Spatial patterns of trees of different size in boreal <i>Pinus sylvestris </i>forest sites with different fire histories

The spatial distribution of living trees of different size-classes was examined on two sites with different fire histories in a mature <i>Pinus sylvestris </i>L. dominated forest in eastern Finland. The study area was a 4-ha square plot, which was divided into two parts based on fire history analysis. In one part, the last fire event was a stand-replacing fire in the early 19^thcentury, after which the whole stand regenerated, while the other part of the study plot was subsequently burnt by a surface fire in 1906. For the analyses, the trees were divided into five height-classes, reflecting competitive hierarchy. Spatial patterns of different tree height-classes were analysed using formulae for spatial autocorrelation with categorical data. In both areas, saplings showed an aggregated and overstorey trees a random spatial pattern at all scales studied. The other tree height-classes showed aggregated or random spatial patterns depending on the area and distance class. Generally, similar tree height-classes tended to be randomly or positively associated (attraction), except for the co-dominant and dominant, and overstorey trees, which were negatively associated (repulsion). The results suggest that the three dimensional spatial organization of the tree population was likely driven by two main interacting processes: (1) Low severity disturbances such as surface fires and treefalls, which create favourable regeneration patches, leading to positive association in smaller tree height-classes and (2) strong neighborhood competition among the largest trees and the competitive effect of these large trees on the small understorey trees, leading to negative association among these components of the tree population. The results also indicate that canopy replacement by trees currently in the understorey is unlikely to occur without a sufficient opening in the forest.