Human-caused ignitions and plant invasion drive fire-regime change in a dryland biodiversity hotspot
Dryland ecosystems cover much of Earths terrestrial surface and are increasingly affected by climate warming, land-use change, and biological invasion. The Sonoran Desert of western North America, a biodiversity hotspot that historically experienced infrequent fire, exemplifies how these global-change processes are driving ecosystem transformation through the emergence of novel disturbance regimes. Using records of more than 73,000 wildland fire events, we quantified anthropogenic and environmental controls on fire ignition, size, and spread across the Sonoran Desert. Human-caused ignitions accounted for 95.5% of all recorded fires since the early 1990s and were concentrated near roads, buildings, and other areas of human activity, particularly during late spring. In contrast, lightning-caused ignitions were rare within the Sonoran Desert and occurred primarily in adjacent montane forests during late summer months. Consistent with broader trends across the western United States, fires were typically largest when they ignited far from roads, where access is limited, and fire management often relies on indirect attack strategies. Pre-fire annual and perennial herbaceous vegetation both promoted fire spread, with detectable effects at cover values as low as 5%, while bare ground reduced fuel connectivity and constrained fire growth. Rather than exhibiting a distinct threshold response, fire spread increased linearly with herbaceous cover, suggesting that even modest increases in fine fuels can alter fire behavior in this historically fuel-limited ecosystem. Together, our results illustrate that human ignitions determine where fires occur, whereas herbaceous fuels determine whether those ignitions spread, driving fire-regime transformation across an iconic North American desert. Managing and adapting to this novel fire regime will require coordinated efforts to reduce human-caused ignitions, limit invasive herbaceous fuels, maintain bare-ground interspaces, and rapidly suppress fires. More broadly, our findings identify the mechanisms controlling fire regime change in dryland ecosystems and provide a framework for anticipating ecosystem transformation under ongoing global change.