Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia

Abstract

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44-45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27‰), 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 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 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 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil 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 demonstrated by high soil moisture, earlier rain events were not fully utilized by the current 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 fully developed plant cover, are discussed in relation to climate change and vegetation modelling.

Growth characteristics associated with biomass production in three varieties of Trichloris crinita (Poaceae), a forage grass native to the arid regions of Argentina

“…Both types of adaptations could increase survival in such a dry and rather unfertile site (Grime 1979). Similarly, along an aridity gradient in Patagonia, Schulze et al (1996) found that plant total biomass decreased as precipitation decreased, but belowground biomass decreased at a lower rate than aboveground biomass, resulting in increasing root/shoot ratios. Fernández and Reynolds (2000) studying growth parameters and drought tolerance in eight desert grasses did not find a trade-off between total biomass or root/shoot ratios and drought tolerance.…”

Responses of Natural Vegetation to Different Stages of Extreme Drought during 2009–2010 in Southwestern China

“…With limited water, the quantity of water absorption is a function of root depth. Strong, deep roots can help the plants store and use the water kept in the soil efficiently [52].…”

Photosynthetic physiology of eucalypts along a sub-continental rainfall gradient in northern Australia

“…Understanding physiological responses of plants to environmental resource gradients is important for predicting spatial variation in ecosystem processes and for predicting ecosystem responses to climate change (Koch et al, 1995;Schulze et al, 1996). A strong rainfall gradient exists in northern Australia (Table 1 and Fig.…”