roots dynamics in Prosopis flexuosa, a phreatophyte in the Monte Desert (Argentina)

GUEVARA, ARANZAZU; GIORDANO, CARLA V.

Encuentro; VII Colloquim on Plant Ecophysiology; 2011

Universidad de Concepción, Chile

We studied the spatial and temporal dynamics of fine roots of a facultative phreatophyte Prosopis flexuosa in its natural hábitat, the central monte desert, an arid ecosystem with 150 mm of anual rainfall. We wanted to determine the environmental factors that control growth and biomass assignment to fine roots in this species, in two populations that differed in water table accessibility and rainfall dependence: a  valley population that have access to both sources of water, and a dune population that depends enterily on rainfall. We installed rizotrons in the field for two consecutive growing seasons, took core samples of fine roots, measured nutrient and water in the soil, growth, physiological state, reproductive and vegetative phenology and nutrient status of aerial parts of adult trees in both populations. We found that although the dune population was subjected more frequently to water stress than the valley population, growth of branches and leaves were not affected, but  vegetative and reproductive phenology was. Leaves and flowers of trees of the dune population sprouted variablily and associated to rainfall events, while the valley population was more coordinated and independent of rainfalls. Root growth started in January with the start of the rainfall period in both populations, and the growth period that finished by autumm in the dune, lasted to the start of winter in the valley. In both populations peaks of fine root growth were associated to peaks of rainfalls, with ca 10 d of delay on average. Root growth dynamics correlated significantly with rainfall dynamics, but not with soil temperature dynamics. Fine roots located spatially in the soil coincidently with the soil water, that was deeper in the dune and shallower in the valley.  Root length density was similar in both populations, but trees of the dune assigned more biomass to fine roots, producing wider or denser roots than the valley population. Although nitrate, ammonium and phosphorous content in the soil was higher in the valley than in the dune (under tree crowns), leaves of trees from both populations did not differed in the nutritional status. These results suggest that water and not nutrient dynamics is the main control of fine root dynamics in this ecosystem.