Three large fire years threaten resilience of closed crown black spruce forests in eastern Canada

Abstract

An emerging paradigm regarding vegetation response to climate warming is that the interaction of weather extremes and disturbance will trigger abrupt changes in ecosystem types by overcoming resilience of dominant species. Black spruce (Picea mariana (Mill.)) ecosystems are widespread across the North American boreal forest, because of ecophysiological adaptations that allowed these communities to thrive in fire‐prone areas. We investigated resilience of spruce‐moss forests to weather‐disturbance interaction after a 3‐year period (2005 to 2007) of major fire activity caused by extreme fire weather in eastern Canada. Pre‐ and post‐fire conifer densities and environmental parameters related to seed rain, post‐fire seedbeds, microclimate, and post‐fire weather were measured in 133 burned stands throughout the closed‐crown forest of Quebec. Critically low black spruce (BS) regeneration was observed in almost all of the stands, leading to a decrease in stand density and a shift of species dominance from BS to jack pine (Pinus banksiana (Lamb.), JP). The studied sites were characterized by thick residual organic matter, resulting in a predominance of charred duff, a seedbed associated with low water retention and high variation in temperature. While high levels of JP seedling establishment were reported on this seedbed, it was unfavorable to BS germination and survival in the context of warm and dry weather that prevailed in post‐fire summers. In these ecosystems, early vegetation establishment patterns are generally reliable predictors of future stand composition and the exclusion of BS will presumably be maintained through succession. During large fire years, high proportions of the landscape are subjected to the interaction of fire regime and weather that creates unsuitable conditions for BS regeneration. Hence, vegetation change is susceptible to happen at a broad scale. Therefore, the frequency of major fire years could have a decisive influence on the rate of vegetation response to climate change in this biome.