Effects of light and nitrogen on morphological plasticity of the moss Calliergonella cuspidata

In a short-term experiment we studied the effects of increased light and nitrogen (N) availability on the moss Calliergonella cuspidata by cutting vascular plants and adding nitrogen in a calcareous fen in the mountainous region of central Switzerland. Shoots of Calliergonella growing in plots with reduced vascular plant competition showed smaller increments in length of the main axis and produced shorter offshoots, but showed higher branching densities at the main axis, higher number of offshoots and higher dry mass per unit length. Although the growth forms of pleurocarpous bryophytes are quite different from stolonand ramet-producing clonal vascular plants, the overall morphological responses to changing light levels are very similar and, at least in part, comparable to the ‘spacer and branching’ strategy exhibited by some stolon-bearing vascular plants. The observed morphological plasticity of Calliergonella would in principle allow for foraging of heterogeneously distributed resources. Nitrogen supply, on the other hand, had no effects on either dry mass or shoot morphology of Calliergonella. Moreover, interactions between the effects of removal of vascular plants and increased nutrient supply on biomass and shoot morphology of Calliergonella did not occur. This could be explained by the short duration of the experiment. However, since above-ground biomass of vascular plants was increased by the nutrient supply, it cannot be excluded that in the long run, increased N inputs will have serious secondary consequences on Calliergonella, e.g. due to heavy shading by vascular plants.