Are deserts different? Perennial plant responses to ten years of elevated CO2 in the Mojave Desert

Background/Question/Methods It is generally assumed that elevated CO₂ increases primary production, but few studies have focused on the effects of elevated CO₂ in desert systems. Elevated CO₂ has been predicted to significantly increase plant production in deserts through stimulatory effects on plant water-use efficiency. We examined the effects of elevated CO₂ at the Nevada Desert FACE (Free-air CO₂ Enrichment) Facility (NDFF) in the Mojave Desert. At the NDFF, ambient and elevated CO₂ levels were 360 and 550 μmol mol^-1 CO₂, respectively. CO₂ treatments were applied continuously from 1997-2007 in intact plots 25 meters in diameter. While other studies focused on soil, root, and annual plant responses to elevated CO₂, our study focused on aboveground perennial plant production. In 1997, diameters and heights of all perennial individuals were recorded. Shoot tip measurements were made on dominant species throughout the ten years, including Larrea tridentata, Lycium pallidum, Lycium andersonii, Ambrosia dumosa, and Pleuraphis rigida (‘Big Five' species). In 2007 at the end of the experiment, diameters and heights were measured and aboveground biomass was harvested for every perennial individual in the plots. Harvest data were used to construct regressions for twenty species to predict starting biomass based on diameters and heights.

Results/Conclusions When analyzing the effects of elevated CO₂ on final harvest biomass in 2007, we found no significant treatment effects on total plant biomass per ring. Final biomass was significantly different for the ‘Big Five' species, where Larrea tridentata comprised most of the biomass; however, species did not respond differently to elevated CO₂. Biomass of individual species either increased or decreased over time; however, elevated CO₂ did not affect growth over time. Our results contrast with other studies showing that elevated CO₂ increases productivity. Other studies in our research group have shown that increases in production are only seen in wet years during the ten-year period of CO₂ treatments at NDFF. This suggests that water is more limiting than CO₂ in this desert system. These results indicate that 1) elevated CO₂ may stimulate primary production in the Mojave Desert under certain conditions, but this stimulation may be highly variable, and 2) given the episodic nature of resource availability in deserts, no sustained increase in aboveground primary productivity has been detected after ten continuous years of exposure to elevated CO₂.