Implications of precipitation redistribution for shifts in temperate savanna ecotones

In contrast to documented increases in woody plant dominance of former sava nnas and grasslands of North America, ecotones between oak (Quercus L.) woo dlands and semi-desert grasslands of the southwestern United States and nor thwestern Mexico have been relatively stable over the past several centurie s. Soil resource partitioning, wherein shallow-rooted grasses use summer pr ecipitation and deep-rooted woody plants use winter precipitation, may have contributed to the stable coexistence of grasses and trees that form savan nas at this ecotone. Thus, predicted changes in regional precipitation patt erns and soil moisture caused by anthropogenic trace gas emissions have the potential to alter interactions between woody plants and grasses with pote ntial ramifications fur their relative abundance and distribution. We used a field experiment to investigate the response of the dominant wood land and savanna tree Quercus emoryi to simulated potential scenarios of pr ecipitation redistribution within the context of shifts in the woodland-gra ssland ecotone. Experimental soil volumes isolated from ambient precipitati on 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 precipit ation increased seedling emergence and recruitment rates as much as threefo ld, whereas emergence and recruitment were independent of altered winter pr ecipitation regimes. Seedling survival, size, growth, and biomass allocatio n were largely independent of shifts in seasonal pre cipitation regimes. We conclude that Q. emoryi recruitment and subsequent abundance and distrib ution would more likely be affected by changes in summer precipitation than changes in winter precipitation. For example, increases in summer precipit ation that increase rates of seedling emergence and establishment may facil itate downslope shifts in the woodland-grassland ecotone. Similarly, this m echanism may explain downslope shifts in this ecotone that occurred coincid ent with particularly high summer precipitation during the "Medieval Warm" period, 645-1295 yr BP. Further, whereas soil moisture resource partitionin g between adult Q. emoryi and coexisting grasses may contribute to savanna maintenance, soil resource partitioning does not occur within the first thr ee growing seasons after Q. emoryi germination. Results illustrate the impo rtance of consideration of the regeneration niche, which is often overlooke d when predicting thr response of woody plants to impending climate change.