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Factors controlling the regeneration process in unevenly aged silver fir forests: inferences from the spatial pattern of trees

The main objective of this study was to prove the relationship between the spatial pattern of trees and distribution of advanced regeneration in structured Abies alba Mill. forests in the Western Carpathians (of southern Poland). In 13 sample plots (0.45-1.00 ha in size), the locations of all live trees above 15 cm in d 1.3 and all stumps of canopy trees (above 50 cm in d 1.3 ) were determined. In addition, in all plots the coordinates of all live and dead pole- stage trees (5-15 cm in d 1.3 ), in seven plots the coordinates of all live and dead younger saplings (0.5-1.3 m in height), and in eight plots the coordinates of all live and dead older saplings (above 1.3 m in height and below 5 cm in d 1.3 ) were registered. The univariate and bivariate Ripley's K-function was applied to test spatial independence of the patterns of trees representing different size categories. In each of the stands, younger and older saplings and pole-stage trees showed an aggregated pattern. The strongest attraction between trees of these size categories was detected at the scale 6, 5 and 10 m, respectively. At the scale below 10 m, a positive spatial correlation was frequently found between large firs and pole-stage trees and saplings. A spatial repulsion between advanced fir regeneration and trees of the middle stand layer was negligible or observable only in a relatively large scale above 15 m. The patterns of saplings and trees of the lower stand layer appeared to be spatially independent in most of the stands being studied. These results suggest that in unevenly aged fir forests, a stable mechanism is generating the aggregated patterns of regeneration functions and the factors involved are spatially linked with the distribution of large firs. Therefore, a hypothesis is advanced that the spatial pattern of regeneration in such forests is controlled by edaphic factors - in particular, the humus form - and that light conditions play only a secondary role.