Semi-arid Central Argentina Research Articles

Land-use change affects soil hydro-physical properties in Mollisols of semiarid Central Argentina

The objective of the present study was to describe the relationships between soil physical properties and to identify variables that determine the pore system and hydrological functions which are specifically important in drought prone semiarid marginal croplands. We compared Mollisols under natural vegetation (NV) with their agriculture (AG) pairs in the central Argentinean Dry Pampas to obtain the range of values for variables that characterize the soil pore system. These were total carbon (OC), bulk density (BD), structural instability index (SII), volumetric weight of aggregates (VAW), total porosity (TP), macropores (Ma), saturated hydraulic conductivity (K), infiltration rate (IR), Dexter's S index (S), and least limiting water range (LLWR). To a depth of more than 23 cm, TP and Ma were considerably lower (~ 35% and 50% less, respectively) in AG versus NV. The changes in porosity and pore size resulted in 74% smaller IR and 65% lower K. The S index had mean values of 0.16 and 0.14 for the upper and middle layer respectively in NV; while for AG, these values were significantly lower (0.09 and 0.10, respectively). The LLWR was also lower in AG (21 and 59% less in the upper and middle layer, respectively), due to high soil resistance even at non-limiting BDs (1.17 and 1.32 g cm−3 for AG2 and AG1, respectively). Soil OC was positively correlated with TP, Ma, and S, and negatively correlated with BD and SII. Multiple regression models for S included SII, TP, and clay contents, while for LLWR the variables were OC, TP, and clay plus silt. Thus, these complex soil quality indices can be predicted by using simpler physical parameters, but texture and organic matter also have a strong effect on them. Moreover, our results contribute to a better understanding of the role of texture and organic matter in controlling soil physical quality and for maintaining soil-based ecosystem services.

Plant interactions balance under biotic and abiotic stressors: the importance of herbivory in semi-arid ecosystems.

Biotic and abiotic stressors commonly co-occur in plant communities and influence interactions between plants. However, their combined effects on plant interactions have not been widely studied and are still unclear. Here, we assessed the balance of interactions between neighboring plants along a grazing gradient and under two water regimes. We conducted a three-year-field experiment in semi-arid central Argentina with transplants of the dominant palatable grass Piptochaetium napostaense growing in Baccharis ulicina and open microsites across a gradient of grazing pressure. Additionally, we established a water addition treatment along that gradient. We recorded herbivory, size, and fecundity of P. napostaense. During the first two years, P. napostaense was consumed less and was larger below Baccharis than in the open. These differences were greatest under high grazing pressure. Differences in fecundity between microsites were only detected under high and medium grazing pressure in the first two years. In the third year, Baccharis lost their leaves for unclear reasons and provided poor herbivory protection; hence, P. napostaense plants in Baccharis were larger than those in the open only under medium and low grazing pressure, and there were no differences in fecundity between microsites under any grazing condition. Water additions exerted no effect on plant interactions. The balance of interactions changed from positive under heavy grazing to neutral at low and no grazing and water availability did not alter that balance. We conclude that herbivore pressure is an important driver of the balance of plant interactions in semi-arid environments.

Vetch-rye biculture is a sustainable alternative for enhanced nitrogen availability and low leaching losses in a no-till cover crop system

