pairs (Melica nutans, M. uniflora; Primula veris, P. elatior ; Veronica chamaedrys, V. montana; Viola riviniana, V. reichenbachiana) acr oss a transect from northern Central to North Europe. The second-mentioned species of each pair is confined in its geographical distribution to the so uthern parts of the studied transect. Sample plot data of deciduous forests were compiled from(1) Germany, S Niedersachsen, (2) Germany, northern Schl eswig-Holstein, (3) Denmark, and (4) Boreo-nemoral Sweden. We compared the ecological optima and amplitudes of the response curves of species along th e gradients for moisture, pH and nitrogen by means of phytosociological dat a, detrended correspondence analysis (DCA) and Ellenberg indicator values. pH measurements from Sweden were significantly correlated with the correspo nding DCA sample plot scores and the plot averages of the Ellenberg values for reaction (pH). In accordance with our main hypothesis, the wide range species appeared to have broader ecological amplitudes on the northern margins of their distrib utional ranges, especially in Boreo-nemoral Sweden, than in the southern pa rts of the study area. Our findings are in contrast to theories claiming a reduced niche breadth of range-margin populations of species compared to ra nge-centre populations. The shifts in ecological behaviour were particularl y obvious with respect to soil acidity. We believe that these shifts are ca used by changes in the competitive relationships between the species: in th e north, the total pool of species in deciduous forests is comparatively sm all. The low species richness is likely to lead to reduced competition and to an expansion of the ecological amplitude, known as competitive release.