Hedging at the edge: intraspecific trait variability promotes long term resilience of marginal populations at the rear edge of a widespread boreal tree species

Background/Question/Methods

Marginal populations might be disproportionately important for the long-term resilience and eco-evolutionary dynamics of widespread species in the face of global changes. Rear-edge populations located at the warm margin of species distribution are small, isolated and face environmental conditions currently at the limit of species bioclimatic envelope but which could become more common with ongoing global change. Intraspecific phenotypic variation contributing to the persistence of peripheral populations is expected to become increasingly important for species resilience where range shifts lag behind climate change velocity. We investigated the putative role of intraspecific phenotypic variation in serotiny, a key adaptive trait, for the long-term maintenance of rear-edge populations of fire-prone jack pine (Pinus banksiana), an obligate pyriscent boreal species. First, we inferred Holocene history of rear-edge jack pine stands based on macrofossil analysis of soil charcoal particles. This allowed us to evaluate the long-term persistence of these marginal stands. Second, we conducted a field study to assess whether variation in cone serotiny is associated with the demographical trajectory of rear-edge stands. Third, we experimentally submitted closed cone to incrementing temperature treatments followed by seed germination tests to investigate ecophysiological determinants of cone dehiscence and seed viability in rear-edge jack pine populations.

Results/Conclusions

Rear-edge jack pine stands in eastern Canada persist since >7000 years despite an infrequent and unpredictable fire environment. The average fire-return interval is 1050 years (stand-scale estimates range: 450-3750 years), which is three times the value reported from range-core stands within the boreal forest (300 years). In rear-edge populations, cone serotiny is lower and more variable compared to range-core populations. Rear-edge stands are older and display reverse J-shape tree size structure indicative of old-growth demographic equilibrium, whereas range-core stands are younger and show a unimodal stand structure depicting a single aging cohort lacking interfire recruitment. Cones from marginal populations dehisce at a lower opening temperature, which increases with cone age. Cones from range-core stands open at a more constant, yet higher temperature. At the rear edge, seed viability remain invariable with cone age (50%), whereas it decreases from 70% to 30% in 20 years at the range core. Unlike semelparous range-core populations, rear-edge populations exhibit a dual life-history strategy that ensures both steady recruitment during fire-free intervals (iteroparity) and successful post-fire regeneration. This capacity to cope with infrequent and unpredictable fire regime should increase the resilience of jack pine populations as global changes alter fire dynamics of the boreal forest.