The reliance of current farming systems on synthetic fertilizers caused concerns about their sustainability, and alternatives to supply nitrogen through biological processes have to be adapted to practical conditions. The present study compared a pure legume and legume-grass biculture as cover crops for sorghum in their supply of nitrogen (N) to the cash crop and their N leaching losses during fallow. A three-year field experiment under no-till with sorghum (Sorghum bicolor Moench.) as main crop and cover crop treatments (C- control=bare fallow; R-rye Secale cereale L.; V-vetch Vicia villosa sp dasycarpa; VR-vetch-rye biculture) was established with a completely randomized block design (four replicates) in semiarid central Argentina. Aerial biomass (AB) and N contents were determined for all crops. Soil moisture to 1m and nitrate-N to 0.60m depth were determined. Water use- and nitrogen use efficiencies (WUE and NUE) were calculated and biological nitrogen fixation (BNF) estimated. BNF depended on nitrate-N contents of the soils; highest values (11 and 10g BNFm−2 for V and VR respectively) were reached at 1.3gNm−2. Sorghum responded to higher N availability with an average of +299 and +512gABm−2 for V and VR compared to C, with higher WUE. The relationship between WUE and nitrate-N was positive with an optimum of 0.048gNm−2mm −1 where WUE reached a maximum of 4.9gABm−2mm−1. Potential N losses by leaching were highest in control, while all cover crop treatments had lower losses. Our results support the hypothesis that a legume-grass biculture was more efficient in the trade-off between nitrogen provision to the cash crop and prevention of N losses by leaching. Although the amount of BNF was lower in the biculture than in pure vetch, it covered sorghum N requirements with less potential leaching losses even in high rainfall fallows.

The effect of fire exclusion on the structure and tree mortality patterns of a caldén (Prosopis caldenia Burkart) woodland in semi-arid Central Argentina

We studied the effects of long-term fire exclusion on the structure and mortality of a caldén (Prosopis caldenia Burkart) woodland, a rare vegetation type remaining only in fragments in semi-arid Central Argentina. We tested the hypothesis that differences in caldén stand structure are consequence of differences in fire history by quantifying DBH and tree height in 30 plots set in caldén woodlands with fire occurring at least every 10 y, with fire excluded for 20–30 y and without fire for at least 60 y. Stem height, density and basal area increased after 20–30 years of fire exclusion. The frequently burned woodland had high mortality in all but the largest (>60 cm in DBH and >8 m in height) size class, whereas the fire-excluded plots exhibited high mortality only in the smallest stems (<15 cm in DBH and <3 m in height). Our results showed that the fire exclusion increased stem basal area and density while decreasing mortality of larger size classes and reducing resprouting of stems of any size; the probable explanation for these changes is a shift in the prevalent thinning mechanisms from fire-induced to competition-induced mortality.

Decomposition and Carbon Dynamics of Crop Residue Mixtures in a Semiarid Long Term No-Till System: Effects on Soil Organic Carbon

Plant residues decomposition transfers organic matter and nutrients to soil, and plays a decisive role in carbon (C) cycling. The aims of our study were to analyze under realistic field conditions the annual decomposition and C dy- namics of crop residues mixtures under long-term no-till management and their effect on soil organic carbon (SOC). Three treatments were evaluated: soybean monoculture, soybean rotation (maize as preceding crop), and maize rotation (soybean as preceding crop). In each treatment soil samples and crop residue samples were collected. In crop residues samples we determined: total residue mixture biomass, soybean residue biomass, maize residue biomass, non identifiable residue biomass, total C, soluble C, insoluble fiber. In soil samples, SOC were analyzed. Decomposition rates were calcu- lated for residues mixture, soybean and maize residue before and after deposition of fresh residue. Decomposition rates of all residues analyzed showed a high variability between treatments. Total C, soluble C and insoluble fiber concentrations of the residues mixtures showed a great similarity among treatments. Contrarily, their annual dynamics differed between sampling dates in all treatments. We concluded that decomposition and C dynamics of crop residues mixtures in long- term no-till systems in the semiarid central Argentina are strongly influenced by: the interaction of the chemical quality of the residues, the proportion of the residues from different crops and/or with different decomposition degree,and the sea- sonal effect. The greater C amount in residues mixture of rotation was not reflected in SOC contents, and further studies are recommended.

