Human Influence on California Fire Regimes

Abstract

Periodic wildfire maintains the integrity and species composition of many ecosystems, including the mediterranean-climate shrublands of California. However, human activities alter natural fire regimes, which can lead to cascading ecological effects. Increased human ignitions at the wildland-urban interface (WUI) have recently gained attention, but fire activity and risk are typically estimated using only biophysical variables. Our goal was to determine how humans influence fire in California and to examine whether this influence was linear, by relating contemporary (2000) and historic (1960-2000) fire data to both human and biophysical variables. Data for the human variables included fine-resolution maps of the WUI produced using housing density and land cover data. Interface WUI, where development abuts wildland vegetation, was differentiated from intermix WUI, where development intermingles with wildland vegetation. Additional explanatory variables included distance to WUI, population density, road density, vegetation type, and ecoregion. All data were summarized at the county level and analyzed using bivariate and multiple regression methods. We found highly significant relationships between humans and fire on the contemporary landscape, and our models explained fire frequency (R2 = 0.72) better than area burned (R2 = 0.50). Population density, intermix WUI, and distance to WUI explained the most variability in fire frequency, suggesting that the spatial pattern of development may be an important variable to consider when estimating fire risk. We found nonlinear effects such that fire frequency and area burned were highest at intermediate levels of human activity, but declined beyond certain thresholds. Human activities also explained change in fire frequency and area burned (1960-2000), but our models had greater explanatory power during the years 1960-1980, when there was more dramatic change in fire frequency. Understanding wildfire as a function of the spatial arrangement of ignitions and fuels on the landscape, in addition to nonlinear relationships, will be important to fire managers and conservation planners because fire risk may be related to specific levels of housing density that can be accounted for in land use planning. With more fires occurring in close proximity to human infrastructure, there may also be devastating ecological impacts if development continues to grow farther into wildland vegetation.

Human-caused fire occurrence modelling in perspective: a review

“…However, studies on high population density in large urban areas, such as those by GonzalezOlabarria et al (2011) for north-eastern Spain, Argañaraz et al (2015) for Argentina, and Beccari et al (2015) for northern Italy, found low fire occurrence. This may have been caused by the lower availability of fuel to support HCFs in highly developed urban areas (Syphard et al 2007(Syphard et al , 2009Price and Bradstock 2014). Donoghue and Main (1985) observed an increase of HCF occurrence related only to non-metropolitan population density.…”

Is there an inherent conflict in managing fire for people and conservation?

“…The presence of humans in the landscape can alter patterns of ignitions (Penman et al 2013a;Collins et al 2015;Mann et al 2016), often resulting in an overall increase in the number of ignitions per year (Syphard et al 2007;Syphard et al 2009;Plucinski et al 2014;Price 2015). Fragmentation and manipulation of forests through expansion of agriculture and human settlements has created discontinuities in fuels thereby influencing fire spread patterns (Finney 2007;Price and Bradstock 2010;Salis et al 2014) and surface fire behaviour (Agee et al 2000).…”

The impact of wildfire on stream fishes in an Atlantic-Mediterranean climate: evidence from an 18-year chronosequence

“…Human activities have altered the regime of wildfires beyond their natural historic range (Syphard et al, 2007;Batllori et al, 2013), often driven by changes in landuse (Pausas et al, 2008). In Mediterranean Europe, the abandonment of farmland and the plantation of commercial forests (for the wood and paper industry) have contributed to the increased frequency and spatial extent of wildfire (Moreira et al, 2001;Pausas, 2004;Pausas et al, 2004a).…”