Intraspecific trait variation as a mechanism of coexistence of congeneric blueberry (Vaccinium) species after forest harvesting

Abstract

Understorey shrubs of boreal forests grow vigorously after canopy removing disturbance such as clearcutting and fire and dominate the post-disturbance community. Many of these dominant shrubs are morphologically and phylogenetically closely related. How do they coexist despite having similar traits and resource needs? One way the coexisting plants may avoid competition is by niche differentiation that may alleviate the negative effects of competition due to high niche overlap. We studied two blueberry species, Vaccinium angustifolium and V. myrtilloides often found in proximity to one another after forest harvesting to test the hypothesis that coexisting congeneric species differentiate their resource acquisition and use through variation in their functional traits (niche differentiation). Discovering the coexistence mechanisms of Vaccinium spp. that flourish after forest harvesting and fire is important from both theoretical and applied perspectives. Vaccinium spp. are dominant shrubs after forest harvesting and fire and constitute a significant food source for wildlife and health-conscious consumers and are also of cultural significance to indigenous peoples. From three post-harvest forest sites in northwestern Ontario, we measured cover of Vaccinium spp. in 81 1 × 1 m quadrats where they occurred alone and together. We then excavated 103 ramets of the two Vaccinium spp. occurring alone and together after measuring three habitat variables (organic matter depth, soil moisture and soil temperature) in the immediate surroundings of the excavated plants. We measured five functional traits: height, canopy diameter, specific leaf area, specific stem density and specific root length. Using these data, we calculated functional hypervolumes (niche space occupancy) of each species occurring alone and together. We also calculated probabilistic niche overlap between the two species as well as their reaction norms to habitat variables. We found that species hypervolumes changed in size, and the probability and degree of overlap between the two Vaccinium spp. changed when occurring alone and co-occurring. Linear regression showed no significant effect of habitat factors on traits. We detected high intraspecific variation in some traits leading to shifts in hypervolumes when they co-occurred. We conclude that at fine spatial scales (homogeneous habitats, open canopy condition) individual trait variation allows for niche complementarity promoting coexistence among these important, early succession, closely related species.