Radiation budget changes with dry forest clearing in temperate Argentina

Land cover changes may affect climate and the energy balance of the Earth through their influence on the greenhouse gas composition of the atmosphere (biogeochemical effects) but also through shifts in the physical properties of the land surface (biophysical effects). We explored how the radiation budget changes following the replacement of temperate dry forests by crops in central semiarid Argentina and quantified the biophysical radiative forcing of this transformation. For this purpose, we computed the albedo and surface temperature for a 7-year period (2003-2009) from MODIS imagery at 70 paired sites occupied by native forests and crops and calculated the radiation budget at the tropopause and surface levels using a columnar radiation model parameterized with satellite data. Mean annual black-sky albedo and diurnal surface temperature were 50% and 2.5°C higher in croplands than in dry forests. These contrasts increased the outgoing shortwave energy flux at the top of the atmosphere in croplands by a quarter (58.4 vs. 45.9Wm(-2) ) which, together with a slight increase in the outgoing longwave flux, yielded a net cooling of -14Wm(-2) . This biophysical cooling effect would be equivalent to a reduction in atmospheric CO2 of 22 Mg C ha(-1) , which involves approximately a quarter to a half of the typical carbon emissions that accompany deforestation in these ecosystems. We showed that the replacement of dry forests by crops in central Argentina has strong biophysical effects on the energy budget which could counterbalance the biogeochemical effects of deforestation. Underestimating or ignoring these biophysical consequences of land-use changes on climate will certainly curtail the effectiveness of many warming mitigation actions, particularly in semiarid regions where high radiation load and smaller active carbon pools would increase the relative importance of biophysical forcing.

Fire patterns in central semiarid Argentina

Wildfires can cause severe impacts on the terrestrial ecosystems depending on their frequency and behavior. We studied the environmental factors influencing the spatial and temporal distribution of fires, their size and duration in the central semiarid Argentina. We identified fires using MODIS satellite data and we analyzed their association with climate and land cover/use patterns. Spatial and temporal fire patterns varied between eastern, central and western regions according to the presence of agriculture, shrublands and water deficits, respectively. The frequency and behavior of fires also varied temporally with water conditions. Years with low effective precipitations were characterized by an important hotspot density and fire number, as well as the months preceded by two months with low effective precipitation (r2: 0.42; p < 0.0001). We observed a spatial delay of fires in a northeast–southwest sense, related to the delay of the spring beginning (r2: 0.7594; p < 0.0001). The mean fire sizes and duration varied significantly among vegetation types (F: 10.76, p < 0.0001 and F: 3.703, p < 0.01). Fires were bigger in shrublands and longer in shrublands or forests regarding agricultural areas or degraded areas (F: 16.0, p < 0.0001). The results obtained would be useful to prevent/control fires and to preserve natural resources and human communities.

Effects of defoliation and competition on the post-fire response of Poa ligularis Ness. in semi-arid central Argentina

The objectives of the study were to evaluate the effects of defoliation on tiller growth of Poa ligularis Ness. at two different intervals after fire as well as how recovery is affected by different levels of herbaceous interspecific competition during the subsequent growing season. Early post-fire defoliation (6 months after burning) reduced (P &lt; 0.05) the height, number of green leaves and total green length of tillers on P. ligularis in comparison to Control plants. Late post-fire defoliation (12 months after burning) did not affect any of these growth attributes. Burnt plants of P. ligularis exposed to the no competition treatment (NC) had a higher number of green tillers per plant than plants exposed to the total competition treatment (TC). Moreover, the burnt plants exposed to the NC were significantly taller (P &lt; 0.05) than plants exposed to TC. The highest (P &lt; 0.05) above- and belowground biomass was recorded in those burnt P. ligularis plants growing in the NC treatment. Our results provide some evidence to show that early defoliation and interspecific competition after fire reduce the tiller growth and also the above- and belowground biomass of P. ligularis plants.

