The Effect of Amount and Frequency of Precipitation on Seedling Establishment and Survival of Lane Mountain Milkvetch (Astragalus jaegerianus Munz)
Final rept. 1 Jun 2005-31 Dec 2006
CALIFORNIA UNIV LOS ANGELES DEPT OF ECOLOGY AND EVOLUTIONARY BIOLOOGY
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The Lane Mountain milkvetch Astragalus jaegerianus Munz is an endangered perennial herb and is one of few plant species narrowly endemic to the central Mojave Desert. Its populations are threatened primarily by physical disturbance and habitat fragmentation. Rare recruitment events and the plants short life span result in extreme fluctuations in population size from high numbers following a successful recruitment year to very low numbers following several years of average or below average precipitation. Our field studies indicate that in addition to total precipitation, frequency and timing of precipitation are critical for successful recruitment. In a greenhouse study, we grew milkvetch under nine artificial rainfall treatments 3 amounts 100, 150, and 200 mmwet season x 3 frequencies 1,2, and 4 times per month of precipitation. Our results indicate that 200 mm of precipitation with a frequency of four times per month is required for seedlings to attain a level of development that enables them to endure a summer drought and resprout in the next growing season. We conclude that exceptionally above average and frequent precipitation is required for recruitment to occur. Such conditions are estimated to occur once in about every 10 years in the central Mojave Desert.