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

Abstract

Premise: Understanding mechanisms fostering long-term persistence of marginal populations should provide key insights about species resilience facing climate change. Cone serotiny is a key adaptive trait in Pinus banksiana (jack pine), which shows phenotypic variation according to the fire regime. Compared to range-core populations within the fire-prone boreal forest, low and variable serotiny in rear-edge populations suggest local adaptation to uncommon and unpredictable wildfire regime. We assessed environmental/physiological factors that might modulate intraspecific variation in cone serotiny. Methods: We experimentally subjected closed cones to incrementing temperatures, then tested seed germination to determine whether and how various ecological factors (cone age, branch height, tree size, tree age) are related to cone dehiscence and seed viability in jack pines from rear-edge and range-core populations in eastern Canada. Results: Cones from rear-edge 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. Cones from rear-edge stands take between 13 and 27 years to reach the level of serotiny achieved at the range core. At the rear edge, seed viability is steady (51%), whereas it decreases from 70% to 30% in 20 years at the range core. Conclusions: We inferred the mechanisms of a bet-hedging strategy in rear-edge populations, which ensures steady recruitment during fire-free intervals and successful postfire 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.

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

“…However, this issue is largely alleviated for jack pine, in which we found the inverse relationship (i.e. older cones require higher heat sum to dehisce with no loss of seed viability; Pelletier & de Lafontaine, in press). Although we acknowledge potential limits to the chosen approach, it is simple, intuitive and time‐efficient, and it has been extensively used to assess serotiny in many pine species.…”

“…This meets conditions 1, 2 and 3. Additionally, Pelletier and de Lafontaine (in press) reported a higher viable seed count within cones from rear‐edge stands compared with those from the core of the range. This suggests that maintaining higher serotiny could come at a cost in jack pine, likely reflecting a trade‐off between maintenance and reproductive output (i.e.…”

Postfire reproduction of a serotinous conifer, the giant sequoia, in the Nelder Grove, California

“…In the western United States, the conservation importance of maintaining or reintroducing high‐severity fire has been recognized for numerous serotinous conifers, such as Rocky Mountain lodgepole pine ( Pinus contorta var. latifolia Engelm., Schoennagel et al., 2003 ), jack pine ( Pinus banksiana Lamb., Briand et al., 2015 ; Pelletier & de Lafontaine, 2023 ), and the giant sequoia ( Sequoiadendron giganteum [Lindl.] J.Buchh., Harvey et al., 1980 ).…”

Forest resilience and regeneration dynamics following wildfire disturbance

“…Anthropogenic or climate‐change‐related alterations to disturbance type and regimes may disrupt this ecological memory and change forest resiliency, represented by alteration to forest composition and structure, or potentially more dramatically by shifting the system into an alternative state (i.e., unforested or compositionally different; Braziunas et al, 2018; Enright et al, 2015; Mcewan et al, 2011; Murray et al, 2012; Rooney et al, 2000; Seidl et al, 2017; Stevens‐Rumann et al, 2018; Turner et al, 2022). This is especially critical for species at their range margins, as they are potentially already experiencing climatic conditions at the limits of their physiological tolerance, enhancing vulnerability to the interactions between changing disturbance regimes and environmental conditions (Pelletier et al, 2022; Pelletier & de LaFontaine, 2023; Thomas, 2010).…”

“…Fire not only stimulates jack pine regeneration, but also creates conditions that may threaten survival, especially under climate warming. In addition to heat‐opening cones and releasing seed (Pelletier & de LaFontaine, 2023), fire exposes bare mineral soil and reduces germination competition (Lamont et al, 2020). Jack pine regeneration following fire disturbance results in pure or mixed (Savva et al, 2007) even‐aged stands (Arseneault, 2001; le Goff & Sirois, 2004), with the majority of seedlings establishing within the first year following disturbance (Greene et al, 2013).…”