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

Abstract

1. 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.2. 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.3. 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 rangecore 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%. 4.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

Jack pine of all trades: Deciphering intraspecific variability of a key adaptive trait at the rear edge of a widespread fire‐embracing North American conifer

“…In this area, most sites are located on rock outcrops within the northern temperate forest, mainly composed of sugar maple ( Acer saccharum ), yellow birch ( Betula alleghaniensis ), and balsam fir. These peripheral rear‐edge stands are smaller, more isolated, and occupy less of the terrestrial landscape than those in the core of the range (Pelletier et al, 2022). According to the Canadian fire regime zonation system (Erni et al, 2020), the rear edge has a very low fire activity with virtually no natural wildfire (e.g., natural fire rotation period >1600 years in preindustrial forests of this area; Elzein et al, 2020), whereas range‐core sites are located in fire units prone to recurrent lightning‐ignited wildfires (Figure 1).…”

“…According to the Canadian fire regime zonation system (Erni et al, 2020), the rear edge has a very low fire activity with virtually no natural wildfire (e.g., natural fire rotation period >1600 years in preindustrial forests of this area; Elzein et al, 2020), whereas range‐core sites are located in fire units prone to recurrent lightning‐ignited wildfires (Figure 1). On average, serotiny is lower and more variable within rear‐edge populations compared to those from the core of the range (Pelletier et al, 2022).…”

“…For each tree, DBH was recorded, and a core was sampled using a Pressler increment borer as close to the ground as possible. Detailed methods used to estimate stand‐scale serotiny are reported elsewhere (Pelletier et al, 2022). Briefly, at each site, up to 100 mature jack pine individuals were assigned to one of six serotiny classes (based on the relative abundance of closed cones), and average class value was reported as a percentage reflecting apparent stand‐scale serotiny.…”

“…Indeed, reduced serotiny was reported in some jack pine stands where the frequency of lethal wildfire is low and unpredictable (Gauthier et al, 1993b, 1996), a noticeable trend at the rear‐edge of the species distribution (Radeloff et al, 2004). For instance, Pelletier et al (2022) reported lower yet more variable serotiny within rear‐edge jack pine populations from eastern Canada compared to range‐core populations. Furthermore, these uneven‐aged stands at the rear edge are in demographic equilibrium with steady regeneration, whereas younger range‐core stands are single cohorts lacking interfire recruitment.…”

“…Furthermore, these uneven‐aged stands at the rear edge are in demographic equilibrium with steady regeneration, whereas younger range‐core stands are single cohorts lacking interfire recruitment. Pelletier et al (2022) argued that, unlike range‐core jack pine populations, rear‐edge populations locally rely on a bet‐hedging evolutionary strategy to ensure both steady recruitment during long fire‐free intervals and eventual successful postfire regeneration. The variation in serotiny in rear‐edge populations could thus represent an adaptive solution to the infrequent and unpredictable fire regime.…”