Stand structural dynamics on drained peatlands dominated by Scots pine

We investigated the structural dynamics of naturally established stands dominated by Scots pine (Pinus sylvestris L.) on peatland sites drained for forestry. The study was based on 3-10 repeated measurements of the distributions of tree diameters at breast height (DBH) of 83 stands located in southern and central Finland. Thinned as well as unthinned stands were included. Site nutrient levels ranged from poor (ombrotrophic) to moderately rich (mesotrophic). The Weibull function was used to parameterize the DBH distributions, and mixed linear models were constructed to characterize the impacts of different ecological factors and management on stand dynamics as described by the variation of the shape parameter of the Weibull function. The analyses were carried out separately for the dominant canopy layer and the under-Storey. The structural unevenness of the stands, indicated by positive skewness of the DBH distributions. initially increased after drainage. Ingrowth of new trees was enhanced more on sparsely forested composite peatland sites (Group II sites) than on more fully stocked sites (Group I sites). Structural heterogeneity started to decrease 20-30 years after drainage, depending on the site type. DBH distributions continued to develop towards normality and then to negative skewness. This trend reflected inherent stand dynamics mediated by increased inter-tree competition. which resulted in mortality of small trees. Thinnings hastened the change in DBH distributions. Highly uneven-sized understories of Norway spruce (Picea abies L. (Karst.)) and pubescent birch (Betula pubescens Ehrh.) became established beneath the dominant canopy layer of Group B sites. (C) 2004 Elsevier B.V. All rights reserved.