The 2008 Joint Meeting of the Society for Range Management and the America Forage and Grassland Council. |
Tuesday, January 29, 2008 - 3:40 PM
Grass and Juniper Responses to Modified Precipitation Seasonality and Warming
David D. Briske1, Mark G. Tjoelker1, and Astrid Volder2. (1) Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX 77843, (2) Department of Horticultural Sciences, Texas A&M University, 2133 TAMU, College Station, TX 77843-2133
The growth, physiology, and competitive ability of a C4 grass (little bluestem; Schizachyrium scoparium) and juniper (eastern redcedar; J. virginiana) were evaluated in a field experiment simulating climate change scenarios of intensified summer drought and increased spring and autumn rainfall, both with and without warming. The C4 grass showed a relatively neutral response to the climate change scenarios. Although aboveground biomass and leaf-level gas exchange tended to decrease in response to intensified summer drought, plant basal area and tiller density were unaffected. However, intensified summer drought did reduce the number of reproductive tillers. Surprisingly, the C4 grass was unresponsive to warming even when precipitation was shifted from summer to spring and autumn. In contrast, juniper showed reduced growth and leaf-level gas exchange in response to the intensified summer drought and increased spring and autumn rainfall, but a substantial growth increase to warming. Warming increased stem height and diameter growth both in monoculture and mixture with the grass and both with and without precipitation redistribution. Juniper gas exchange rates increased in response to warming only when grown with grass, but not in monoculture. Grass-juniper interactions shifted from facilitation during the spring to competition in the summer and autumn. Intensified summer drought accentuated competition effects on leaf gas exchange and growth, but the wetter spring and autumn did not enhance facilitation. The grass showed reduced tiller density and above-ground biomass when grown in mixture with juniper. We conclude that the positive growth response of juniper to warming will more then offset the negative response to intensified drought to enhance the potential for continued encroachment. C4 grasses will likely persist in savannas characterized by warmer, drier climates, but reduced production may compromise their competitive ability to further promote juniper expansion.