Implications of precipitation redistribution for shifts in temperate savanna ecotones

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 overlooked when predicting the response of woody plants to impending climate change.