PARENT SESSION
Organized Oral Session 41: Ecological responses to precipitation: Scaling patterns and processes from the genome to the ecosystem
Organizer(s): ME Loik and SD Smith
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

Multi-scale responses of carbon and water fluxes to magnitude and frequency of precipitation pulses in the northern Chihuahuan desert.

Snyder, Keirith^*,1, Maxwell, Connie¹, Cable, Jessica ², ¹ USDA-ARS Jornada Experimental Range, Las Cruces, NM² University of Arizona, Tucson, AZ

ABSTRACT- Changes in the frequency and magnitude of precipitation may be as important as changes in total seasonal precipitation amounts, especially in water-limited environments. We manipulated both the frequency and magnitude of precipitation over two growing seasons in the northern part of the Chihuahuan desert at the Jornada Experimental Range. A 46% increase relative to mean summer rainfall was applied at two different frequencies and magnitudes over twelve weeks during the summer growing season to plots containing both mesquite (Prosopis glandulosa) and black grama grass (Bouteloua eriopoda). Plots received either: ambient precipitation (controls), ambient plus frequent small (5-6 mm) rainfall events applied weekly, and ambient plus infrequent large (20-24 mm) events applied monthly. Fluxes of carbon and water were measured at different scales using static chambers and an IRGA. Isotopic partitioning of evapotranspiration (ET) fluxes indicated that the proportion of ET accounted for by mesquite transpiration increased in large infrequent rainfall treatments relative to small frequent rainfall treatments; however, mesquite was able to use water from both large and small magnitude rainfall treatments. Evapotranspiration flux in large chambers containing mesquite, grass and soil was higher in large infrequent watering treatments than in the other two treatments in August and September, but not July. However, increased uptake of CO₂ was observed only in large infrequent watering treatments in August. In contrast to this, smaller chambers containing only grass and soil showed increased uptake of CO₂ in both small frequent rainfall and large infrequent rainfall treatments. These data suggest that changes in the depth and duration of soil wetting in conjunction with the degree of mesquite encroachment may have important implications for carbon and water cycling.

Key words: evapotranspiration, stable isotopes, carbon flux, shrub encroachment