The influence of landscape-level heterogeneity in fire frequency on canopy composition in the boreal forest of eastern Canada

Question: Will a three-fold variation in fire frequency among large patches (approximate to 10(5)-10(6) ha) of boreal forest generate differences in canopy composition and, more specifically, will it influence the relative abundance of species with regard to their typical position along the succession gradient. Location: A landscape of 1.6 Mha in the boreal forest of eastern Canada ( Quebec). Methods: We sampled 160 circular plots in closed-canopy forest in which we measured canopy vegetation composition, local fire history ( time since last fire) and edaphic conditions. We conducted multivariate analyses ( NMDS, multiresponse permutation procedure) and pair-wise comparisons to probe differences in canopy composition between areas of contrasting fire frequency before and after controlling for the influence of local environmental factors. Results: There are significant differences between areas of contrasting fire frequency in terms of relative species abundance, even after analytically removing the effect of important local environmental factors. In old stands, Picea mariana is significantly more abundant in high fire frequency areas while Abies balsamea is significantly more abundant in low fire frequency areas, both before and after controlling for local environmental factors, including time since last fire. Young stands do not differ in terms of individual species relative abundance but show more variability among stands in low fire frequency areas. Main conclusion: The low fire frequency areas allow late-successional specialist A. balsamea to dominate over ubiquitous successional generalist P. mariana because of the typically longer time elapsed since the last fire. This suggests that succession from P. mariana to A. balsamea can occur long after what is typically covered by dendroecologically reconstructed fire history in this type of boreal landscape (> 200-300 yr).