Reproductive maturity in boreal trees, Northwest Territories, Canada

In boreal North America, much of the landscape is covered by fire-adapted
forests dominated by serotinous conifers. For these forests, reductions in
fire return interval could limit reproductive success, owing to
insufficient time for stands to reach reproductive maturity i.e., to
initiate cone production. Improved understanding of the drivers of
reproductive maturity can provide important information about the capacity
of these forests to self-replace following fire. Here, we assessed the
drivers of reproductive maturity in two dominant and widespread conifers,
semi-serotinous black spruce and serotinous jack pine. Presence or absence
of female cones were recorded in approximately 15,000 individuals within
old and recently burned stands in two distinct ecozones of the Northwest
Territories (NWT), Canada. Our results show that reproductive maturity was
triggered by a minimum tree size threshold rather than an age threshold,
with trees reaching reproductive maturity at smaller sizes where
environmental conditions were more stressful. The number of reproductive
trees per plot increased with stem density, basal area, and at higher
latitudes (colder locations). The harsh climatic conditions present at
these higher latitudes, however, limited the recruitment of jack pine at
the treeline ecotone. The number of reproductive black spruce trees
increased with deeper soils, whereas the number of reproductive
jack pine trees increased where soils were shallower. We examined the
reproductive efficiency i.e., the number of seedlings recruited
per reproductive tree, linking pre-fire reproductive maturity of recently
burned stands and post-fire seedling recruitment (recorded up to 4 years
after the fires) and found that a reproductive jack pine can
recruit on average three times more seedlings than a reproductive black
spruce. We suggest that the higher reproductive efficiency of jack pine
can explain the greater resilience of this species to wildfire compared
with black spruce. Overall, these results help link life history
characteristics such as reproductive maturity to variation in post-fire
recruitment of dominant serotinous conifers.