Climate change is increasingly altering forest disturbance regimes, including the size and frequency of wildfires, as well as post-fire forest dynamics. In the Northern Rockies, forest resilience to wildfires depends upon the nature of the fire (i.e. severity, size) and pre-fire forest composition, which affect rates of post-fire tree establishment by determining seed availability. Seed production, germination, and seedling survival are also particularly sensitive to seasonal and annual climate.
We studied the impacts of wildfire and climate variability on lower treeline forests by quantifying how post-fire tree establishment varies with seasonal-to-annual climate conditions. We sampled 611 seedlings across 16 sites in lower treeline forests that burned in eight separate moderate- to high-severity wildfires since 1992. We reconstructed establishment rates of the dominant lower treeline species, Pinus ponderosa and Pseudotsuga menziesii, using dendrochronology to precisely age seedlings at the root-shoot boundary. To test for significant relationships between recruitment and climate, we compared recruitment events (i.e., years with > twice the median annual recruitment) to annual climate conditions, including annual climatic moisture deficit (CMD, mm), annual growing degree days (base 5°C, GDD5), summer heat:moisture index (SHM), and mean warmest month temperature (MWMT, °C). We used superposed epoch analysis (SEA) to characterize climatic conditions before, during, and after recruitment events.
We identified 15 recruitment events across all 16 sites, with episodic regeneration largely occurring within five years of a wildfire. Precise dendrochronological dating of tree seedlings was robust to validation by independent recounts and counts of nursery-grown samples, whereas field-based whorl counts largely underestimated tree age, by as many as 15 years. SEA indicated that climate prior to and during years of recruitment events was significantly different from average (p < 0.05). Annual moisture deficits and temperature (GDD5) were significantly lower than average two years prior to and during the year of ponderosa pine recruitment events. The significance of antecedent climate suggests an important role of cone production (in surviving trees) for post-fire regeneration, while cooler conditions coincident with recruitment events likely reflect the importance of seasonal to annual climate for germination. Temperature (GDD5) was significantly higher than average during the year of Douglas-fir recruitment events. Our results highlight important mechanisms linking annual climate variability to the temporal pattern of post-fire conifer regeneration, contributing to a growing understanding of the resilience of lower treeline forests to future changes in climate and disturbance activity.