Biophysical controls on forest structure and fire severity in Yosemite National Park

Background/Question/Methods

We studied whether the biophysical setting or experienced fire history exerted greater control over the physical structure of forests in Yosemite National Park, California, USA.  In mountainous regions, topography creates a mosaic of biophysical settings that vary in precipitation, temperature, heat load, and slope position that controls forest biomass accumulation.  In frequent fire landscapes, fire can override much of the influence of the biophysical setting in shaping forest structure.  We tested the hypothesis that in the absence of fire, the biophysical setting explains forest structure but fire becomes the dominant driver of structure when it is re-introduced.   Our mountainous study areas (10895 and 16209 ha) in Yosemite National Park experienced several decades of fire suppression followed by 50 primarily low and mixed severity fires since 1984.  We measured three aspects of forest structure – canopy cover >2 m and 2–8 m and dominant tree height – with airborne LiDAR data and estimated fire severity since 1984 with Landsat data.  We modeled the relationship of the biophysical setting – actual evapotranspiration, climatic water deficit, slope position, slope, and solar radiation – on forest structure and fire severity for unburned stands (controls) and stands that burned once and twice.

Results/Conclusions

We found that the biophysical setting explained the majority of forest structure both with and without fire.  For example, the biophysical setting alone explained 71.3% of the canopy cover >2 m for unburned stands.  For stands that burned once, the biophysical setting alone explained 62.6% of variance and adding fire severity as a predictor explained 69.6% (58.5% and 63.7%, respectively, for two fires).  Biophysical setting explained 60.6% of fire severity (single fire) and 89.2% of whether a location never burned, or burned once or burned twice.  The biophysical setting apparently creates a feedback loop in which it strongly influenced the pre-fire forest structure, the biophysical setting and forest structure influenced fire location and severity which in turn determined post-fire forest structure.  The stochastic effects of weather during fires appear to have been a less influential control on forest structure and fire severity.  Our results suggest that the topographic heterogeneity in mountainous regions produce a relatively stable biophysical mosaic that creates deterministic controls on forest structure both in the absence and presence of low and mixed severity fires.  These results can be used to improve forest management and help predict the effects of climate change in topographically complex landscapes.