干旱区生态环境知识资源中心

We established a large-scale manipulative experiment in a semi desert grassland on the Santa Rita Experimental Range to determine how the recruitment and physiology of woody plants (Prosopis velutina Woot.) are affected by invasive grasses, seasonal precipitation regimes, and underlying soil characteristics. We established 72 2.8-m(2) plots beneath six large rainout shelters divided evenly between a clay-rich and a sandy loam soil less than 1 km apart. Monospecific stands of the the invasive African grass Eragrostis lehmanniana and the native grass Heteropogon contortus were established into four plots each, and four plots were left bare under each shelter. Our watering protocol simulated 50 percent increases and decreases in average summer precipitation. Here we compare soil water content and temperature in Eragrostis and bare plots during a large, isolated irrigation event that we applied to the plots in June 2002. Daily average and maximum temperatures near the soil surface declined following the irrigation compared to nonirrigated, external plots, and were cooler for several days afterwards. Soil moisture contents declined and maximum soil temperatures increased more rapidly in plots dominated by Eragrostis than in bare plots. Near-surface soil temperatures are apparently too high for establishment of Prosopis seedlings in June prior to the onset of summer rains. Eragrostis may further prevent successful Prosopis establishment by shortening the period over which moisture and soil temperatures are suitable for germination and survival of Prosopis seedlings following a pulse of summer rain.