Almost a century of active fire suppression has led to unintended biomass accumulation that consequently increased the average size and intensity, as well as ecological impact, of modern fires (Keane et al 2002). Paradoxically, some of the recent increases in fire activity in parts of the continent, notably in the west, may be due to a legacy of fire management policies that were too successful (Stephens and Ruth 2005). Although this phenomenon has been largely attributed to a warmer and drier climate, it is increasingly recognized that, in some areas, humans may have played a role (Stephens et al 2007, McWethy et al 2013). Over the past three decades, however, substantial increases in wildfires have been reported, notably in the western United States (Westerling et al 2006) and in the boreal zone (Kasischke and Turetsky 2006). An initial surge in area burned at the time of settlement due to slash-and-burn practices in some areas (Weir and Johnson 1998) was followed by a large-scale reduction in fire activity that was largely the result of fire suppression and human modifications to the natural vegetation cover (Marlon et al 2008). Several investigations of wildland fire in North America show a dramatic effect on fire activity following the arrival of Europeans. In addition to these direct effects on fire activity, humans may exert pervasive and enduring indirect effects on fire regimes by altering, or removing, the natural vegetation (e.g., change in forest structure, invasive annuals, logging, conversion to irrigated agriculture) (Cochrane 2003, Pausas and Keeley 2009, Perry et al 2012, Fréjaville and Curt 2015). Globally, humans are usually responsible for the majority of ignitions however, many regions where human ignitions are prevalent are also subject to intense fire management programs aimed at extinguishing fires before they become large (Stocks et al 2002, Stephens 2005). Conversely, in other regions or ecosystems of the world, such as in parts of North America, land-use change and a culture of aggressive fire suppression may limit fire activity (Cumming 2005, Finney et al 2009). This is the case, for example, in African savannas, where cultural practices of frequent burning make it the archetype of a 'human-tended' fire regime (Archibald et al 2012). In many parts of the world, people increase fire activity beyond what is naturally expected by deliberately or accidentally setting fire to natural vegetation (Carcaillet et al 2009, Bowman et al 2011). Humans affect fire regimes worldwide, sometimes causing profound changes to ecosystem structure and function (McWethy et al 2013). Consequently, projections of future fire activity should consider human impacts on fire regimes to ensure sound adaptation and mitigation measures in fire-prone areas. 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.

Hexels effects