Size and frequency of rainfall events in a semiarid Chaco savanna, Argentina

CASILLO, J.; KUNST C.; SEMMARTIN M.

Rosario

Congreso; Congreso Internacional de Pastizales; 2011

International Rangeland Congress

Introduction In arid environments, rainfall regime (annual precipitation, seasonality, size and frequency of rainfall events) control key ecosystem features (e. g. primary production, community structure). Soil moisture pulses of varying magnitude can trigger different responses (e. g. emergence or woody plant recruitment) (Schwinning and Sala, 2001). In arid environments, there is still a significant debate whether the recruitment of woody species is episodic or a continuous process (Wiegand et al., 2004). The savanna of the Chaco region (Argentina) lacks this basic information. Our objective was to describe the size and frequency of rainfall events in a Chaco savanna. Material and methods Study was located in Experimental Ranch ‘La Maria’ (28 º 3 'S and 64 º 15' W). Mean annual precipitation (1936 to 2006) is 569 mm falling almost in summer (SD 208 mm, interanual CV 36.22%, mean confidence interval 519.88 – 618.11 mm.year-1). A subset of daily precipitation data (1981 to 2006) was classified as discrete size classes with a fixed increment in 8 mm and plotted by the mean value of each class (e.g. first class of more frequent events were the number of rain events between 0 and 8 mm with a mean of 4 mm) (Fig. 1). Results and discussion The frequency curve presented a J form (Fig. 1). The distribution of rainfall events resulted similar to those reported for other arid environments (high frequency of small rainfall events and low frequency of large rainfall events) (Reynolds et al., 2004). Interanual coefficient of variation was larger than that observed for Texan savannas, reported as one of the highest for semiarid regions of the world (Le Houerou and Norwine, cited by Brown and Archer, 1999). In arid environments, important ecological processes such as emergence and recruitment may be affected by changes in frequency of rain events from year to year (Bowers et al., 2004).   Conclusions Since global climate models predict a shift to larger and less frequent rain events due to climate change (Easterling et al., 2000) further studies are necessary to determine to what extent rainfall variability influences tree/grass ratios in semiarid Chaco savannas.