The impact of watering regime and ambient relative humidity on the effect of density on growth in two boreal forest mosses, Dicranum majus and Rhytidiadelphus loreus

The impact of watering regime, ambient relative humidity and shoot density on production of offshoots, biomass increase and elongation growth in two b oreal forest mosses, Dicranum majus and Rhytidiadelphus loreus, was studied under controlled conditions. Experimental colonies of the mosses at three shoot densities were subjected to humidity conditions differing with respec t to desiccation rate, desiccation severity and length of the moist period, by varying the ambient relative humidity (RH), and the frequency and conti nuity of days when the shoots were watered. Overall, increased density had a negative effect on relative growth rate (RGR) and green biomass in both s pecies. However, under several watering regimes, optima for both RGR and gr een biomass were found at intermediate densities, especially at low RH. Pro duction of offshoots decreased with density in D. majus. Length growth rate (LGR) increased with RH in both species, and LGR also increased with densi ty in D. majus. The differences in response to watering regimes were large, but few consistent patterns emerged with respect to the impact of the leng th of wet and dry periods, watering frequency, and the desiccation rate and intensity. Nevertheless, RGR increased with the length of the wet-dry cycl e when the total number of wet and dry days remained equal.