Fire results in increased physiological vigor for surviving Joshua trees (Yucca brevifolia) and creosote bushes (Larrea tridentata) in the Mojave Desert

Background/Question/Methods

Recent invasions of exotic vegetation (e.g. Bromus rubens) leading to continuous fuel loads have resulted in increased frequency of large fires in the Mojave Desert. Natural revegetation of burned areas will depend on propagules from surviving plants within the burned areas and plants on the burn boundaries. However, environments immediately surrounding the plants surviving within burns and at the burn boundary have been altered by fire. Fire reduces plant competition for water and other resources, deposits resources from burned plants, and exposes the soil surface to increased erosion, potentially altering hydraulic properties of the soil. Surviving vegetation in burned areas may benefit from the fire through increased available soil moisture and nutrients which would translate to increased rates of photosynthesis measurable both directly and through increases in non-structural carbohydrates. Two species of evergreen plants, Joshua tree (Yucca brevifolia Engelm.) and creosote bush (Larrea tridentata (D.C.) Cov.) were sample seasonally throughout 2010. Plants were sampled at Beaver Dam Wash in southwest Utah in four independent fires all having burned during summer 2005. Control plants were selected in undisturbed sites and paired with adjacent burned areas. Plant physiological status was measured as pre-dawn water potential, ETR (electron transport rate), and foliar concentrations of nitrogen, phosphorus, and non-structural carbohydrates.

Results/Conclusions

Pre-dawn water potentials were less negative in burned areas for Joshua tree but not for creosote bush. Joshua tree also had a 6% increase in photosynthetic electron transport rates over control plants. Both Joshua tree and creosote bush in burned areas had greater foliar nitrogen (17 and 22% respectively) and foliar phosphorus (7 and 20% respectively). Creosote bush in burned areas had 22% more foliar starch. In summary plants sampled in burned areas tended to have better water and nutrient relations and higher carbon metabolism. Increased physiological vigor may translate to increase  reproductive output from surviving plants in burned areas that may contribute to re-vegetation of the native plant community.