Reseating of ecological gradients. II. The effect of scale on symmetry of species response curves

Abstract

Response curves are presented for 15 species of vascular plants, bryophytes, and lichens relative to the water‐table gradient in a boreal Norwegian mire. The gradient is scaled in two ways; position of median water‐table in cm below the surface of the bottom layer, and in units of compositional turnover (rescaled by DCA). The two scalings produce quite different curves above the maximum water‐level, which corresponds to median water‐level 10–14 cm below the surface of the bottom layer. The significant drop in compositional turnover above this level can be ascribed to vertical differences in water‐table having a lesser effect on plants in aerated hummocks than on plants in temporarily water‐logged hollows. Skewness of response curves was reduced by gradient rescaling, most strongly after removal of influence of other gradients. It is argued that response curves will generally be Gaussian if: (1) the response is with respect to a dominant ecological factor, (2) the range of variation along the gradient is sufficient, (3) the distribution of samples is adequate, and (4), the gradient is scaled in units of compositional turnover.