ROOTING DEPTH, WATER AVAILABILITY, AND VEGETATION COVER ALONG AN ARIDITY GRADIENT IN PATAGONIA

Above- and belowground biomass distribution, isotopic composition of s oil and xylem water, and carbon isotope ratios were studied along an a ridity gradient in Patagonia (44-45 degrees S). Sites, ranging from th ose with Nothofagus forest with high annual (770 mm) to Nothofagus scr ub (520 mm), (290 mm) and Stipa (160 mm) grasslands and into desert ve getation (125 mm), were chosen to test whether rooting depth compensat es for low rainfall. Along this gradient, both mean above- and belowgr ound biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at -27 parts per thousand), i ndicating no major differences in the ratio of assimilation to stomata l conductance at the time of leaf growth. The depth of the soil horizo n that contained 90% of the root biomass was similar for forests and g rasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m ). In all habitats, roots reached water-saturated soils or ground wate r at 2-3 m depth. The depth profile of oxygen and hydrogen isotope rat ios of soil water corresponded inversely to volumetric soil water cont ents and showed distinct patterns throughout the soil profile due to e vaporation, water uptake and rainfall events of the past year. The iso tope ratios of soil water indicated that high soil moisture at 2-3 m s oil depth had originated from rainy periods earlier in the season or e ven from past rainy seasons, Hydrogen and oxygen isotope ratios of xyl em water revealed that all plants used water from recent rain events i n the topsoil and not from water-saturated soils at greater depth. How ever, this study cannot explain the vegetation zonation along the tran sect on the basis of water supply to the existing plant cover. Althoug h water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not f ully utilized by the current plant cover during summer drought. The ro le of seedling establishment in determining species composition and ve getation type, and the indirect effect of seedling establishment on th e use of water by fully developed plant cover, are discussed in relati on to climate change and vegetation modelling.