Carbon contents and respiration rates of aggregate size fractions under no-till and conventional tillage

The purpose of the present study was to evaluate the long-term (14 yr) effect of no-till (NT) compared with conventional tillage (CT) on the distribution of dry sieved aggregate size fractions, their carbon (C) and nitrogen (N) contents and respiration activity. Soil samples were taken from a long-term (14 years) tillage experiment with a rotation of 6 cash crops, 4 years pasture and another 6 cash crops in the sandy plains region of semiarid central Argentina, on an Entic Haplustoll. Sampling was carried out at 0.06 m intervals to 0.18 m depth, with 4 replicates per tillage treatment. Bulk density (BD), C and N were determined on air dried samples. Dry aggregate size distribution (fractions: >4, 1–4, and <1 mm) and C and N contents of these size fractions were determined. Samples of fractions and of undisturbed soil were incubated and respiration was measured by trapping evolved carbon dioxide (CO 2) in NaOH. The NT soil had 25% more macroaggregates in surface 0.06 m samples than CT, while at 0.06-0.12 and 0.12-0.18 m, these increased by 38 and 17% respectively. This indicated that the effect of NT on aggregate size distribution extended towards deeper soil regions. The intermediate aggregate fraction of surface soil under NT had 8.2 g kg −1 more C than under CT, while for >4 and <1 mm aggregates the differences between tillage systems were smaller (6.8 and 7.6 g kg −1 respectively). The data confirmed our hypothesis with respect to higher C accretion in intermediate aggregates under NT, approaching values of a permanent pasture soil. Large aggregates did not sequester more C than small aggregates, as would be expected according to the conceptual model of aggregate hierarchy. At 0.06-0.12 m depth these differences in C concentrations were less pronounced. Thus, although the effect of tillage on aggregate size distribution extended below the upper 0.06 m, C sequestration occurred mainly in this depth interval. While intermediate aggregates represented 12 and 15% of total C under NT and CT respectively, their contribution to soil respiration was19 and 21% for the same treatments. This would indicate that C losses from soil through mineralization are mostly associated with intermediate aggregate size.

Effect of a postfire environment on the establishment of Prosopis caldenia seedlings in central semiarid Argentina

Abstract The objective of this work was to evaluate postfire environmental effects on the emergence, survival and growth of Prosopis caldenia seedlings in relation to different controlled fire frequencies, seed scarification methods, and planting site. Seedling emergence was significantly higher in experimental units exposed twice and three times to controlled fire than in unburned experimental units. The highest average seedling survival was recorded with triple exposure to controlled fires. Emergence, survival and growth of seedlings from seeds exposed to acid scarification and 600°C for 5 min were higher in the burned experimental units than in the unburned ones. In the former, seedling survival was higher beneath a P. caldenia canopy than in an adjacent open site, though seedling emergence was similar in both planting sites. Our results suggest that postfire conditions characterized by a reduction in the vegetative cover and competition interference and an increase in soil temperatures and nutrients levels (e.g. nitrogen and phosphorus) may facilitate the establishment of P. caldenia seedlings in the Caldenal.

Radial growth and biological rotation age of Prosopis caldenia Burkart in Central Argentina

Prosopis caldenia Burkart (caldén) is the most important woody species in the caldenales, the open xerophytic woodlands covering approximately 33 000 km 2 in the semiarid central region of Argentina. Dendrochronological techniques were used to estimate mean radial growth and the biological rotation age of P. caldenia in San Luis, Central Argentina. Inter-annual variations in ring widths were measured on cross-sections from dominant and co-dominant trees at four sampling sites from across the natural distribution of P. caldenia in the province of San Luis. For a period of 65 years, the mean cumulative radial growth between these four stands varied from 174.9 to 246.09 mm, whereas the mean annual radial growth ranged from 2.7 to 4.0 mm yr −1. Biological rotation ages were estimated based on the temporal evolution of the annual basal area mean and current increments. At the four stands, the biological rotation age occurs at ages equal to or greater than 66 years. In comparison with the introduced species presently used for commercial reforestation in the region, P. caldenia shows similar rates of radial growth, is better-adapted to the extreme climatic variability of semiarid Central Argentina, and its wood largely exceeds in quality the wood of those species used for commercial reforestation.

