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 factors in the field, synonymous to the term realized niche) of four closely related species pairs (Melica nutans, M. uniflora; Primula veris, P. elatior; Veronica chamaedrys, V. montana; Viola riviniana, V. reichenbachiana) across a transect from northern Central to North Europe. The second-mentioned species of each pair is confined in its geographical distribution to the southern parts of the studied transect. Sample plot data of deciduous forests were compiled from(1) Germany, S Niedersachsen, (2) Germany, northern Schleswig-Holstein, (3) Denmark, and (4) Boreo-nemoral Sweden. We compared the ecological optima and amplitudes of the response curves of species along the gradients for moisture, pH and nitrogen by means of phytosociological data, detrended correspondence analysis (DCA) and Ellenberg indicator values. pH measurements from Sweden were significantly correlated with the corresponding 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 distributional ranges, especially in Boreo-nemoral Sweden, than in the southern parts of the study area. Our findings are in contrast to theories claiming a reduced niche breadth of range-margin populations of species compared to range-centre populations. The shifts in ecological behaviour were particularly obvious with respect to soil acidity. We believe that these shifts are caused by changes in the competitive relationships between the species: in the north, the total pool of species in deciduous forests is comparatively small. The low species richness is likely to lead to reduced competition and to an expansion of the ecological amplitude, known as competitive release.