Abstract

1. Effect of rainfall pulses on soil CO2 fluxes and ecophysiology of Bouteloua eriopoda in a northern Chihuahuan Desert Grassland: a synthesis of research to date Thomey, ML, Collins, SL, Vargas, R1, Johnson, JE, Brown, RF, Natvig, DO, Friggens, MT Department of Biology, University of New Mexico 1Department of Environmental Sciences policy and Management, University of California, Berkeley Abstract Increasing global temperatures are projected to alter the intensity of the hydrological cycle (1). For example, precipitation patterns are predicted to become more variable dominated by extreme rainfall events punctuated with longer dry intervals (4). Changes in the magnitude and frequency of precipitation pulses is especially important in arid land systems where soil water content is a key control on the components of carbon cycling (3,5). To determine the effects of altered precipitation regimes on a Chihuahuan Desert grassland ecosystem, we experimentally altered precipitation frequency and intensity for two years during the summer monsoon season (July-Sept). Experimental rainfall treatments included: 1) ambient precipitation plus one 20 mm rain event per month (n = 5) and 2) ambient precipitation plus four 5 mm rain events per month (n = 5). Control plots received only ambient precipitation (n = 3). Here we present two methods employed to measure the effects of altered precipitation regimes on soil water dynamics and the response of soil respiration and primary productivity of the dominant Chihuahuan Desert grass, Bouteloua eriopoda. First, we continuously measured soil respiration and applied wavelet coherence analysis to the time series derived from manipulated monsoon scenarios to determine the periods (e.g. days or weeks) at which each treatment influenced the synchrony between soil respiration and PAR (photosynthetic active radiation), used as a surrogate for photosynthesis. Second, we measured instantaneous leaf-level gas exchange and predawn leaf water potential to determine how experimental rainfall treatments influenced the mechanistic processes driving trends in long-term/continuous measurements of aboveground net primary productivity (ANPP) and soil respiration. We found that a small number of large events (more extreme events) increased the synchrony between soil respiration and the biophysical mechanisms that act at the 1-day period. Moreover, higher magnitude pulses increased soil moisture that corresponded with an increase in soil respiration and primary productivity. Overall, our results show that desert grasslands dominated by B. eriopoda are highly sensitive to changes in precipitation regime. Climate models consistently indicate a future with altered precipitation patterns and an increase in extreme precipitation events. Understanding how the spatial and temporal patterns in precipitation will affect arid land ecosystems is important because water availability affects ecosystem carbon exchange through shifts in plant and microbial activity. Through this and similar rainfall manipulation experiments, we can begin to address the consequences of environmental change in arid land ecosystems. Results Results a) d) e) b) c) f) Figure 4 – Instantaneous (a-c) and long-term/continuous (d-f) plant and soil measurements for the 2007-2008 monsoon season. Bars represent the mean ± SE and significant differences (p < 0.05) are indicated by different letters Figure 2- Time series covering two monsoon seasons (July-September 2007 and 2008) of rainfall manipulation experiments. Daily means are shown. a-b, Precipitation. c-d, soil water content (SWC) of the 0-30 cm layer. e-f, soil temperature at 8 cm depth. g-h, Soil respiration (SR). Blue line, control grasslands (n = 5) with ambient precipitation; green line, treatment grasslands (n = 5) with ambient plus 5 mm weekly; red line, treatment grasslands (n = 5) with ambient plus 20 mm per month. DOY, day of the year. • Conclusions • A small number of large (more extreme events) : • increased the synchrony between soil respiration and the biophysical mechanisms that act at the 1-day period likely being photosynthesis that was supported by field measurements of Anet and ANPP. • 2) instantaneously increased rates of photosynthesis, predawn water potential and soil water content and this corresponded with a significant increase in ANPP. • 3) increased continuous measurements of soil respiration that mirrored continuous measures of soil water content Figure 1- Monsoon Manipulation Rainfall Experiment at the Sevilleta LTER. • Study site and Methods • Monsoon Manipulation Rainfall Experiment • at the Sevilleta LTER in New Mexico, USA • (Fig 1.) • -We installed multiple soil temperature (Ts), • moisture (SWC) and CO2 sensors (Vaisala • GMM 222) to continuously measure soil • CO2 concentrations. • -Precipitation was manipulated during the • Monsoon season (July-Sept). Ambient • precipitation (control), ambient plus 5 mm • Weekly, and ambient plus 20 mm per month • (treatments). References 1) Easterling, DR, Meehl GA, Parmesan C, Changnon SA, Karl TR, Mearns, LO. 2000. Climate extremes: observations, modeling and impacts. Science 289:2068-2074. 2) Grinsted A, Moore JC, Jevrejeva S. 2004. Application of the cross wavelet transform and wavelet coherence to geophysical time series. Nonlinear processes in geophysics 11: 561-566. 3) Huxman TE, Snyder KA, Tissue D, Leffler AJ, Ogle K, Pockman WT, Sandquist DR, Potts DL, Schwinning S. 2004. Precipitation pulses and carbon fluxes in semiarid and arid ecosystems. Oecologia 141:254-268. 4) Kharin VV, Zwiers FW, Zhang X, Hegerl GC. 2007. Changes in temperature and precipitation extremes in the IPCC ensemble of global coupled model simulations. Journal of climate 20: 1419-1443. 5) Noy-Meir I. 1973. Desert ecosystems: environment and producers. Annual review of ecology and systematics 4:25051. 6) Torrence C, Compo GP 1998. A practical guide to wavelet analysis. Bulletin of the american meteorological society 79: 61-78. - We used wavelet coherence analysis (2,6 ) to explore the relationship between soil moisture, soil respiration and PAR (used as a surrogate for photosynthesis). - We measured instantaneous leaf-level gas exchange (Anet) and water potential (Ψpd) to determine how rainfall treatments influenced the mechanistic processes driving long-term measures of ANPP and continuous measures of soil respiration. Figure 3- Wavelet coherence analysis between temperature independent SR and PAR of rainfall manipulation experiments. The graphs represent two monsoon seasons (July-September 2007 and 2008) at (a,b) control, (c,d) small water addition, and (e,f) large water addition. The color codes for power values are from dark blue (low values) to dark red (high values). Black contour lines represent the 5% significance level and the edge black line indicates the cone of influence (COI). Values nearer the boundaries of the COI should be interpreted with caution. DOY means day of the year.