Osvaldo A. Fernandez

Effects of Defoliation at Varying Soil Water Regimes on Aboveground Biomass of Perennial Grasses

We evaluated the effects of defoliation under varying soil water regimes on aboveground biomass of perennial grasses native to semiarid rangelands of central Argentina. The palatable species Stipa tenuis and S. clarazii were either defoliated or not for two years within a uniform competitive background of S. gynerioides, an unpalatable species which remained nondefoliated until the end of the study in 1997. Stipa gynerioides increased biomass when soil water was plentiful compared to water stress conditions. This species showed a greater percentage of medium- to large-size plants under rainfed and irrigated conditions than under water stress conditions. Greater shading of S. gynerioides as soil water content increased may have contributed to determine a greater proportion of small-size plants in S. clarazii and of dead plants in S. tenuis in the irrigated than in the water stress treatment. Results suggested that grazing of perennial forage grasses under high soil water contents speeded rather than slowed their degradation when they were competing within a matrix of ungrazed, unpalatable perennial tussock grasses.

Leaf water potential trends in three grasses native to semiarid Argentina.

The natural grazing land of the semiarid region of central Argentina is subject to long and intense periods of drought during the hot season. The aim of the present study was to analyze the pattern of leaf water status for the cool-season grassesPiptochaetium napostaense and Stipa tenuis and the warm-season grass Pappophorum subbulbosum, which are important forage species in this grazing area. Leaf water potential, soil water potential, and relative humidity were measured during 1983 and 1984. At soil water potentials above -0.5 MPa, the leaf water potential of the 3 species showed high values at 0800, after nocturnal rehydratation, and minimum values at 1400, coincident with maximum atmospheric water demand. P. subbulbosum showed a greater capacity to withstand the drop in water potential at 1400 than did the cool-season species. During periods of low soil water potential the range of daily variation in leaf water potential was reduced in the 3 species because of marked decreases in the maximum and minimum values. Leaf water potential in the cool-season species correlated better with the deficit of atmospheric vapor pressure than with soil water potential. Leaf water potential of P. subbulbosum on the other hand showed a higher correlation with soil water potential than with atmospheric vapor pressure deficit.

The effect of water stress on top and root growthin Medicago minima

Abstract Plants of Medicago minima var. minima grown in rootboxes of soil were subjected to water stress treatments in a growth chamber. The soil was returned to field capacity after drying cycles of 7, 14, 21, 28 or 35 days. After a 105-day-study-period water stress significantly decreased (p

Acrolein reduces biomass and seed production of Potamogeton pectinatus in irrigation channels

Chemical weed control with acrolein has been shown to be a lower cost method for reducing submerged plant biomass of sago pondweed in the irrigation district of the Lower Valley of Rio Colorado, Argentina (39°10′S–62°05′W). However, no experimental data exist on the effects of the herbicide on plant growth and its survival structures. Field experiments were conducted during 3 yr to evaluate the effect of acrolein on growth and biomass of sago pondweed and on the source of underground propagules (i.e., rhizomes, tubers, and seeds). Plant biomass samples were collected in irrigation channels before and after several herbicide treatments. The underground propagule bank was evaluated at the end of the third year. Within each treatment, plant biomass was significantly reduced by 40 to 60% in all three study years. Rapid new plant growth occurred after each application; however, it was less vigorous after repeated treatments. At the end of the third year at 3,000 m downstream from the application point, plant biomass at both channels ranged from 34 to 3% of control values. Individual plant weight and height were affected by acrolein treatments, flowering was poor, and seeds did not reach maturity. After 3 yr, acrolein did not reduce the number of tubers. However, they were significantly smaller and lighter. Rhizomes fresh weight decreased by 92%, and seed numbers decreased by 79%. After 3 yr of applications, operational functioning of the channels could be maintained with fewer treatments and lower concentrations of acrolein. Nomenclature: Acrolein; acrylaldehyde; 2-propenal; sago pondweed, Potamogeton pectinatus L. Additional index words: Aquatic weed control, chemical control, submerged plants, propagule banks. Abbreviations DW, dry weight.

Arid and Semiarid Rangelands of Argentina

Two thirds of continental Argentina are arid and semiarid rangelands. These rangelands include five phytogeographic regions: (1) Puna, (2) Chaco Occidental, (3) Monte, (4) Caldenal, and (5) Patagonia. This review includes and begins with a brief overview of the climate, soil, and vegetation characteristics of each region. After that, the major causes of degradation or desertification of these territories are indicated, together with the previous and current impacts on the water resources; the physicochemical and biological soil characteristics, and vegetation structure; productive activities; economy; and society. Fortunately, in contrast with various other arid and semiarid regions in the world, a threshold of losses of renewable natural resources has not yet been reached as a result of such degradation or desertification of the study ecological systems. Beyond this threshold, reestablishment of benefits which could have been obtained from rational (not abusive) utilization of those resources will be impossible. Our aim is an improved land use that allows sustainable production, the magnitude of which will depend upon its previous degree of degradation or desertification.