MICROCLIMATE AND UNDERSTORY STRUCTURE OF LIVE OAK (QUERCUS FUSIFORMIS) CLUSTERS IN CENTRAL TEXAS, USA

Plateau live oak (Quercus fusiformis) trees that grow in the upland sa vannas on the Edwards Plateau of central Texas, USA, are thought to se rve as nurse plants for Ashe juniper (Juniperus ashei) and other shrub s. To explore nurse-plant effects of live oaks, we characterized the l ight and temperature conditions in oak clusters with respect to azimut h and canopy positions and related these patterns to woody understory structure and development. The microclimate in oak clusters differed f rom that in the surrounding grasslands, and light attenuation and temp erature lair and soil) varied significantly with both cardinal directi on and canopy position within the oak clusters during August, Septembe r. and October. Understory height was significantly affected by canopy position and generally decreased from the cluster center to the edge of the oak canopy. However, there were no detectable differences in un derstory height or width (i.e., extent of understory development away from the trunk! among the four cardinal directions (N, E, S, W) within the oak cluster Comparisons between oak clusters with and without a w oody understory showed that the presence of the woody understory stron gly affected light penetration to the soil surface but influenced air and soil temperature to a lesser extent. These findings suggest that, although live oak-induced modifications in microclimate can influence understory (i.e.,juniper) growth and development, there is little indi cation that microclimate effects are an important component of facilit ation by live oak nurse trees in mature woody clusters in this savanna ecosystem.