THE ROLE OF LEAF AND CANOPY-LEVEL GAS-EXCHANGE IN THE REPLACEMENT OF QUERCUS-VIRGINIANA (FAGACEAE) BY JUNIPERUS-ASHEI (CUPRESSACEAE) IN SEMIARID SAVANNAS

Photosynthesis, transpiration, and leaf area distribution were sampled in mature Quercus virginiana and Juniperus ashei trees to determine t he impact of leaf position on canopy-level gas exchange, and how gas e xchange patterns may affect the successful invasion of Quercus communi ties by J. ashei. Sampling was conducted monthly over a 2-yr period in 12 canopy locations (three canopy layers and four cardinal directions ). Photosynthetic and transpiration rates of both species were greates t in the upper canopy and decreased with canopy depth. Leaf photosynth etic and transpiration rates were significantly higher for Q. virginia na (4.1-6.7 mu mol CO2 . m(-2). s(-1) and 1.1-2.1 mmol H2O . m(-2). s( -1)) than for J. ashei (2.1-2.8 mu mol CO2 . m(-2). s(-1) and 0.7-1.0 mmol H2O . m(-2). s(-1)) in every canopy level and direction. Leaves o n the south and east sides of both species had higher gas exchange rat es than leaves on the north and west sides. Although Quercus had a gre ater mean canopy diameter than Juniperus (31.3 vs. 27.7 m(2)), J. ashe i had significantly greater leaf area (142 vs. 58 m(2)/tree). A simple model combining leaf area and gas exchange rates for different leaf p ositions demonstrated a significantly greater total canopy carbon diox ide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g CO2 . tree(-1). d(-1), respectively). Total daily water loss was also great er for Juniperus (125 vs. 73 L . tree(-1). d(-1)). Differences in leaf gas exchange rates were poor predictors of the relationship between t he invasive J. ashei and the codominant Q. virginiana. Leaf area and l eaf area distribution coupled with leaf gas exchange rates were necess ary to demonstrate the higher overall competitive potential of J. ashe i.