OOS 38-5
Fire effects on boreal treeline responses to climate change
There is a widespread expectation that treelines will advance in response to warming climate trends in arctic and alpine regions, consistent with past patterns of Holocene tree migration. However, changes in climate are also bringing changes in disturbance regimes, with the potential for strong effects on treeline dynamics. Fire is a climate-sensitive disturbance that disrupts the feedbacks that constrain community change, thus creating opportunities for rapid shifts in composition. At the same time, fire directly affects key demographic processes that determine treeline dynamics (e.g. seed dispersal, recruitment, survival). Thus, fire can act as a catalyst of treeline change as well as a direct driver that interacts with climate to shape demographic outcomes. Here we report on case studies of natural fires near treelines in NW North America to examine how fire effects on seed availability, seedling recruitment, and tree survival shape patterns of treeline change. We used a set of seeding and transplant experiments at paired burned and unburned sites in Alaska (n = 7) and Yukon Territory (n = 8) to test the relative importance of demographic bottlenecks related to seed availability, seedbed quality, and early survivorship in determining how fire may shape treeline dynamics.
Results/Conclusions
Seed applications caused the greatest increase in observed germination at sites in the North Yukon where there was a short interval (15 years) between successive fire events, but not in adjacent burned sites with a long fire return interval (>80 years). Concurrent measurements of seed rain indicate that repeat fire events deplete local seed stores, causing acute seed limitation for the serotinous treeline species, Picea mariana. In Alaska (n = 7), seed applications stimulated recruitment only where severe fires had removed patches of the surface organic layer. At these sites, tree density increased more than 2-fold compared to the pre-fire community, and species composition altered from being 100% conifers to dominance (>50%) by deciduous trees. In both regions, adjacent unburned stands showed no successful seedling recruitment, demonstrating the role of fire in creating windows of opportunity for seedling recruitment. Rates of seedling recruitment from sowing trials were positively correlated with survivorship and growth of transplanted seedlings, emphasizing the recruitment phase as a key bottleneck in post-fire regeneration at treeline. These studies illustrate that fire events can lead to divergent outcomes of retraction or expansion at treeline, depending on fire effects on seed availability and seedbed quality.