PS 3-25
Fire effects and post-fire tree regeneration in alpine treeline ecotones in the northern Cascades Range, USA
Increased tree establishment in subalpine parkland and an upward movement of treeline are expected in a warming climate, and may already be occurring in some ecosystems, but other factors, such as changing disturbance regimes, have received less attention. We examined the effects of fire on subalpine parkland with dominant tree species of Abies lasiocarpa, Larix lyallii, Pinus albicaulis, and Picea engelmannii in the northern Cascade Range of Washington. We used field data from two burned areas, 17 and 25 years after fire, to investigate relationships between fire severity and post-fire herbaceous species composition and tree regeneration across the subalpine parkland ecotone. We sampled 68 and 45 plots within 1994 Butte Creek and 1985 Hubbard Creek fire perimeters, respectively, to assess herbaceous species composition across a range of fire severities. We used plant functional groups, classified based on growth form, to investigate how the pre-fire setting (closed forest, open forest, alpine woodland, krumholtz, alpine treeless vegetation) interacts with the severity of fire to affect the abundance of plant functional groups. We also examined the influence of pre-fire overstory tree structure and fire severity on post-fire seedling density and composition, across 151 and 100 plots within Butte Creek and Hubbard Creek, respectively.
Results/Conclusions
Plant functional groups differed between burned and unburned sites and along the gradient from closed forest to treeless alpine vegetation. We found significantly more cushion plants in unburned sites than burned sites. Sites that burned with high and very high severity had significantly more graminoids, herbaceous perennials, and tall shrubs. Likewise, closed forest and open forests had significantly more graminoids in burned than unburned areas. Our initial results indicate that post-fire succession dynamics in A. lasiocarpa forests in the eastern Cascades may be strongly influenced by heterogeneous patterns of severity resulting in heterogeneous survival of seedlings and overstory trees. Preliminary analysis indicates that P. albicaulis seedlings abundance was negatively correlated with fire severity in open forest (10-40% tree cover), but showed no significant relationship in alpine woodlands. P. albicaulis seedling abundance was negatively correlated with fire severity in alpine woodlands but showed no significant relationship in open forest. Climate change may increase area burned and frequency of fire in high-elevation ecosystems, which could have long-term impacts on ecosystem functions such as wildlife habitat use, hydrology, and nutrient cycling. Our results provide a starting point for understanding the effects of fire on herbaceous vegetation and post-fire forest succession in subalpine parkland.