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 dist urbance 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 held measurements) a nd demographic changes in this experimental population and an unmanipu lated reference population were followed from 1993 to 1995. The mean s ize of mature segments decreased in both populations during the study, due to an extended drought period in 1994. The number of mature segme nts in the unmanipulated population remained almost unchanged. Branchi ng 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 wer e higher both in the first and second year after treatment. We conside r enhanced light penetration deeper into the moss carpet as the main c ause of the observed differences between the populations. The number o f Hylocomium splendens growing points in the experimental population r eached 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 pr oduced by regeneration are smaller than a population's average segment size. Our study demonstrates that the resilience of a population afte r disturbance depends on the population's current size structure, the frequency and severity of disturbance, and environmental conditions. E xperimental disturbance also influenced sporophyte production; in 1995 the frequency of sporophytes was more than ten times as high in the e xperimental 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 ramific ation frequency, sporophyte production in Hylocomium splendens is posi tively dependent on the size of the source segment, and a well-defined reproductive threshold, a minimum resource (size) level for successfu l reproduction, seems to occur.