The partitioning of water uptake between growth forms in a Neotropical savanna: do herbs exploit a third water source niche?

In addition to trees and grasses, the savannas of central Brazil are characterised by a diverse herbaceous dicot flora. Here we tested whether the coexistence of a highly diversified assemblage of species resulted in stratification or strong overlap in the use of soil water resources. We measured oxygen and hydrogen isotope ratios of stem water from herbs, grasses and trees growing side by side, as well as the isotopic composition of water in soil profile, groundwater and rainfall, and predawn (Ψ(pd)) and midday (Ψ(md)) leaf water potentials. We used a stable isotope mixing model to estimate vertical partitioning of soil water by the three growth forms. Grasses relied on shallow soil water (5-50 cm) and were strongly anisohydric. Ψ(pd) and Ψ(md) decreased significantly from the wet to the dry season. Trees extracted water from deeper regions of the soil profile (60-120 cm) and were isohydric. Ψ(pd) and Ψ(md) did not change from the wet to the dry season. Herbs overlapped with grasses in patterns of water extraction in the dry season (between 10 and 40 cm), but they took up water at soil depths intermediate (70-100 cm) to those of trees and grasses during the wet season. They showed seasonal changes in Ψ(pd) but not in Ψ(md). We conclude that vertical partitioning of soil water may have contributed to coexistence of these three growth forms and resulted in a more complex pattern of soil water extraction than the two-compartment model of soil water uptake currently used to explain the structure and function of tropical savanna ecosystems.

Variation in the use of orchid extrafloral nectar by ants.

The relation between ant-plant specificity and the use of host plants as a resource was investigated in the facultative, myrmecophytic orchid, Caularthron bilamellatum (Rchg.f.) Schult. Using stable isotopes, we determined the portion of the ants' diets derived from host plants. We documented that six ant species inhabiting the orchid: (1) derived nutritional benefit from host orchids, and (2) had species-specific levels of extrafloral nectar use. Proportionate contribution of extrafloral nectar to ant diets ranged from 11 to 48%. These results demonstrate extreme interspecific differences in the nutritional benefits received by ants from host orchids. Interspecific differences in nutritional benefits from orchid nectar may be affected by colony size, nutritional needs, behavioral ecology of the ants, and the abundance of alternate food sources.