Effect of fire on growth of three perennial grasses from central semi-arid Argentina

The effect of fire on growth of Piptochaetium napostaense, Stipa tenuis, and Stipa gynerioides, three important native perennial grasses in the semi-arid region of central Argentina, was evaluated under different fire temperature regimes: 300–400°C (low temperature regime), 500–600°C (high temperature regime) and no fire (control). Fire treatments were applied with a portable propane plant burner in April and December 1994, May 1995, and January 1996. Overall results indicate that during the first months after fire occurrence, average total green length of S. tenuis, P. napostaense and S. gynerioides tillers was severely reduced ( p<0.05) by fire. This effect was more pronounced in plants burned with the high temperature treatment. The observed patterns of response to fire for height of tillers were very similar to those already reported for total green length of tillers. Towards the end of each growth cycle, the number of green leaves per tiller of burned plants of P. napostaense, S. tenuis, and S. gynerioides were similar or greater ( p<0.05) than the number of green leaves on tillers of control plants. Relative growth rates for total green length and for height in tillers of burned plants of S. tenuis, P. napostaense and S. gynerioides were greater than in tillers of control plants. Our results indicate that fire affected differentially the growth of the studied species; S. gynerioides was more affected by fire than P. napostaense and S. tenuis. The species most tolerant to fire was P. napostaense.

Impact of Fire on Soil Nutrients in Central Semiarid Argentina

The modification of the soil chemical properties by fire is directly related to the degree of soil heating and to the weight and spatial distribution of vegetation and litter accumulated. This research was aimed at studying the immediate effects of two controlled burns on the organic carbon (OC), and on the total and available nitrogen (N), phosphorus (P), and sulfur (S) of surface soils (0-5cm) of the southern Caldenal region, Argentina. The pH, CEC, and available cations were also evaluated. Follow-up samplings were carried out yearly, during the three years between the burns. The initial unburned shrub vegetation-covered soil had a greater concentration of total and available nutrients than the herbaceous-covered soil. Soil pH and CEC values were slightly lower in shrub soil. The first controlled burn generally caused a beneficial effect on the total elements either immediately or one or two years after the burn. It also caused a considerable flush of the available nutrients that was more persistent under the shrubs. The changes were temporary and generally, the nutrients levels returned to those of the unburned soils during the second or third year after the burn. On the other hand, a second moderate controlled fire experienced three years later was shown to have no measurable effect on most soil properties, but a decrease of total S in grass soil and NO 3 - -N in woody soil was observed after burning, pointing out the risk of a soil fertility depletion. Consequently, further studies in the Caldenal region are needed to evaluate changes in the soils under circumstances of different fire frequencies.

Effect of Fire on Perennial Grasses in Central Semiarid Argentina

Fire is a key factor in the temperate semiarid region of central Argentina. The objectives of this work were to evaluate the effect of different fire intensities applied during different seasons under field conditions on the mortality of Piptochaetium napostaense (Speg.) Hack., Stipa tenuis Phil., and Stipa gynerioides Phil., 3 of the dominant grasses within the region and to determine their thermal death points in the laboratory. Ten plants of each species were exposed to low fire intensity (300-400 degrees C), high fire intensity (500-600 degrees C), and no fire (control) in April and December 1994, May 1995, and January 1996. Fire treatments were applied with a portable propane plant burner. The thermal death point was determined (during fall and spring) using the Wright's technique. Although mortality with high fire intensity was always higher than mortality with low fire intensity for all species, differences were not significant (p > 0.05). Pooling both treatments, the highest (p 0.05) for all burning dates. Only after the May burn, was average mortality of P. napostaense and S. tenuis higher (p < 0.05) than average mortality of S. gynerioides. The thermal death point was similar in all studied species. It was 65 degrees C during the fall, and 68 degrees C during the summer. This could explain, at least in part, similar mortalities (except after the May burn) between species and the date of burning found in this study. DOI:10.2458/azu_jrm_v54i5_pelaez

