The influence of the spatial pattern of trees on forest floor vegetation and silver fir (Abies alba Mill.) regeneration in uneven-aged forests

In the Western Carpathians (of southern Poland), in three representative multilayered fir stands, all trees above 6.9 cm in d1.3 were mapped. Subsequently, forest floor vegetation cover, vascular plant species, and fir seedlings (below 0.5 m in height) were systematically sampled on small circular plots in a 5 m × 5 m grid. On the basis of location in the stand and indicator values of plant species registered within the plot, an index of influence and a mean indicator value of soil moisture, soil trophy, and soil pH was assigned to every plot. The mean values of these variables were compared in plots with abundant and scarce fir regeneration. Spatial correlation methods were used to test the relationships between the tree pattern and fir seedling density, the vegetation cover, and the species composition of the forest floor. In the studied stands, despite relatively homogeneous spatial structure, both species composition of the forest floor vegetation and vegetation cover, as well as fir seedling density, exhibited a patchy spatial variation. Patches with a higher proportion of larger trees were characterised by less vegetation cover, more frequent occurrence of plant species associated with mesic sites, and more abundant fir regeneration. In gaps, canopy openings, and fragments with relatively small trees, fir regeneration was frequently absent. Occurrence of extremely dense forest patches, creating very unfavourable niches for plants in general, and competition of the forest floor vegetation could be acknowledged as the prime reasons for the lack of fir regeneration only in 30–50% of the area of the stands. It is suggested that adequately explaining the spatial variation of regeneration occurrence would require the inclusion of a complex of factors that potentially limit the survival of juvenile seedlings in fertile microhabitats. If occurrence of fir regeneration on fertile sites is controlled in larger measure by factors associated with edaphic variability than by light conditions, then cuttings aimed at prompting regeneration may be successful only where a regeneration bank has earlier been established. Otherwise, such removal of trees may cause an increase in percent of coverage by forest floor vegetation and, through unfavourable eutrophisation of initially acidified microhabitats, additionally hinder the establishment of fir seedlings.