Neighborhood interactions influencing tree population dynamics in nonpyrogenous boreal forest in northern Finland

Open-canopy moss-rich woodlands dominated by Picea abies and Betula pubescens in northern Finland may undergo cyclic development with reciprocal replacement of the tree species due to the positive feedbacks on soil conditions. Although relations to the abiotic environment are well understood, intra- and interspecific interactions during development of sparse boreal forests have received less attention. We studied tree regeneration, growth and survival with respect to size and density of neighboring trees in four stands representing roughly four stages of the Picea-Betula forest cycle. We conducted spatial analysis (Ripley's K-function) of mapped locations of live and dead stems to reconstruct the distribution of stems prior to mortality, and to infer possible causes of tree population decline. The prevalence of standing dead stems enabled us to test if mortality was associated with density and size of neighboring trees. Logistic regression was used to test for relationships between tree survival and local crowding indices. We also examined spatial autocorrelations of individual size characteristics to determine the mode and spatial extent of tree interactions. Picea abies had reduced recruitment in open areas occupied by mosses and dwarf-shrubs, and preferentially regenerated near B. pubescens, whereas B. pubescens formed small clumps (and occasionally these consisted entirely of stems from a single tree) that showed local repulsion from large P. abies trees. Size of neighboring trees was the primary determinant of individual growth and survival, whereas neighborhood density per se had only a weak effect. Picea abies had negatively correlated sizes among close neighbors (0-4 m radius) indicating that dominant trees suppress their smaller neighbors. Negative autocorrelations prevailed at the transition stages where the patches of smaller trees were concentrated around evenly spaced large trees. Tree sizes became spatially independent at the mature phase. We hypothesize that both low light and soil nutrient availability causes the P. abies population decline. Dominant trees in this high latitude forest have large light depletion zones and shallow root system to promote strong above- and below-ground competition with younger trees. Higher mortality rates within canopy patches were not compensated for by recruitment in gaps, causing P. abies population to decline steadily.