Stable isotope analysis was used to determine sources of water used by coexisting trees and grasses in a temperate savanna dominated by Quercus emoryi Torr. We predicted that (1) tree seedlings and bunchgrasses utilize shallow sources of soil water, (2) mature savanna trees use deeper sources of water, and (3) trees switch from shallow to deep water sources within 1 year of germination. We found that Q. emoryi trees, saplings, and seedlings (about 2 months, 1 year, and 2 years old), and the dominant bunchgrass [Trachypogon montufari (H.B.K.) Nees.] utilized seasonally available moisture from different depths within the soil profile depending on size/age relationships. Sapling and mature Q. emoryi acquired water from >50 cm deep, 2-month-old seedlings utilized water from <15 cm, and 1- and 2-year-old seedlings and grasses used water from between 20 cm and 35 cm. This suggests that very young seedlings are decoupled from grasses in this system, which may facilitate germination and early establishment of Q. emoryi within extant stands of native grasses. The potential for subsequent interaction between Q. emoryi and native grasses was evidenced by similar patterns of soil water use by 1- and 2-year-old seedlings and grasses. Q. emoryi seedlings did not switch from shallow to deep sources of soil water within 2 years of germination: water use by these seedlings apparently becomes independent of water use by grasses after 2 years of age. Finally, older trees (saplings, mature trees) use water from deeper soil layers than grasses, which may facilitate the stable coexistence of mature trees and grasses. Potential shifts in the seasonality of precipitation may alter interactions between woody plants and grasses within temperate savannas characterized by bimodal precipitation regimes: reductions in summer precipitation or soil moisture may be particularly detrimental to warm-season grasses and seedlings of Q. emoryi.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. In contrast to documented increases in woody plant dominance of former savannas and grasslands of North America, ecotones between oak (Quercus L.) woodlands and semi-desert grasslands of the southwestern United States and northwestern Mexico have been relatively stable over the past several centuries. Soil resource partitioning, wherein shallow-rooted grasses use summer precipitation and deep-rooted woody plants use winter precipitation, may have contributed to the stable coexistence of grasses and trees that form savannas at this ecotone. Thus, predicted changes in regional precipitation patterns and soil moisture caused by anthropogenic trace gas emissions have the potential to alter interactions between woody plants and grasses with potential ramifications for their relative abundance and distribution.We used a field experiment to investigate the response of the dominant woodland and savanna tree Quercus emoryi to simulated potential scenarios of precipitation redistribution within the context of shifts in the woodland-grassland ecotone. Experimental soil volumes isolated from ambient precipitation and soil moisture were hand-watered between July 1994 and October 1996. Control plots received mean annual precipitation (602 mm), whereas treated plots received all possible combinations of 50% additions and reductions to mean summer and winter precipitation (n = 4). Increases in summer precipitation increased seedling emergence and recruitment rates as much as threefold, whereas emergence and recruitment were independent of altered winter precipitation regimes. Seedling survival, size, growth, and biomass allocation were largely independent of shifts in seasonal precipitation regimes.We conclude that Q. emoryi recruitment and subsequent abundance and distribution would more likely be affected by changes in summer precipitation than changes in winter precipitation. For example, increases in summer precipitation that increase rates of seedling emergence and establishment may facilitate downslope shifts in the woodland-grassland ecotone. Similarly, this mechanism may explain downslope shifts in this ecotone that occurred coincident with particularly high summer precipitation during the "Medieval Warm" period, 645-1295 yr BP. Further, whereas soil moisture resource partitioning between adult Q. emoryi and coexisting grasses may contribute to savanna maintenance, soil resource partitioning does not occur within the first three growing seasons after Q. emoryi germination. Results illustrate the importance of consideration of the regeneration niche, which is often overlo...

In southeastern Arizona, Prosopis juliflora (Swartz) DC. and Quercus emoryi Torr. are the dominant woody species at grassland/woodland boundaries. The stability of the grassland/woodland boundary in this region has been questioned, although there is no direct evidence to confirm that woodland is encroaching into grassland or vice versa. We used stable carbon isotope analysis of soil organic matter to investigate the direction and magnitude of vegetation change along this ecotone. δC values of soil organic matter and roots along the ecotone indicated that both dominant woody species (C) are recent components of former grasslands (C), consistent with other reports of recent increases in woody plant abundance in grasslands and savannas throughout the world. Data on root biomass and soil organic matter suggest that this increase in woody plant abundance in grasslands and savannas may increase carbon storage in these ecosystems, with implications for the global carbon cycle.

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