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 the impact of leaf position on canopy‐level gas exchange, and how gas exchange patterns may affect the successful invasion of Quercus communities 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 greatest in the upper canopy and decreased with canopy depth. Leaf photosynthetic and transpiration rates were significantly higher for Q. virginiana (4.1–6.7 μmol CO2·m−2·s−1 and 1.1–2.1 mmol H2O·m−2·s−1) than for J. ashei (2.1–2.8 μmol CO2·m−2·s−1 and 0.7–1.0 mmol H2O·m−2·s−1) in every canopy level and direction. Leaves on the south and east sides of both species had higher gas exchange rates than leaves on the north and west sides. Although Quercus had a greater mean canopy diameter than Juniperus (31.3 vs. 27.7 m2), J. ashei had significantly greater leaf area (142 vs. 58 m2/tree). A simple model combining leaf area and gas exchange rates for different leaf positions demonstrated a significantly greater total canopy carbon dioxide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g CO2·tree−1·d−1, respectively). Total daily water loss was also greater for Juniperus (125 vs. 73 Ltree−1·d−1). Differences in leaf gas exchange rates were poor predictors of the relationship between the invasive J. ashei and the codominant Q. virginiana. Leaf area and leaf area distribution coupled with leaf gas exchange rates were necessary to demonstrate the higher overall competitive potential of J. ashei.