In contrast, the lightning-caused ignitions were distributed more remotely in Sierra Nevada and North Interior, with snow water equivalent, lightning strike density, and Normalized Difference Vegetation Index (NDVI) as primary drivers. Model diagnosis showed that precipitation, slope, nighttime light, and road network shaped the statewide spatial distribution of human-started ignitions. The human-caused ignitions dominated the areas closer to populated regions and along the traffic corridors. The models captured well the spatial patterns of human and lightning started wildfire ignitions in California. We developed maximum entropy models to estimate wildfire ignition probability and understand the complex impacts of anthropogenic and biophysical drivers, based on a historical ignition database. We here modeled and analyzed human- and lightning-caused ignition probability across the whole state and sub-ecoregions of California, USA. To better design effective fire prevention and management strategies, it is critical to understand ignition patterns and predict the probability of wildfire ignitions from different sources. Considerable studies have advanced our knowledge on patterns and drivers of total area burned and fire frequency, but much is less known about wildfire ignition. Consequently, projections of future fire activity should consider human impacts on fire regimes to ensure sound adaptation and mitigation measures in fire-prone areas.Īs a key component of wildfire activities, ignition is regulated by complex interactions among climate, fuel, topography, and humans. Our results suggest that there are few purely natural fire regimes in North America today. These remote areas, however, also often have lower lightning densities, leading us to believe that they may be ignition limited at the spatiotemporal scale of the study. Intriguing exceptions to this relationship are the continent's least disturbed areas, where fewer humans equate to less fire. A surprisingly coherent negative relationship between fire activity and humans was observed across the United States and Canada: fire probability generally diminishes with increasing human influence. Through a statistical control approach, whereby we account for the effect of other explanatory variables, we found evidence of non-negligible human–wildfire association across the entire continent, even in the most sparsely populated areas. We circumvent these challenges by investigating the broad-scale impact of humans on fire activity using parallel statistical models of fire probability from 1984 to 2014 as a function of climate, enduring features (topography and percent nonfuel), lightning, and three indices of human activity (population density, an integrated metric of human activity, and a measure of remoteness ) across equally spaced regions of the United States and Canada. In North America, several studies have evaluated the effects of society on fire activity however, most studies have been regional or subcontinental in scope and used different data and methods, thereby making continent-wide comparisons difficult. Humans affect fire regimes by providing ignition sources in some cases, suppressing wildfires in others, and altering natural vegetation in ways that may either promote or limit fire.
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