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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 (Narea), 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 Narea 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 Narea is positively related to photosynthetic capacity, plants with high leaf Narea 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 Narea rather than by stomatal conductance limitations associated with water availability. [ABSTRACT FROM AUTHOR]

This study tested the hypothesis that grasses with the C 4 photosynthetic pathway are avoided as a food source by insect herbivores in natural communities. Insects were sampled from ten pairs of C 3 -C 4 grasses and their distributions analyzed by paired comparisons tests. Results showed no statistically significant differences in herbivore utilization of C 3 -C 4 species. However, there was a trend towards heavier utilization of C 3 species when means for both plant groups were compared. In particular, Homoptera and Diptera showed heavier usage of C 3 plants. Significant correlations between insect abundances and plant protein levels suggest that herbivores respond to the higher protein content of C 3 grasses. δ 13 C values for six of the most common grasshopper species in the study area indicated that three species fed on C 3 plants, two species fed on C 4 plants, and one species consumed a mixture of C 3 and C 4 tissue.