Carlos Alberto Busso

Drought and clipping effects on tiller demography andgrowth of two tussock grasses in Utah

Abstract Tiller demography and growth were determined for clipped and unclippedplants of crested wheatgrass (Agropyron desertorum) and bluebunch wheatgrass Pseudoroegneria spicata under drought, natural or irrigated conditions from 1984 until 1986. Mild water stress during the 1984 growing season did not reduce herbage accumulation at the end of that season on plants of both species. However, green leaf number, rate of leaf initiation, height and total green leaf area were all reduced on tillers of both species when predawn leaf xylem pressure potentials fell below — 2·5 MPa during two or more growth periods. In the 3rd year of repeated treatments, the lowest daughter tiller production and growth were observed under the simultaneous influence of drought and clipping. Repeated late and severe grazing of these species under long-term droughts (2 or more years) could then be expected to rapidly reduce their persistence in the community.

Conservation value of timber quality versus associated non-timber quality stands for understory diversity in Nothofagus forests

Conservation strategies of forested landscapes must consider biodiversity of the included site types, i.e. timber-quality forests and associated non-timber-quality stands. The objectives were to characterize forest overstory structure in timber-quality versus associated non-timber-quality stands; and to compare their understory communities. Six forest types were sampled in Nothofagus forests of Tierra del Fuego (Argentina): two timber-quality N. pumilio forests, and four associated non-timber-quality stands (edge, N. antarctica, wetlands and streamside forests). Overstory structure and understory vegetation (species richness, frequencies, cover and biomass) were characterized during spring and summer seasons. Analysis of variance and multivariates were carried out. Overstory structure differed across the site types, with higher tree size, canopy closure and tree volume in timber-quality stands. Fifty-one understory plant species were observed, but understory variables varied with site types, especially wetlands (highest native and exotic richness, cover and biomass, and 25% of exclusive species). Site types were grouped in three: N. antarctica stands, streamside stands and the other N. pumilio forests according to multivariate analysis. Forty three percent of plants were distributed in all site types, and all timber-quality forest understory species were present in some associated non-timber-quality stands. Timber-quality N. pumilio forests have a marginal value for understory conservation compared to associated non-timber-quality stands, because these last include all the plants observed in timber-quality forests and also possess many exclusive species. Therefore, protection of associated non-timber-quality stands during forest management planning could increase understory conservation at landscape level, and these could be better reserves of understory diversity than retentions of timber-quality stands.

Defoliation tolerance and ammonium uptake rate in perennial tussock grasses

Abstract Stipa clarazii, Ball. has been shown to be more tolerant to defoliation and a superior competitor to S. tenuis Philo and S. ambigua Speg. 3 perennial grasses native to semiarid rangelands in central Argentina. Mechanisms contributing to its great defoliation tolerance and competitive ability, however, are largely unexplored. We examined tolerance to defoliation and ammonium uptake rates on defoliated and undefoliated plants of those species at 10, 25, and 50 ppm NH4+ using (NH4)2SO4 solutions containing 60 atom %15N excess. By mid-spring, greater regrowth following defoliation in S. clarazii than in S. tenuis or S. ambigua indicated greater defoliation tolerance in the first than in the other 2 species. Stipa clarazii had similar of higher ammonium uptake rates than S. tenuis and S. ambigua. Higher ammonium uptake rates in S. clarazii thus appear to be one of the mechanisms most likely contributing to its greater competitive ability and defoliation tolerance when compared to the other 2 species. Defoliated plants of all 3 species had similar or greater ammonium uptake rates than undefoliated plants. These results suggest that photosynthetic canopy reestablishment may be achieved without sacrificing root function in these perennial grasses, at least as long as carbon reserves do not become a limiting factor. Ammonium uptake rates increased when NH4+ concentrations increased in the labeled solutions in S. clarazii, S. tenuis and S. ambigua. This result demonstrates the capacity of the root system for increasing nutrient acquisition during periods of high resource availability.

Direct assessment of competitive ability and defoliation tolerance in perennial grasses

Stipa clarazii is a dominant, late-seral perennial grass species under exclosure or lightly grazed rangelands in semiarid Argentina, whereas S. tenuis and S. ambigua are earlier-seral perennial tussock grasses. Recent studies have demonstrated that late-seral species are more competitive and can have either similar or greater herbivory tolerance than earlier-seral species. We hyphothesized that (1) tolerance to defoliation is greater on defoliated plants of S. clarazii than on those of S. tenuis and S. ambigua, (2) competitive ability is greater in S. clarazii than in S. tenuis and S. ambigua when plants remain undefoliated or are exposed to a comparable defoliation intensity, and (3) competitive ability decreases in S. clarazii when this species is selectively defoliated within a non-defoliated, nearby neighbourhood of S. tenuis or S. ambigua. The study objectives included obtaining a direct measure of competitive ability and defoliation tolerance in the three perennial grasses when they grew either in i...

