Water use by woody plants on contrasting soils in a savanna parkland: assessment with delta H-2 and delta O-18

In savanna parklands of southern Texas, patches of grassland and 'discrete clusters' of small trees and shrubs occur on sandy loam surface soils under lain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in ' groves' occur on deep (2 m) sandy loam soils without an argillic horizon. d elta(2)H and delta(18)O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in p lant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs, wooded landscap e elements. Groundwater was isotopically similar to weighted rainfall, sugg esting local recharge at this site. Linear regressions of soil water delta( 2)H on delta(18)O yielded slopes less than the meteoric water line, indicat ing significant evaporative losses of soil moisture in all landscape elemen ts. Interspecific differences in root density distribution were significant ; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. delta(2)H and delta(18)O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the pr ofile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to h ave a functional significance that was not revealed by biomass or density d eterminations. Root densities of species in discrete clusters (claypan pres ent) were typically greater than those of the same species in groves (clayp an absent), especially in the upper 80 cm of the soil profile. Consistent w ith rooting profiles, delta(2)H and delta(18)O values of plant water indica ted that trees and shrubs in discrete clusters with fine-textured subsoils obtained most of their water at depths <1.5 m. As with groves, there was no indication of water resource partitioning between species. In summary, we saw no isotopic evidence that co-occurring woody plants at this savanna par kland site were partitioning soil moisture vertically during late summer/ea rly fall, despite marked differences in their root density distributions. T his supports other lines of evidence which indicate that species interactio ns in tree/shrub clumps are competitive, and that species composition is th erefore unstable in those landscape elements.