Effect of fire on perennial grasses in central semiarid Argentina

Fire is a key factor in the temperate semiarid region of central Argentina. The objectives of this work were to evaluate the effect of different fire intensities applied during different seasons under field conditions on the mortality of Piptochaetium napostaense (Speg.) Hack., Stipa tenuis Phil., and Stipa gynerioides Phil., 3 of the dominant grasses within the region and to determine their thermal death points in the laboratory. Ten plants of each species were exposed to low fire intensity (300-400 degrees C), high fire intensity (500-600 degrees C), and no fire (control) in April and December 1994, May 1995, and January 1996. Fire treatments were applied with a portable propane plant burner. The thermal death point was determined (during fall and spring) using the Wright's technique. Although mortality with high fire intensity was always higher than mortality with low fire intensity for all species, differences were not significant (p &gt; 0.05). Pooling both treatments, the highest (p &lt; 0.05) average mortality for P. napostaense (55%) and S. tenuis (85%) was observed after the May burn. Average mortality for S. gynerioides was similar (p &gt; 0.05) for all burning dates. Only after the May burn, was average mortality of P. napostaense and S. tenuis higher (p &lt; 0.05) than average mortality of S. gynerioides. The thermal death point was similar in all studied species. It was 65 degrees C during the fall, and 68 degrees C during the summer. This could explain, at least in part, similar mortalities (except after the May burn) between species and the date of burning found in this study.

Seasonal variation of the seed bank of Medicago minima and Erodium cicutarium as related to grazing history and presence of shrubs in central Argentina

Medicago minima (L.) Grufberg and Erodium cicutarium (L.) L'Herit are very abundant on the rangelands of central semi-arid Argentina, especially on wet springs. Available information on these species deals with their ecophysiology, demographic response to grazing, nutritive value, etc., but no information within the region has been detected on their seed bank dynamics and dormant status of buried seeds. The main objective of this work was to study the seasonal variation of the seed bank of these species as related to shrub and grazing influence. Dormancy states and viability of buried seeds were also studied. Results indicate that the seed bank is abundant, especially in the case of M. minima, and it seems to be more abundant in the grazed area than in the ungrazed area. The seed bank showed low seasonal variation and shrubs seem to have only minor effects. Two simultaneous dormancy states were detected in both species and all seeds tested for dormancy resulted viable.

Impact of Fire on Soil Nitrogen Forms in Central Semiarid Argentina

Burning effects on soil N are particularly important in the Caldenal region of Argentina because N is likely to be limited and is easily lost by volatilization during fire. Burning may cause an initial increase in the mineral N content of the surface soil and, perhaps more important, can reduce the proportion of N in hydrolyzable (that is, more mineralizable) forms. The modification in soil N fractions by fire is directly related to the degree of soil heating and to the proportion and amount of N fractions remaining in the ash. This research was aimed at studying the effects of a controlled burn on total N, inorganic (NO3 and NH4+) and organic (amino acids, ammonium, hexosamines) N forms, as well as hydrolyzable and nonhydrolyzable N compounds of top soils of the southern Caldenal. Soil samples were taken from beneath the canopy of shrubs and from the areas with herbaceous cover, before and after burning. In order to show that a greater proportion of N added to regularly burned soils will result in lower levels of mineralizable N, differences in the changes of the various pools of plant tissue N upon burning at different ignition tem peratures are also reported. The initial unburned top soils had no significant differences in total, inorganic, and organic N form concentrations between shrub and herbaceous vegetation-covered soil. Burning did not affect the net content of total N in soil under woody vegetation, but in the grassland soil a decrease of total N concentration was observed. Nitrate concentration increased and NH4+ concentration decreased in both shrub and grassland soils after burning. Controlled burning caused losses of amino acid and ammonium organic N forms from the soil under grass plants and gains of hexosamine N in both shrub-and herbaceous-coveredsoils. There was an observed tendency to increase the relative proportions of non hydrolyzable N forms in both the shrub and grassland soils after burning. Inorganic and organic N form concentrations were similar in initial shrub litter and grass, but litter had higher concentrations of total and nonhydrolyzable N than those of grass. Shrub litter ash obtained after muffling at 300 C contained higher NH4+-N, aminoacid N, ammonium N, and total and nonhydrolyzable N concentrations than those from grass. An increase of 100 C in the ignition temperature produced clear ashes, rich in mineral N with little concentration of total N and C. The organic N forms affected most by the ignition temperature increase were amino acid N and ammonium N, respectively, particularly in woody litter. The most striking feature was the constituent increase in the percentages of nonhydrolyzable N forms of the vegetation covers on passing from initial to 300 C to 400 C, particularly for the litter and its ashes. These results suggest that the soil mineralizable N pool may be reduced by frequent burning or by more severe burning than observed presently in the Caldenal region.

