Competition and facilitation co-regulate the spatial patterns of boreal tree species in Kanas of Xinjiang, northwest China

Competition and facilitation are the primary mechanisms structuring communities and driving spatial distribution patterns of trees. However, we still know relatively little about the underlying effects of these two processes on natural forest ecosystems and this is particularly true in boreal forest ecosystems. Our lack of understanding regarding the factors regulating spatial patterns of boreal forest ecosystems is important given their role in carbon sequestration and timber products. Using data from a 12-ha boreal forest dynamic plot in the Kanas Nature Reserve of Northwest China, we assessed the population distribution and diameter at breast height (DBH) structure, as well as intra- and interspecific interactions, of the four most abundant tree species (Larbc sibirica, Picea obovata, Pinus sibirica, and Betula pendula). Our results indicate that the spatial distribution patterns of these species were aggregated at small spatial scales because of dispersal limitation, and that the degree of aggregation decreased with increasing spatial scale. Aggregation was the highest for the less common species (i.e., B. pendula pendula). Saplings (1 cm <= DBH < 5 cm) and juveniles (5 cm <= DBH < 15 cm) were more aggregated than larger individuals (DBH >= 15 cm). Only P. obovata adults (DBH >= 15 cm) showed a regular (i.e., hyperdispersed) distribution because of the intense intraspecific competition at small scales (i.e., < 5 m). Intraspecific interactions between individuals were either positive or not significant. In terms of interspecific competition, there were significant negative associations among large (DBH >= 15 cm) and small adults (5 cm <= DBH < 15 cm) for the two shade tolerant species (P. obovata and P. sibirica). In contrast, we observed positive interspecific interactions between individuals of different DBH classes and different shade tolerance species. Thus, most of the intra- and interspecific interactions showed positive or no significant correlations. Negative associations were generally less common than positive associations both within and between species. Our results show that competition and facilitation simultaneously influence the spatial patterns of plants in this community. Moreover, positive interactions between tree individuals in this temperate region are likely to be more common than competitions for survival and growth of tree stems.