Schulze, E.-D., H.A. Mooney, O.E. Sala, E. Jobbagy, N. Buchmann, G. Bauer, J. Canadell, R.B. Jackson, J. Loreti, M. Osterheld, and J. R. Ehleringer. 1996. Rooting depth, water availability,and vegetation cover along an aridity gradient in Patagonia. Oecologia 108:503-511.

TAbove- and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44° to 45° South). Sites, ranging from high annual rainfall with Nothofagus forest (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm rainfall), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, mean above- and belowground biomass decreased as well as leaf area index, but average carbon isotope ratios of sun leaves remained constant (at -27 o/oo), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80 to 0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2 to 3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2 to 3 m soil depth had originated from rain periods earlier in the season or even from past rain seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the top soil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats as seen in high soil moisture, earlier rain events were not fully utilized by the present plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by a fully developed plant cover are discussed in relation to climate change and vegetation modelling.