Effect of fire intensity on bud viability of three grass species native to central semi-arid Argentina

The effect of fire on axillary bud viability ofStipa tenuis, Piptochaetium napostaense, andStipa gynerioides, three important native perennial grasses in the semi-arid region of central Argentina, was studied under different fire intensities: 300–400°C (low fire intensity), 500–600°C (high fire intensity) and no fire (control). Fire treatments were applied in April 1994 and May 1995 with a portable propane plant burner. After each burn the number of dead buds ofS. gynerioideswas higher (p< 0·05) at the centre of the plants than at the periphery.Stipa tenuisshowed a similar response with the exception of the 1995 high intensity treatment. No differences (p> 0·05) in mortality were detected between buds located at the centre of the plants and those at the periphery forP. napostaensein either burn. In general, the number of dead buds in the high fire intensity treatment was slightly higher than the number of dead buds in the low fire intensity treatment in all of the studied species. Treatment effects were similar inS. tenuis, P. napostaenseandS. gynerioides. After each burning date, tillers of plants exposed to fire had more (p< 0·05) dead buds and fewer (p< 0·05) metabolically active buds than the tillers of control plants. Within each fire intensity treatment during both study periods there were no statistical differences (p> 0·05) among species in the number of total, metabolically active, dormant and dead buds. These results would suggest that fire limits postfire regrowth ofS. tenuis, P. napostaenseandS. gynerioides.

Effect of fire on woody species in central semi-arid Argentina

The objective of this work was to study the effects of different fire intensities and season of burning on five important woody species in the central semi-arid rangelands of Argentina. Overall results in this experiment indicate that a single fire reduces woody species abundance. This effect persists for several growing seasons after the fire and is related to fire intensity. Mortality rates, with the exception ofLarrea divaricata, were low and all the species produced new sprouts after the fire. This sprouting capacity may be the key to the abundance and persistence of woody species in these communities. A single fire does not start a replacement sequence and the plant community persists with some variation in the abundance of the principal woody species.

Water relations between shrubs and grasses in semi-arid Argentina

The two-layer hypothesis argues that in shrublands shrubs take up most of the water from the lower layers of the soil whereas grasses take up most of the water from the upper layers of the soil. We tested two predictions from this hypothesis in a shrubland of central semi-arid Argentina: (1) the correlation between plant water potential and soil water potential in the upper layers is stronger for grasses than for shrubs and (2) rooting depth is greater for shrubs than for grasses. The apparent pattern of water use by the deciduous shrub ( Prosopis caldenia) and the two perennial grasses ( Stipa tenuis and Piptochaetium napostaense) under study was consistent with the two-layer hypothesis whereas the evergreen shrub ( Condalia microphylla) did not support this model. Our results suggest that the pattern of water use by grasses and shrubs in shrublands may be dependent upon strategies to counteract water stress and the phenological patterns developed by species within each life form.