Variation in woody plant δ¹³C along a topoedaphic gradient in a subtropical savanna parkland

δ¹³C values of C₃ plants are indicators of plant carbon-water relations that integrate plant responses to environmental conditions. However, few studies have quantified spatial variation in plant δ¹³C at the landscape scale. We determined variation in leaf δ¹³C, leaf nitrogen per leaf area (N_area), and specific leaf area (SLA) in April and August 2005 for all individuals of three common woody species within a 308 × 12-m belt transect spanning an upland-lowland topoedaphic gradient in a subtropical savanna in southern Texas. Clay content, available soil moisture, and soil total N were all negatively correlated with elevation. The δ¹³C values of Prosopis glandulosa (deciduous N₂-fixing tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) leaves increased 1-4‰ with decreasing elevation, with the δ¹³C value of P. glandulosa leaves being 1-3‰ higher than those of the two shrub species. Contrary to theory and results from previous studies, δ¹³C values were highest where soil water was most available, suggesting that some other variable was overriding or interacting with water availability. Leaf N_area was positively correlated with leaf δ¹³C of all species (p < 0.01) and appeared to exert the strongest control over δ¹³C along this topoedaphic gradient. Since leaf N_area is positively related to photosynthetic capacity, plants with high leaf N_area are likely to have low p _I/p _a ratios and therefore higher δ¹³C values, assuming stomatal conductance is constant. Specific leaf area was not correlated significantly with leaf δ¹³C. Following a progressive growing season drought in July/August, leaf δ¹³C decreased. The lower δ¹³C in August may reflect the accumulation of ¹³C-depleted epicuticular leaf wax. We suggest control of leaf δ¹³C along this topoedaphic gradient is mediated by leaf N _area rather than by stomatal conductance limitations associated with water availability.