Neighborhood dissimilarity consistently attenuates competition stress on tree growth under altered water availability in a natural boreal forest

Abstract

Tree growth regulates forest productivity and carbon sequestration. Although there has been evidence showing positive effects of neighborhood diversity on tree growth, few have experimentally demonstrated how tree growth may respond to neighborhood diversity in natural forests under altered water availability, which is anticipated to occur more frequently with ongoing global climate change. We examined three-year individual tree basal area growth under the water alteration treatments of reduced (-25% throughfall during the growing season), ambient and added (+25% throughfall during the growing season) rainfall in the broadleaf stands, conifer stands and their mixtures of a young natural boreal forest. We quantified neighborhood drought tolerance dissimilarity, phylogeny dissimilarity and neighborhood crowding based on individual tree sizes and the distance between them. We found that the tree growth on average did not change significantly with water alteration but decreased with neighborhood crowding, with the negative crowding effect being more pronounced in water reduction treatment than in ambient and water addition treatments. Higher neighborhood dissimilarity of drought tolerance and phylogeny promoted tree growth consistently across all water conditions. Moreover, the positive effects of neighborhood dissimilarity of drought tolerance and phylogeny became more pronounced with increasing neighborhood crowding. Our results suggest that promoting species mixtures in boreal forests may sustain higher boreal forest productivity under changing water availability.