TESTING FOR POSITIVE DENSITY-DEPENDENT PERFORMANCE IN FOUR BRYOPHYTE SPECIES

Abstract

The impact of environmental conditions on the relative importance of competitive and facilitative interactions and their responses to shoot density was studied in four boreal forest bryophytes. Increased density is believed to have both a positive effect, by increasing water retention, and a negative effect, by reducing light availability. Mortality of shoot apices, offshoot production, and rates of biomass increase of both individual shoots and whole colonies were recorded in experimental monospecific, known-density colonies of Dicranum majus, Plagiochila asplenioides, Ptilium crista-castrensis, and Rhytidiadelphus loreus under controlled relative humidity (four levels) and irradiance (two levels). A phenomenological model was developed, and predictions were made about the responses to increased density under different humidity and resource (light) levels. Mortality was low, but offshoot production generally decreased with density. The effects of density on growth rates varied among species and environmental conditions. As predicted, in dry environments under the high light level, growth rates of D. majus and R. loreus peaked at an intermediate density at which the positive effects of a close packing of shoots balanced the increased competition for light. In humid or dark environments, the relationships between growth rates and density were mostly monotonic and negative. Growth rates of P. crista-castrensis decreased with density under most environmental conditions, whereas density was not shown to have an effect on growth rates of P. asplenioides. The results show that relatively high light levels, compared to those found in their natural spruce forest habitat, are necessary for aggregation to facilitate growth of these bryophytes. Thus, our results indicate that competition for light, rather than a low water availability, limits the performance of individual shoots in colonies of these bryophytes under a wide range of densities and humidity conditions.