Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia
Authors: Schulze, ED, HA Mooney, OE Sala, E JobbÃ¡gy, N Buchmann, G Bauer, J Canadell, RB Jackson, J Loreti, M Oesterheld, JR Ehleringer
.Above- and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44 deg to 45 deg South) ranging from high annual rainfall with Nothofagus forest (770 m) to Nothofagus scrub (560 mm), Festuca- (290 mm) and Stipa- (160 mm) grasslands and into desert vegetation (125 mm rainfall) in order to test if rooting depth compensates for low rainfall. Along this gradient there were decreases in mean above- and belowground biomass as well as in leaf area index, but average carbon isotope ratios of sun leaves remained constant (at -27 o/oo) indicating no major differences in water use efficiency 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 oxygen and hydrogen isotopic ratios of soil water showed distinct patterns of repeated enrichments and depletions of the heavy isotope during evaporation and rainfall events respectively. The isotope ratios indicated that moisture at 2 to 3 m soil depth had originated from rain periods earlier in the season or even from past rain seasons. This study cannot explain the vegetation zonation on the basis of water supply to the existing plant cover because high soil moisture from earlier rain events was not fully utilized by the plant cover during summer drought, and water was accessible to roots in deeper soil layers in all habitats. 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, is discussed in relation to climate change and vegetation modeling.