Forest resilience is eroding in many places as climate and disturbance regimes change globally. Trees are already beginning to shift their geographic distributions in response, and each species is migrating individually based on functional traits and biotic interactions. Thus, new tree-species assemblages will likely begin to appear that have never co-occurred in the past. Research increasingly focuses on where and why today’s forests are vulnerable to change, but there is substantially less emphasis on how novel forests will reorganize from current systems. Boreal Alaska is an excellent place to explore forest reorganization because it has warmed two-times faster than the global average, causing advanced ecological change. Further, boreal wildfires, which can create opportunities for reorganization, are increasing in frequency and size. Finally, lodgepole pine (Pinus contorta var. latifolia), a widespread Rocky Mountain conifer, has been migrating north for 12,000 yrs and is currently only 80km from the Alaskan border. Drawing from the invasion literature, we propose four mechanisms that could dictate how novel tree species assemblages form with changing climate and disturbance and use a simulation experiment to evaluate how these mechanisms might influence lodgepole-pine colonization in boreal Alaska.
Results/Conclusions
We simulated 192 stands (1-ha) comprised of black spruce (Picea mariana), trembling aspen (Populus tremuloides), and Alaska birch (Betula neo-alaskana), spanning a 600km gradient in boreal Alaska. Stands were simulated under all combinations of four lodgepole-pine propagule intensities (0.001%, 0.01%, 0.1%, 1% chance of seed landing per 2-m2) and three fire return intervals (no fire, 100yr, 30yr) (n=2,304 total). First, propagule intensity was a critical determinant of colonization success. Lodgepole pine only reliably colonized stands where its seed rain was largest. Second, wildfire was necessary to break legacy locks of the current system. Under the highest propagule intensity, lodgepole pine were almost three times more dense in stands that burned compared to those that did not. Further, when stands burned every 30yrs, lodgepole pine became nearly as dense as black spruce because short-interval fire exceeded spruce’s capacity to recover. Third, climate variability mediated effects of propagule intensity and fire. Lodgepole pine more successfully colonized warmer stands with higher annual precipitation. Fourth, lags existed where lodgepole pine colonized but stabilized at low densities for a few decades before rapidly increasing again after the next fire event. Simulations reveal how biotic and abiotic factors could interact to shape 21st-century forest reorganization.