Print PageIncreased wildfire has been observed with the displacement of native cottonwood-willow (Salix and Populus spp.) gallery forests by invasive, non-native tamarisk (Tamarix spp.) in desert riparian zones of North America. Greater post-fire recovery of Tamarix relative to native species suggests a Tamarix fire trajectory where repeated fire excludes native riparian species. 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?
Patterns in fuel consumption and recovery of Tamarix vs. native species as a function of Tamarix density were described in a survey of 30 recent riparian burns. Tamarix vs. native (Salix and Populus) flammability was compared by igniting foliar samples in a muffle furnace at 650°C. Prescribed burns were conducted at two sites: Lovelock (2008) and Valley of Fire (2008), NV. Fire intensity was measured with thermocouples and dataloggers, and clip plots estimated biomass per area. In Lovelock, root carbohydrates were sampled from Tamarix exhibiting a gradient of biocontrol Diorhabda herbivory impact, and a mathematical model was developed to test whether the interaction of herbivory and fire were additive or synergistic.
Results/Conclusions
Tamarix fire intensity is biomass dependent: 6m and 35m flame lengths were observed in 14,500 Kg-Ha-1 plots in Lovelock and 212,779 Kg-Ha-1 plots in Valley of Fire, respectively (ANOVA P < 0.05). Tamarix foliage ignites in 3.2 seconds, 3.1 seconds faster than Populus, and 5.1 seconds faster than Salix (ANOVA P < 0.001). Fuel consumption increases (R2 = 0.28 , P <0.0001 ) and native recovery decreases as a function of Tamarix density (R2 = 0.23 , P =0.0078). These results support a positive feedback between Tamarix and fire intensity that excludes native vegetation. At the Lovelock site, Diorhabda herbivory was directly related to root crown starch content (R2 = 0.79), and Tamarix mortality was a function of both fire intensity and herbivory, which interacted synergistically. Synergisms between fire and herbivory may alter the fire trajectory from the perpetuation of Tamarix to coexistence, as increased Tamarix mortality over time will decrease biomass and fire intensity, allowing native survival and recovery. Although complete eradication of Tamarix may not be possible given the irreversible nature of the hydrologic changes that promoted its invasion, native coexistence may return some habitat value to Tamarix invaded areas.
