Moving-or-staying dilemma: reproductive traits reveal a latitudinal trade-off in black spruce

Successful regeneration depends on the reproductive traits and adaptations of species to the local environment. Rapid climate change is altering the fire regime in boreal regions, affecting the reproductive potential and potentially reshaping forest composition. We use the wide latitudinal gradient of the boreal forest to assess how reproductive traits respond to climate. We test whether and why populations from contrasting latitudes differ in reproductive traits due to seed resource allocation strategies.

This study assessed variability in reproductive traits among five populations of black spruce (Picea mariana (Mill.) B.S.P.) ranging from 48 to 53° parallels in Quebec, Canada. We tested differences in cone and seed traits, germination performance, and seed biochemical composition.

Compared to northern sites, southern sites had bigger seeds (14% longer and 16% wider) and 50% more seeds per cone (45 vs 30). Seed mass and wing loading decreased with latitude. A higher nutrient content, especially sucrose (5.8 vs. 2.6 mg·g-1) and phytic acid (8.74 vs. 0.93 mg·g-1), was detected in the southern sites, which also showed a higher germination rate (13%). Northern sites had lower germination rates (9%) and seeds dominated by starch (0.94 mg·g-1). Northern sites required higher temperatures to open their cones than southern sites (≥ 55.1 vs. 48.9 °C). Reproductive traits indicate a shift in resource allocation, with southern sites producing larger, nutrient-rich seeds that promote rapid establishment, while smaller, starch-dominated seeds in northern sites are associated with dispersal capacity.

Our results reflect trade-off between seed dispersal and persistence in regeneration, supporting a moving-or-staying dilemma in black spruce. Latitudinal variation in seed traits represents ecological adaptations to the local environment. Understanding the reproductive ability of species is critical for predicting the responses of black spruce to future climate for planning adaptive forest management of boreal stands.