The changing role of fire in whitebark pine population dynamics: Implications for conservation

Background/Question/Methods

Whitebark pine is a foundation species in subalpine forest ecosystems of western North America that performs many important ecological functions and services.  Whitebark pine populations are currently undergoing range-wide decline due to interactions between multiple stressors, notably white pine blister rust (a non-native pathogen), mountain pine beetle (a native insect), and climate change.

In many settings whitebark pine is a pioneer species and, in such areas, whitebark pine regeneration is closely associated with early-successional habitats created by fire.  The clear association of whitebark pine regeneration with fire has led many to conclude that 20^th century fire suppression was an important contributor to whitebark pine population decline, and has motivated the increasingly widespread use of prescribed fire as a whitebark pine conservation strategy.

We examined contemporary relationships between whitebark pine regeneration and fire-created early-successional habitats at two different scales. At large scales we used the Monitoring Trends in Burn Severity database and a published whitebark pine range map to estimate the area of whitebark pine habitat burned in the contiguous United States over a 26 year period (1984-2009). At local scales we investigated patterns of whitebark pine seedling presence within subalpine parkland in the Bob Marshall Wilderness, Montana, USA, that burned in the 1994 Helen Creek fire. 

Results/Conclusions

Approximately 15% of whitebark pine range within the US burned between 1984 and 2009; the largest proportions of area burned occurred in 1988, 1994, 2003, 2006, 2007– primarily years with large amounts of total area burned in the Northern Rockies or Pacific Northwest, suggesting that availability of early-successional habitat for whitebark pine regeneration is not currently limiting.  Whitebark pine regeneration was present in 35 of 91 plots within the Helen Creek fire perimeter.  Presence of whitebark pine regeneration was most strongly associated with convex landforms, and pre- and post-fire basal area.  Whitebark pine seedling presence was only weakly related burned sites (compared to unburned sites within the fire perimeter) and burn severity.

We suggest that interactions with white pine blister rust and the mountain pine beetles have fundamentally altered role of fire-created early-successional habitats in whitebark pine population dynamics relative to historical conditions.  Specifically, contemporary whitebark pine recruitment is likely more limited by seed production and dispersal than by availability of early-successional habitats suitable for seedling establishment.  Prescribed and natural fires may only contribute to whitebark pine conservation when augmented with management to reduce supply-side recruitment limitation, such as planting or direct seeding.