Across the northern boreal forest, conifer-dominated communities are structured by landscape-scale fires, where the species present have adapted to a historical disturbance regime. With climate warming, fire activity is increasing in many areas of the boreal forest, altering historic regimes. In concert, we expect tree populations at the northernmost edge of the boreal forest to increase in density (stand infilling) and advance (range expansion) into uncolonized tundra and alpine ecosystems as environmental constraints on tree growth and recruitment lessen. In contrast to this expectation, contemporary forest advance has only been observed in approximately half of studied systems. Increasingly, research is revealing the importance of biotic and non-climatic factors in mitigating the response of species, like trees, to direct climatic effects. Yet, what role does fire play in these processes, and how does increasing fire activity interact with reproductive processes and regeneration under climate change? We explore these questions across two scales: i) a population of black spruce (Picea mariana) in subarctic Yukon, where recent fires have occurred after a much shorter time interval than the region has historically experienced; and ii) the northern edge (treeline) of the circumpolar boreal forest.
Results/Conclusions
Tree colonization was drastically reduced following two closely timed fires in northern Yukon. On-site germination experiments demonstrated that black spruce recruitment was limited by seed availability, not substrate. Across the circumpolar treeline, the production of viable seed was insufficient for range expansion, including in sites dominated by fire-adapted species. If these marginal stands burn under increased fire activity, we are likely to observe a regeneration failure due to low seed availability. In summary, we have found that seed availability can constrain stand regeneration and population expansion via (at least) two processes: i) increased fire activity can interrupt reproduction when a fire occurs before a population reaches reproductive maturity; and ii) reproductive bottlenecks (i.e., insufficient viable seed production) can limit increases in range-edge populations. Together, these studies suggest that novel fire regimes can lead to a regeneration failure or degradation of northern boreal or treeline forests in some areas, rather than the increased tree density and forest extent predicted with climate warming. Future research on how the timing of tree reproduction is responding both to warming and to the selective pressure of more frequent fires will improve our ability to predict where seed availability may constrain forest regeneration and expansion.