Population Biology of the Clonal Moss Hylocomium splendens in Norwegian Boreal Spruce Forests. 4. Effects of Experimental Fine-Scale Disturbance

We studied demographic effects of density reduction by fine-scale disturbance on the perennial clonal moss species Hylocomium splendens in a field experiment in an SE Norwegian boreal forest. Half the bryophyte cover was removed from 10 plots in a fine-scaled pattern. Size of ca 800 mature segments (estimated from morphological field measurements) and demographic changes in this experimental population and an unmanipulated reference population were followed from 1993 to 1995. The mean size of mature segments decreased in both populations during the study, due to an extended drought period in 1994. The number of mature segments in the unmanipulated population remained almost unchanged. Branching rates and population growth rates were significantly higher in the experimental population. Rates of ramification were higher than in the unmanipulated population the first year after treatment, and rates of regeneration from fragments (in the gaps) and intact shoot chains were higher both in the first and second year after treatment. We consider enhanced light penetration deeper into the moss carpet as the main cause of the observed differences between the populations. The number of Hylocomium splendens growing points in the experimental population reached 68% of the pre-treatment number in two years. Mean segment size decreased more strongly in the experimental than in the unmanipulated population. Further size reductions are expected, because segments produced by regeneration are smaller than a population's average segment size. Our study demonstrates that the resilience of a population after disturbance depends on the population's current size structure, the frequency and severity of disturbance, and environmental conditions. Experimental disturbance also influenced sporophyte production; in 1995 the frequency of sporophytes was more than ten times as high in the experimental population than in the unmanipulated reference population. Stimulation of fertile branch production (and gametangia) by enhanced radiation, and higher probability of successful fertilization, e.g., due to amelioration of the microclimate, may be involved. Like ramification frequency, sporophyte production in Hylocomium splendens is positively dependent on the size of the source segment, and a well-defined reproductive threshold, a minimum resource (size) level for successful reproduction, seems to occur.