Landscape heterogeneity due to mixed-severity fire after a prolonged fire suppression in a Sierra Nevada mixed conifer forest

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Asha Paudel and Scott H. Markwith, Geosciences, Florida Atlantic University, Boca Raton, FL

Background/Question/Methods
Frequent low- to moderate-severity fire is important for maintaining ecosystem diversity of mixed conifer forests of the Sierra Nevada range, CA, USA. Prolonged fire suppression led to ingrowth of conifers seedlings, converting the open heterogenous forest structure into uniformly dense and layered forest. Fuel build-up and increasing drought and extreme heat frequencies caused by climate change are increasing the threat of stand replacing fire. However, a clear understanding of the effects of mixed-severity fire on landscape-scale heterogeneity is lacking.
Our research question was: Does repeated mixed-severity fire in mixed conifer forests reduce landscape-scale patch heterogeneity due to extensive high-severity fire converting forest to shrubland?
The study area consists of a mosaics of burned and unburned patches within Lassen and Plumas National Forests, CA. We used secondary geospatial landcover data classified by cover type after multiple fires between 2000 and 2014 and before any fires from in the years 1991-2000. We calculated various landscape metrics using FRAGSTATS for comparison before and after fires.
Results/Conclusions
Results at the scale of burn patches showed that reburning at low- to moderate-severity nearly doubled vegetation cover type heterogeneity, but at high-severity reduced heterogeneity by half. However, at the full landscape scale mixed-severity fire, including all burn severities, increased vegetation cover type heterogeneity. Fragmentation indices calculated at vegetation class level showed that fire created larger patches of shrub and fragmented continuous conifer forest. This study provides insights concerning vegetation pattern change due to frequent large fire events and suggest that the effects on landscape heterogeneity varies with scale of analysis. Hence, more than one scales should be considered to measure heterogeneity and vegetation pattern change before prioritizing management actions of the mixed conifer forests of the Sierra Nevada.