Deserts of North America are at an ecological “tipping point” in which fires that historically were rare and small are increasing in frequency, size and intensity as a result of changing climate conditions and the rapid expansion of invasive grasses. Critical to controlling the spread of invasive grasses is the innate ability of the native plant and native rodent community to resist invasive grasses and prevent their spread. Recent evidence suggests that fire partially diminishes the ability of native plants and native rodents to prevent the establishment and spread of invasive plants. However, it may take more than one fire to create this effect. Interestingly, one of the key traits of invasive grasses is their high flammability, which promotes recurrent fires.
The primary objective is to test the idea that invasive grasses overcome biotic resistance and spread across desert landscapes by promoting repeat fires that eliminate biotic resistance by the native community. Our methods include Objective Based Image Analysis (OBIA) of drone multispectral imagery and on-the-ground vegetation measurements of repeat fire and native rodent treatments in replicated full factorial experimental plots. Repeat burn treatments conducted in 2016 tested how fire spread after initial burn treatments conducted in 2011.
The experimental exclusion of rodents drastically increased the spread of fire in our experimental plots. When rodents were present the burn pattern is much more patchy reducing the amount of total area burned. These effects are due to differences in cheatgrass (Bromus tectorum) growth in the presence or absence of rodents. Vegetation density measurements show an increase in the average number of individual cheatgrass tillers by almost 2.5-fold in rodent excluded plots from plots which have rodent access. When considering that the average number of individual tillers increases over 10-fold 5 years after a single burn regardless of rodent treatment there is clear patterns between burn pattern and repeat burn coverage area, and rodent presence or absence. Cheatgrass’ post-fire expansion appears to promote repeat fires by increasing fine fuel loads, especially when rodent access is limited.