Dalmatian toadflax (Linaria dalmatica) response to wildfire in a southwestern USA forest

Severe wildfires often facilitate the spread of exotic invasive species, such as Dalmatian toadflax (Linaria dalmatic). We hypothesized that toadflax growth and reproduction would increase with increasing burn severity in a ponderosa pine (Pinus ponderosa)-dominated forest. We measured toadflax density, cover, flowering stalks, and native species richness and cover on 327 plots for 3 y after a 2001 wildfire. Toadflax stem density, cover, and flowering stalks increased in 2003, then decreased in 2004 in all burn severity classes, but remained higher than initial 2002 values. Toadflax spread to previously uncolonized areas, though stem density decreased in unburned plots. Transition matrices showed that more plots on moderately (73%) and severely (74%) burned areas classified as high toadflax density in 2002 remained high density in 2004. Deterministic matrix modeling using 2002 to 2004 transition probabilities projected that the percentage of high-density plots would stabilize on moderately and severely burned sites at 41 and 61%, respectively. In contrast, 20-y rates of change (λ) for unburned and low-severity burn sites were < 1.0, and stabilizing at 2% for unburned plots and 19% for low-severity burn plots. Post-wildfire conditions in high-severity burned areas favour increased density, cover, reproduction, and spread of Dalmatian toadflax, while native species richness was reduced, suggesting that the invasive species would persist, at least in the short term, at the expense of natives.