Tamarix invasion and fire in desert riparian ecosystems

Background/Question/Methods

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. My doctoral research investigated 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.  This work synthesizes several experiments and addresses 2 questions: 1) Is there a positive feedback between Tamarix and fire intensity that excludes native vegetation?  2) Can the Tamarix fire trajectory be altered to allow the coexistence of natives?

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 Tamarixis caused by higher fire intensity.

Results/Conclusions

A regular fire cycle of 5-10 years in Tamarix–invaded riparian areas suggests that the positive feedback cycle is fairly stable, but biological control by the tamarisk leaf beetle (Diorhabda carinulata) may alter the system’s response to fire. Prescribed fires at Great Basin and Mojave sites showed that the combined effects of fire and herbivory on Tamarix mortality were synergistic and have great potential to alter the trajectory from the perpetuation of Tamarix to native coexistence, as increased Tamarix mortality over time will decrease biomass and fire intensity, allowing native recovery.  Collectively, these studies further our understanding of the effects of invasive non-native plants on fire regimes.