Hedging at the rear edge: Intraspecific trait variability drives the trajectory of marginal populations in a widespread boreal tree species

Rear-edge populations at the warm margin of species distribution are small, isolated and face environmental conditions at the limit of species bioclimatic envelope. Intraspecific phenotypic variation contributing to the persistence of peripheral populations is expected to become increasingly important under future climate conditions to avoid local extirpation where range shifts lag behind climate change velocity. We investigated the putative role of intraspecific phenotypic variation for the maintenance of rear-edge populations of fire-prone jack pine (Pinus banksiana), an obligate pyriscent boreal species. We assessed whether variation in cone serotiny is associated with the population trajectory of marginal stands located south of the boreal biome, in the temperate forest where natural wildfires are infrequent and unpredictable. To this end, we estimated stand-scale serotiny, minimal age and tree size structure in 26 jack pine stands from the rear edge (n = 17 sites) and the core (n = 9 sites) of the species' range in eastern Canada. On average, rear-edge jack pine populations are less serotinous albeit more variably compared with range-core populations where serotiny is more uniformly high. Rear-edge stands are generally older and display reverse J-shape tree size structure indicative of a multi-aged demographic equilibrium, whereas range-core stands are younger and show a unimodal stand structure depicting a single ageing cohort generally lacking interfire recruitment. Eco-evolutionary dynamics shifts from a dependency on wildfires in range-core populations to stands that can regenerate and persist without recurrent fires at the rear edge, where stand-scale serotiny reaches values below 85%. Synthesis. Unlike range-core populations, rear-edge jack pine populations can locally rely on a dual life-history strategy to ensure both steady recruitment during fire-free intervals and successful postfire regeneration. This capacity to cope with infrequent and unpredictable fire regime should increase the resilience and resistance of jack pine populations as global changes alter fire dynamics of the boreal forest. More generally, unique intraspecific phenotypic variation in rear-edge populations contributes to long-term species persistence in marginal environmental conditions that might scale up with global changes. The conservation of rear-edge populations and their genetic legacy appears crucial for the resilience of species.

Automated summary

Rear-edge populations at the warm margin of species distribution are expected to rely more on intraspecific phenotypic variation for their persistence. In this study, the role of variation in cone serotiny for the maintenance of rear-edge populations of fire-prone jack pine was investigated. The findings suggest that rear-edge populations can rely on a dual life-history strategy to ensure survival and regeneration in the absence of frequent wildfires.