Drought induced mortality and dieback in desert perennial plants LUKE, TL., HOUSMAN, ML., ANDERSON, RA. Western Washington University Biology Department Introduction Desert scrub habitat in the Sonoran Desert of southern California flourished with healthy perennials, visually observed, measured, and photographed by RA Anderson from the late 1970’s through In 2002 and 2003, the desert scrub plant communities suffered a severe “once-in-a-century” (Figure 3) drought—perhaps associated with global climate change—and as a result, the mortality and dieback rates of perennial plants spiked throughout the Mojave and Sonoran Deserts in 2002 (Breshears et al 2005; Hereford et al 2006, Mitri, et al 2007). In 2005, after near-record rains, the plants already appeared to be in recovery from dieback on our study site (see photos below) and in a higher elevation site studied by others (Mitri, et al 2007) in Joshua Tree National Park. And after another substantial rainy season, in spring 2010 we were able to document and measure over 1400 dead and live plants, and documented plant-to-plant and plant-to-wash distances on a 4 hectare plot on which the plants had been mapped and measured in the early 1980’s. Study Site In the Colorado River Valley Subdivision of the Sonoran Desert Scrub Biome, in the Chuckwalla Valley, in Riverside County (33.73° N, ° W) near Desert Center, CA, which is halfway between Los Angeles, CA and Phoenix, AZ. The 4 ha study site is on a bajada, which is the coalescing of several shallowly sloping alluvial fans, forming a broad spread of sand, clay, and gravel. Many shallow, ephemeral riparian washes interlace across this broad alluvial fan (which is somewhat reminiscent of permanent, braided stream habitats in the southeastern USA). The study plot is dominated numerically and spatially by the ironwood tree Olneya tesota, the creosote bush Larrea tridentata, and bursage or Ambrosia, Ambrosia dumosa (see photos). The site is somewhat ecotonal, between the Larrea-Ambrosia flats to the southeast and the Olneya-associated, deeper washes to the southwest. The study plot is dissected by many shallow washes, which total to about 25.8% of the 4 ha plot. Methods Documented on-site observations in , 1990, 1991, 2000, 2005, and In 1983 and 2010, surveyed and mapped every perennial on 4 hectare plot including: Length, width, height Potential perennial competitors within 4 meters Distance to nearest ephemeral wash The height and diameter of the drought-killed stems compared with the regrowth can be markedly different. Note the smiling visage of 5’6 ” tall researcher T.L. Luke for size perspective. Larrea tridentata recovery from drought, represented by two typical individuals in (a) 2005 and (b) in The extent of recovery (the proportion of diameter dead to diameter live) was significantly greater when the nearest neighbor was not another L. tridentata. (N = 221, F = 5.01, p= 0.026) Fully recovered Ambrosia dumosa in Figure 4. Desert Center received 27mm less rainfall per year than the mean of Hayfield Reservoir, Blythe Airport, and Eagle Mountain from 2004 to Data from NCDC weather stations. Figure 3. The cumulative annual precipitation from 1975 to 2009 from the mean calculated by NCDC weather stations at Hayfield Reservoir, Blythe Airport, and Eagle Mountain, all within 70 km of Desert Center drought Figure 2. Average annual temperature from 1975 to 2009 for three NCDC weather stations within 70 km of the study plot: Hayfield Reservoir (W and 415m elevation), Blythe Airport (E at 120m elevation), and Eagle Mountain (NW and 290m elevation) Figure 1. Annual temperatures in Desert Center (250m elevation) are 0.5 to 1°C higher than the average recorded at the nearby weather stations at Hayfield Reservoir, Blythe Airport, and Eagle Mountain. 10m N Figure 7: Example overview of a portion of the study plot, on a bajada in the Chuckwalla Valley. Notice the distribution of perennials on the edge in comparison to the center of the islands. Blue signifies the ephemeral washes (water flow is from southwest to northeast) and white represents the islands. Green, purple, and black circles correspond to Olneya tesota, Larrea tridentata, and Ambrosia dumosa, respectively. Discussion 2002 incurred a severe drought unlike any other in the past 35 years (Figure 3). In response A. dumosa, L. tridentata, and O. tesota suffered dieback, but the two latter species have since recovered to a greater extent than A. dumosa. A. dumosa located on the edge of a wash are more likely to survive (Table 2). L. tridentata exhibits a similar pattern in recovery regimes (Table 4). Recovery rates of L. tridentata reflect intraspecific competition (see photo of L. tridentata). However, interspecific competition was not significant between any combination of the three perennials. The complete 2010 plot survey provides us with possibilities to detail for the first time an ephemeral riparian zone. Across all plant species, individuals occupy a greater percentage of the edge of a wash with access to water even though there is less total available area at the edge than in the center of islands (Figure 5). Further analyses of O. tesota may reveal a positive influence on other species specifically on the northern shady perimeter. A larger sample size of O. tesota from the outer 50 m of the plot perimeter will allow analyses of dieback and mortality in island centers. A. dumosa dieback and recovery may be correlated to size and areas of high density of A. dumosa. The great basin desert scrub habitat ranging from southern California to southeast Oregon provides the potential for a 1500 km and 1000 m elevation gradient for a comprehensive trophic study. The report we provide here is just the beginning. Figure 6: The percentage of live and dead (drought-killed) perennials in the Chuckwalla Valley. Total numbers of plants sampled: A. dumosa = 741, L. tridentata =502 and O. tesota =69. Adjacent bars with different letters (a,b,c,d) indicate statistically significantly differences, based on chi-square analyses. a b c d c d Center Edge SDMean Table 4: The extent of recovery from drought by Larrea tridentata (amount of regrowth as proportion of pre-drought diameter) on island edge versus island centers. (N=316) (t-value = , p = 0.001) 66.40%84.10%Dead EdgeCenter Table 2. Mortality was higher for Ambrosia dumosa near island centers than along island-wash edges. Chi-square analysis revealed statistically significant difference. (N=730, p = ) Olneya tesota (a) mortality due to wood harvest (b) mortality due to isolation from wash and (c) live in Figure 5: Where perennials grow on plot. Although most of the non-wash habitat is not edge, most plants grow at the edge of washes. The center of an island for A. dumosa, L. tridentata, and O. tesota was defined at 1.33 m, 2m, and 3m from the wash and inward, respectively. Sample sizes are A. dumosa = 730, L. tridentata =499, and O. tesota =68 Table 1. Wash features. N=919 measurements near plants on site Range SD Mean Width (cm)Depth (cm) References Breshears, D.D., et al Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences (USA) 102: Hereford, R., et al Precipitation history and ecosystem response to multidecadel precipitation variability in the Mojave Desert region, Journal of Arid Environments. 67: Miriti, M.N., et al Episodic death across species of desert shrubs. Ecology. 88: Acknowledgements: We wish to thank LJ Grant and WB Boyle for their assistance in data collection. The water in the Chuckwalla Valley flows northeastward from upslope and from the Chuckwalla Mountains (and under the I-10 freeway) past the study area and ends up on the Ford Dry Lake at the valley floor. Chuckwalla Mts. Washes are shallow on plot a b