Fire prone invasive grasses create novel threats to ecosystems by increasing fuel load and continuity, which leads to changes in fire regimes to the detriment of native species. In the US alone, there are 189 grass species listed as invasive or noxious, with more than 20% of these species believed to alter fire regimes based on local studies or expert knowledge. However, at regional scales, the impacts of invasive grass on fire have been quantified for only a single species, Bromus tectorum. Here, we quantify changes in fire size and frequency in invaded and native landcover types for nine grass species across the conterminous US. We leverage agency records of fire occurrence and size in conjunction with abundance records of invasive grass species to determine if points associated with these invasive grasses burned more frequently than uninvaded pseudo absence points on comparable landscapes in the same ecoregion. By comparing the proportion of invaded points that burned to the proportion of uninvaded points that burned, we estimate the effect of the invasive grass on regional fire regimes.
Results/Conclusions
Of the nine invasive grass species tested (Neyraudia reynaudiana, Bromus rubens, Bromus tectorum, Imperata cylindrica, Phalaris arundinacea, Taeniatherum caput-medusae, Pennisetum cilare, Arundo donax, and Cynodon dactylon) all species except Bromus rubens are linked to significantly higher fire frequency, and all except Pennisetum ciliare and Cynodon dactylon are associated with larger fire size, suggesting that the ability of invasive grasses to alter fire regimes is pervasive across species and ecoregions. In areas associated with high invasive grass abundance, fire frequency as much as tripled and mean fire size as much as doubled. While many authors have highlighted the potential for an invasive grass-fire cycle, previous studies have focused on these impacts in semi-arid shrubland ecosystems in the conterminous US. This is the first study to quantitatively demonstrate the widespread impact of invasive grasses on fire size and frequency across a range of US ecosystems, including wetlands, pine savannas, and oak woodlands. As invasive grasses continue to spread, it will be important to include these grass fire interactions in future fire risk models.