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Title: Perennial plant mortality in the Sonoran and Mojave Deserts in response to severe, multi-year drought

item MCAULIFFE, J. - Desert Botanical Garden
item Hamerlynck, Erik

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2010
Publication Date: 2/12/2010
Citation: Mcauliffe, J.R., Hamerlynck, E.P. 2010. Perennial plant mortality in the Sonoran and Mojave Deserts in response to severe, multi-year drought. Journal of Arid Environments. 74:885-896.

Interpretive Summary: Individual studies have shown that recent protracted drought has resulted in widespread adult plant mortality across the arid Southwestern U.S., but to date, no widespread study has been undertaken to establish the patterns of regional variation in drought effects. We found that plant mortality was far greater in deciduous shrubs that rely primarily on cool season (Oct. – March) than those that rely mainly on warm season (April – Sept) precipitation, and that only the most severely drought-impacted areas of southern eastern California and southwestern Arizona showed appreciable mortality of the evergreen drought-tolerant creosotebush (Larrea tridentata). In addition, overall mortality and canopy die back best matched long-term metrics of drought intensity, which suggested plant mortality played out over areas with 5 years of dramatically reduced precipitation. Evidence of past mortality, likely from the 1950’s drought, showed that mortality in these desert communities are distinctly pulsed in nature. This is important because it suggests population and community structure in these water limited systems are not due to demographic processes that are in equilibrium, but are highly episodic in nature.

Technical Abstract: Severe drought that began in late 1998 caused considerable mortality of shrubs in the Mojave and Sonoran deserts. A region-wide survey in 2003 of 15 sites in southwestern Arizona, southern California, and southern Nevada showed that in some places, mortality of the small, drought-deciduous shrubs Ambrosia deltoidea and A. dumosa was 100% or nearly so. In contrast, the larger, drought-enduring evergreen Larrea tridentata fared much better, although mortality at one site in the southern Mojave Desert eventually reached 65% by 2005. In addition, data from 21 other perennial species showed that other species of smaller, drought-deciduous shrubs suffered the highest mortality. The drought’s impacts were most severe in the southern part of the Mojave Desert and the adjacent Lower Colorado section of the Sonoran Desert in southernmost California and Arizona. Several lines of evidence, including regional analyses of precipitation anomalies and comparison of responses of cool- versus warm-season active, drought-deciduous plants indicated that deficits of cool season precipitation were the most important contributor to plant mortality. Soil conditions influenced plant responses to drought. At lower, more arid elevations, plants tended to be in better condition on weakly-developed soils of sandy to gravelly Holocene alluvial deposits than on soils with strongly developed, clay-enriched horizons of older Pleistocene deposits. This situation reversed in higher, less arid locations, with plants on well-developed soils of older Pleistocene deposits faring better. Across the entire region, the best predictor of plant responses was the Standard Precipitation Index (SPI) calculated for a 60-month time frame ending in March 2003 and indicated the importance of the cumulative effect of multiple, successive years of drought in causing plant mortality or significant declines in condition. SPI calculated for shorter antecedent time frames (6, 12, 24, 36, and 48 month) were not good predictors of plant responses. At one site, numerous remains of dead L. tridentata in an advanced state of decay indicated another significant episode of mortality in the past, possibly during extended drought of the 1950s. The cumulative SPI (60-month time frame) ending in 1957 for the nearest precipitation recording station (SPI = -2.03) showed that the area experienced drought of the intensity capable of causing mortality in L. tridentata. Episodes of drought-induced, perennial plant mortality represent extensive, region-wide ecological disturbances and may be one of the most important processes affecting plant populations and community composition in deserts.