Mixture of Norway spruce and other species modify its negative intraspecific interactions in Bialowieza Primeval Forest, Poland

The present spatial structures of old-growth forest stands at fine- and landscape scales mainly result from competitive or facilitative interactions among conspecifics and heterospecifics during succession. Profound influence of these important ecological processes may be characterized by analyzing spatial distribution of individuals at fine scale [1, 2]. Bialowieza Primeval Forest (BPF) is one of the best-preserved forests in the lowland part of Europe which represents an important remnant of temperate broad-leaved forests that once covered most of this continent. Therefore, spatial pattern analysis of focal species in BPF as one of the UNESCO World Heritage Sites may provide significant insights into mechanisms and processes that maintain species associations in BPF. Main goals of this study were to assess fine-scale spatial patterns of Norway spruce mixed stands and explore how inter- and intraspecific interactions of spruce and other species shape their spatial structure. To accomplish these goals, we investigated the spatial patterns of spruce and its accompanying species within 117 plots that had been established in mixed stands of spruce in BFP. The spatial patterns of trees, considering species and biophysical characteristics, were analyzed by univariate and bivariate pair correlation function g(r), density correlation function CmK(r), and mark correlation function kmm(r) [3, 4]. In mixed spruce stands, the upper layer was dominated by Scots pine, alder, and hornbeam with a share of more than 31.5% of all species; while spruce was the suppressed species as the most abundant species (over 55.1%) (Fig. 1a). The unimodal shapes of diameter distributions of spruce and other species (117 plots, total area 5.85 ha) with skewness to the right underline the uneven-aged structure of the studied forest (Fig. 1b). Although the spatial distribution of other species was more clumped than spruce at small spatial scales, the spruce-other species interactions indicated lower interspecific facilitation (Fig. 2). Moreover, the results showed that diameter at breast height DBH, of spruce individuals that had conspecifics at neighborhood was significantly less than DBH of spruce that had individuals of heterospecific species. In addition, we found that density of neighboring individuals of heterospecifics had negative effects on the DBH of spruce (Fig. 3). We conclude that primeval mixed stands of spruce show aggregated spatial distribution at small spatial scales because of high shade tolerance of spruce. We also found that the aggregation of individuals of spruce around their conspecifics negatively influences the DBH, while this negative effect becomes less when other species are located within neighborhood of spruce.