Testing for positive density-dependent performance in four bryophyte species

The impact of environmental conditions on the relative importance of compet itive and facilitative interactions and their responses to shoot density wa s studied in four boreal forest bryophytes. Increased density is believed t o have both a positive effect, by increasing water retention, and a negativ e effect, by reducing light availability. Mortality of shoot apices, offsho ot production, and rates of biomass increase of both individual shoots and whole colonies were recorded in experimental monospecific, known-density co lonies of Dicranum majus, Plagiochila asplenioides, Ptilium crista-castrens is, and Rhyridindelphus loreus under controlled relative humidity (four lev els) and irradiance (two levels). A phenomenological model was developed, a nd predictions were made about the responses to increased density under dif ferent humidity and resource (light) levels. Mortality was low, but offshoo t production generally decreased with density. The effects of density on gr owth rates varied among species and environmental conditions. As predicted, in dry environments under the high light level, growth rates of D. majus a nd R. loreus peaked at an intermediate density at which the positive effect s 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-cast rensis 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 foun d in their natural spruce forest habitat, are necessary for aggregation to facilitate growth of these bryophytes. Thus, our results indicate that comp etition for light, rather than a low water availability, limits the perform ance of individual shoots in colonies of these bryophytes under a wide rang e of densities and humidity conditions.