Root proliferation in perennial grasses of low and high palatability

Root proliferation of desirable (Stipa clarazii andS. tenuis) and undesirable (S.ambigua)perennial grasses was studied in semiarid rangelands of Central Argentina(40°39′S, 62°54′W) in 1998. On 17 September, soil coreswereremoved from the edge of the plant, metal structures lined with screen mesh(hereafter called bags) were buried in the holes, and root-free soil was placedinto these structures. Numbers of green tillers and circumference per plant hadpreviously been determined. Since plants were of unequal size among species,root length and root dry weight data are reported on a per green tiller basis.Half of the plants was defoliated to 5 cm stubble height on 17September and/or 12 October, while the other half remained undefoliated(controls). Bags were destructively harvested either 20 days after the firstdefoliation (first sampling) or 56 days after the second defoliation (secondsampling) by digging out soil very carefully around each bag. Roots were washedfrom soil, root length estimated by the line intercept method, root dry weightdetermined after oven-drying, and root length per unit root dry weightcalculated from the two measured variables. Root length and dry weight weremorethan 96% greater on defoliated and undefoliated plants ofS. clarazii than on those of S.tenuisor S. ambigua for both sampling dates. Root length perunitroot dry weight, however, was more than 43% greater (p < 0.05) inS. tenuis than in S. clarazii andS. ambigua during the second sampling. Defoliated plantshada similar root length and root dry weight than undefoliated plants in all threespecies, although plants of S. tenuis defoliated twiceshowed a greater (p < 0.05) root length than undefoliated controls. Rootlength and root dry weight were similar between sampling periods, except onundefoliated plants of S. tenuis which had a greater (p<0.05) root length and root dry weight at the first than at the second sampling.Although root length per unit root dry weight may be greater inS. tenuis than in S. clarazii andS. ambigua, greater root length and dry weight increasesinS. clarazii after defoliation appear determinant incontributing to explain its greater competitive ability and defoliationtolerance when compared with the other two species.Nomenclature of taxa followed.

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.

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

Effects of vegetation cover on recruitment of Ulmus pumila L. in Horqin Sandy Land, northeastern China

Understanding the effects of vegetation cover on seedling survival is helpful for promoting vegetation restoration in environmentally fragile zones. This study was conducted in the desertified, moving sand dunes of Horqin Sandy Land, Inner Mongolia, northeastern China. We hyphothesized that (1) seed density (i.e., number/m2) increases as vegetation cover increases, and (2) there will be more surviving seedlings in locations with higher vegetation covers. Total vegetation cover and initial densities of seeds, germinated seeds and surviving seedlings of Ulmus pumilia were evaluated under various vegetation covers in trying to clarify the effects of vegetation cover on the early stages of the plant life history. In agreement with the first hypothesis, initial seed densities were greater (P<0.05) under higher vegetation covers. The relationship between vegetation cover and initial seed density was represented by a quadratic regression, where a threshold occurred with a vegetation cover of 36% (P<0.05). The higher total vegetation covers, however, did not result in increased densities of germinated seeds (P>0.05), which on average represented 16.7% of initial seed densities. Even more, three months after the study initiation, total vegetation covers were similar (P>0.05) at all positions in the dunes, and they determined a similar number (P>0.05) of surviving seedlings at those positions (i.e. the second hypothesis had to be rejected). The mean number of seedlings that survived at all positions was only 4.5% of germinated seeds. The number of surviving elm seedlings (0 to 1.7 seedlings/m2) under various vegetations covers (12.2% to 20.8%) at all dune positions by late summer would most likely not contribute to vegetation restoration in the study area.

Influence of Soil Water Status on Arbuscular Mycorrhizas in Three Perennial Grasses in Central Argentina

In this field study, we determined the effects of water stress, rainfed or irrigated conditions on the arbuscular mycorrhizal fungal (AMF) root colonization in the late-seral Stipa clarazii, the earlier-seral S. tenuis and the early-seral S. gynerioides at different developmental stages and sampling dates. We had two goals: (1) to document the levels of AMF root colonization, and (2) to examine the plant stress responses during imposed water stress in the three Stipa species. We tested the hypothesis that the effects of water stress on Stipa grasslands depend upon the interactions between plant AMF status and plant species. No consistent patterns in AMF root colonization frequency were observed relative to soil water levels. This suggests that AMF root colonization is highly plastic and responds to environmental factors that we do not yet understand. Mycorrhiza colonization was greater in the late-seral than in the earlier-seral species. Higher AMF root colonization levels in S. clarazii may partly explain its higher competitive ability in comparison with the earlier-seral species.

Cattle raising in central, semiarid rangelands of Argentina.

M ost rangelands are managed inappropriately in Argentina. This article provides some simple guidelines that can ensure a better grazing of rangeland vegetation and simultaneously increase beef production. Our main objective was to prove that a few simple management guidelines and a short-duration, high-intensity grazing system would increase beef production per acre, while at the same time maintaining the forage resource in the community. Studies were conducted in the phytogeographical province of the Monte1 (Fig. 1, 40°39′S, 62°54′W) in central Argentina. Average annual temperature is 54° to 57°F and rainfall is scanty with 8 to 12 inches annually concentrated in winter and spring; average annual evapotranspiration is about 31 inches per year. This is an extensive, almost continuous, and rather uniform area of shrublands. It constitutes the most arid rangeland of the country.2 Monte vegetation is a steppe scrub dominated by microphyllous, xerophytic shrubs from 39 to 118 inches high2 (Figs. 2 and 3). The most characteristic plant community dominating large areas of the Monte is composed of Larrea divaricata, Larrea cuneifolia and Larrea nitida; Larrea is the most abundant genus. The herbaceous understory is represented by Pappophorum subbulbosum, Pappophorum mucronulatum, Bouteloua aristoides, Bouteloua barbata, Trichloris crinita, Eragrostis argentina, Stipa clarazii, Stipa tenuis, Poa ligularis, and others.3 The soil type is a typical haplocalcid, with an A horizon that is 20 cm Cattle Raising in Central, Semiarid Rangelands of Argentina