Comparison of reproductive timing to snow conditions in wild onions and White-crowned Sparrows at high altitude

-Timing of reproduction was assessed for wild onions and White-crowned Sparrows in relation to snow conditions on the same subalpine meadow in the Sierra Nevada for 21 years. Flowering date and clutch 'initiation date were both highly correlated with snow conditions, being later as snowpack was deeper. Interannual variation in schedule was 46 days for onions and 33 days for sparrows. There was nearly a fivefold difference in snowpack depth, and date of snow disappearance varied interannually by 72 days. Compensation for late-lying snows occurred in both species but was greater in sparrows than in onions because the nest-building behavior of sparrows was flexible. In years of deeper snow, sparrows were able to lay eggs earlier because they built more nests than usual in trees and shrubs rather than waiting for grou~dcover to develop. Key worM: Allium, Zonotrichia, snowpack, high altitude, proximate factors, reproduction. Montane settings are useful for the study of environmental adaptation in organisms because their brief, sharply delimited growing seasons and variable climates can be potent agents of natural selection. Diurnal and seasonal cycles of abiotic factors, principally air temperature (Ta), moisture, and wind speed, shift in level and amplitude as elevation increases (Rosenberg 1974). The resulting decrease in mean Ta, high winds, decreased availability of soil moisture due to freezing, and variable snowpackcan greatly influence the phenology, distribution, and productivity of plants (Billings and Bliss 1959, Scott and Billings 1964, Weaver and Collins 1977, Ostler et al. 1982). Annual schep.ules of reproduction and surVival of hibernating mammals (Morton and Sherman 1978) as well as reproductive success ofboth sedentary (Clarke and Johnson 1992) and migratory birds (Morton 1978, Smith and Andersen 1985) also are known to be affected, especially by spring storms and snowpack depth. It follows that long-term studies of the annual rhythm of reproduction of organisms at high altitude should provide valuable information for understanding the pathways and scope of adaptations to climatic conditions and for determining the efficacy of environmental variations to act as cues or proximate factors in the· control of reproduction schedules. Such studies are few, however, and, to my knowledge, there are none that have compared plants and animals on the same study area. Herein I present 21 seasons of data that index reproductive schedules of the wild onion (Allium validum) and the Mountain White~crowned Sparrow (Zonotrichia leucophrys oriantha) at the same location in the Sierra Nevada in relation to interannual variations in snow conditions. MATERIALS AND METHODS The study site, Tioga Pass Meadow (TPM), is a subalpine meadow with an area of about 50 ha (0.5 X 1.0 km) and elevation of 3000 m located in the upper end of Lee Vining Canyon, Mono County, California. It is bounded by Tioga Lake on the northern edge and the boundary of Yosemite National Park at Tioga Pass on the southern edge. Allium validum, which grows in large clumps in wet meadows at elevations of 1200-3350 m in the Sierra Nevada, is usually 0.5-1.0 m in height and has numerous small (6-10 mm) flowers organized into teIDlinal umbels (Munz 1970). For 21 summers, 1968-70, 1973, 1976, and 1978~93, when I was on TPM daily, I kept notes on the flowering schedule of one particular, fairly compact patch of A. validum that covered an area of about 0.1 ha (.2.5 X 40 m) near the center of TPM and that usually IBiology Department, Occidental College, Los Angeles, California 90041-3392.