PARENT SESSION
Contributed Oral Session 12: Arid Lands: Plant Response to Precipitation
Monday, August 8, 8:00 AM - 11:25 AM, Meeting Room 520 B, Level 5, Palais des congrès de Montréal

Precipitation pulses in a simulated desert grassland: The role of pulse seasonality and bunchgrass species on canopy-scale carbon exchange.

Potts, Daniel^*,1, Huxman, Travis¹, English, Nathan¹, Weltzin, Jake², Williams, David³, ¹ University of Arizona, Tucson, AZ² University of Tennessee-Knoxville, Knoxville, TN³ University of Wyoming, Laramie, WY

ABSTRACT- Episodic precipitation events (pulses) in aridlands are principle drivers of energy and material fluxes in these water-limited ecosystems. Seasonal timing of precipitation pulses may interact with species- and community-specific metabolic responses of plants and soil microorganisms to influence ecosystem C-metabolism. For example, in southern Arizona desert grasslands, the interactive effect of early- versus middle-growing season pulses and the presence of a non-native bunchgrass, Eragrostis lehmanniana on ecosystem C-metabolism is unknown. During June and August 2003, we made canopy-scale diurnal measurements of carbon exchange prior to and for 15 days following simulated 39 mm pulses on replicated, monotypic stands of native (Heteropogon contortus) and non-native (E. lehmanniana) warm season bunchgrasses growing in a rainfall manipulation experiment on the Santa Rita Experimental Range near Tucson, AZ. Mid-day net ecosystem exchange of CO₂ was influenced by season (Repeated measures ANOVA, F_1,2 = 16.8, p = 0.06) but not species. Nighttime CO₂ efflux was influenced by season and a season-dependent species effect (F_1,2 = 25.2, p = 0.04 and F_1,2 = 20.9, p = 0.04 respectively). Additionally, we compared integrated carbon exchange for the 15-day period following the June and August precipitation pulses. Season influenced ecosystem C-metabolism response to the precipitation pulse (ANOVA, F_1,2 = 18.23, p = 0.003), however species did not. During June, 95% confidence intervals of 15-day integrated carbon exchange estimates for both native and non-native bunchgrass treatments included zero suggesting that early-growing season pulses may yield negligible ecosystem carbon gains.

Key words: net ecosystem exchange, precipitation pulse, Eragrostis lehmanniana