Restoring natural gap variability: Relationships between forest type, fire severity, forest structure, and gap patterns in Yosemite National Park

Background/Question/Methods

Fires shape the structure of forests and acts as a keystone process. Understanding how fire modifies forest structure, however, has been difficult because of the inherent complexity of the interaction between fires and forests, which is difficult to elucidate from field studies.  We studied the relationship between fire and forest gap structure across a 96.9 km^-1 area in Yosemite National Park, USA, that was subject to 32 fires ≥40 ha between 1984 and2010.  Our study area included substantial stands of Ponderosa pine, white fir-sugar pine, and red fir forests that lay both inside and outside of fire perimeters.  Measurements of differential Normalized Burn Ratio (dNBR) were made from Landsat images to measure fire severity.  Airborne LiDAR data acquired in 2010 were used to measure both the vertical structure of foliage as well as the structure of canopy patches and gaps.  Our specific questions were (1) What patterns of burn severity were created across our study area? and (2) How did different fire severities change the proportion and structure of canopy patches and gaps within each of the forest types?

Results/Conclusions

We merged the Landsat and LiDAR data to measure change in forest structure -- a unique fusion of remote sensing data to measure process (fire severity) and response (forest structure).  We identified five classes of vertical structure and three arrangements of canopy patches and gaps associated with different combinations of forest type and absence of fire and different fire severities.  Lower severity fires were associated with less canopy material in lower height strata, increased number and size of gaps, and decreased size of canopy patches.  Moderate and high severity fires were associated with open stands either with no standing trees or scattered individual trees.  We found three arrangements of canopy and gaps: canopy-gap arrangements with gaps enclosed in otherwise continuous canopy (no fire and low fire severities); patch-gap arrangements with alternating tree clumps and gaps with neither dominating (moderate fire severities); and open-patch arrangements with trees scattered across open areas (high fire severities).  While the overall trends were expected, we were surprised to observe substantial structural differences associated even with lower severity fires and the longevity of structural changes with time since fire.  To support forest regeneration and wildlife habitat goals, Yosemite Park managers have established goals for gap structure, and these results will assist them in using fire as a management tool to meet those goals.