Exotic plant invasions commonly increase wildfire frequency and intensity. While extensive research has examined relationships between plant invasion and fire regimes in upland systems, few studies have investigated these relationships in riparian ecosystems. In this dissertation, I investigate the mechanisms underlying increases in riparian wildfire associated with the replacement of native cottonwood-willow (Salix and Populus spp.) forests by invasive tamarisk (Tamarix spp.) in desert riparian ecosystems in the southwestern U.S. Several experiments answer the question: what is the role of Tamarix in altering wildfire dynamics in riparian systems?

The superior post-fire recovery of Tamarix observed by numerous investigators suggests a positive feedback cycle where repeated fires exclude native riparian species and promote Tamarix. The results of a regional analysis showed fire to be more common and likely to spread through the riparian corridor where Tamarix occurred. Ignitability experiments showed that Tamarix foliage is more flammable than native foliage. A survey of 30 riparian burns showed that native mortality increased with Tamarix cover. Therefore, a positive feedback between Tamarix and fire excludes native vegetation because Tamarix promotes fire and exhibits higher post-fire recovery than natives.

Because fire intensity is a common mechanism driving positive feedbacks between invasive species and fire, prescribed burns were conducted at two sites (Great Basin 2006 and Mojave 2008) to determine the relationship between Tamarix vegetation architecture and fire intensity. Fire intensity was positively related to Tamarix cover and biomass, thus greater native mortality in denser Tamarix is caused by higher fire intensity.