The Effect of Density on Size Frequency Distributions in Chalk Grassland Bryophyte Populations

In a field experiment, shoot densities of three pleurocarpous chalk grassland bryophytes, Calliergonella cuspidata, Ctenidium molluscum and Rhytidiadelphus squarrosus were reduced by 50%. The size distribution of shoots was analysed 6.5 months later and compared with control plots. Five target shoots were added to each plot and their growth over the study period was measured. The inequality of the shoot sizes was not higher at the end of the growing season than at the start, which is explained by the minor role of asymmetrical competition for light in these bryophyte stands. Inequalities increased after thinning, mainly due to new recruitment. Many of the thinned plots returned fairly rapidly to the original natural densities, suggesting that shoot densities are regulated by an intrinsic mechanism.