WATER RELATIONS OF JUNIPERUS-VIRGINIANA AND ANDROPOGON-GERARDII IN ANUNBURNED TALLGRASS PRAIRIE WATERSHED

We assessed several factors influencing the success of Juniperus virgi niana in unburned tallgrass prairie by mapping the distribution of ind ividuals in a 21-ha watershed on the Konza Prairie Research Natural Ar ea in NE Kansas. We also compared the water relations of this tree wit h those of the dominant grass Andropogon gerardii. No general relation ships were detected between the distribution of J. virginiana and plan t water status, plant size, soil type, slope, aspect or elevation. Lea f xylem pressure potential (psi) in upland trees was significantly low er (0.44 MPa) than in lowland trees, but only during the driest portio n of the growing season. In contrast, topographic position influenced psi more strongly in A. gerardii with psi in upland plants as much as 1.0 MPa lower than in lowland plants. During wetter periods, photosynt hetic rates in A. gerardii were higher than in J. vigriniana in upland s, but the reverse occurred during the driest period of the growing se ason. The ability of J. virginiana to maintain higher psi and higher p hotosynthetic rates relative to A. gerardii during periods of water li mitation may contribute to its success in tallgrass prairie protected from fire.