O.R. Elia

Emergence and seedling survival of calden in the semiarid region of Argentina.

Calden (Prosopis caldenia Burk.) is one of the woody species that is increasing in abundance due to poor grazing management in the semiarid region of central Argentina. The objectives of this work were to evaluate emergence and seedling survival of calden under grazed-low herbaceous foliar cover, and ungrazed-intermediate and ungrazed-high herbaceous foliar cover. Seedling emergence was recorded weekly after 3 planting dates: October 1987, September 1988, and October 1989. Seedling survival was recorded every 2 weeks until all seedlings died. Precipitation, soil water potential, and soil temperature were monitored during the study period. Since calden only reproduces from seeds, optimal conditions for germination under different controlled photoperiod and temperature regimes were also studied. In the laboratory, greatest germination of freshly harvested unscarified seeds occurred at a diurnally alternating temperature regime of 15 and 30 degrees C with light provided for 9 hours. Lead germination occurred at 15 degrees C and continuous darkness. Acid scarification significantly increased germination percentage and germination rate. Unscarified seeds incubated in light exhibited a lower rate of germination than scarified seeds under all temperatures. In the field, soil water availability was usually similar among the 3 experimental conditions. Soil temperature was only occasionally higher in the grazed-low herbaceous cover site. Only for 1987 data, emergence was greater on the grazed area with relatively low herbaceous foliar cover and similar within the ungrazed area. Treatment effect was very little on seedling survival, being lower in the grazed area for only 2 evaluation dates in 1989. Regardless of treatment the emerged seedlings survived 40 days on the average. The highest calden seedling mortality was always coincident with higher soil temperature and lower soil water availability.

Botanical composition and seasonal trends of cattle diets in central Argentina.

About 40,000 km2 of the Caldenal in central semiarid Argentina are rangelands where the most important economic activity is cow-calf operations. Some information on forage species, mainly regarding taxonomy, ecophysiology, and nutritive value, is available, but detailed studies on composition of free-ranging cattle diets are lacking. The objective of this work was to study the botanical composition and seasonal trends of cattle diets in the southern Caldenal. Diets were studied through microscopic analysis of cattle feces collected monthly in a typical plant community during a 12-month period. Grasses were the bulk of the diets, except in November when more than 50% of the diet was Medicago minima (L.) Grufberg. Highest consumption among the grasses was of Piptochaetium napostaense (Speg.) Hack., one of the dominants in the grass layer. High consumption of Pappophorum mucronulatum Nees, one of the few warm-season grasses in the region, was found during the summer. Calden (Prosopis caldenia Burk.) pods, consumed in late summer and fall, were the only woody fragments found in appreciable amounts. Classification and ordination techniques were used to analyze seasonal trends. In spite of a relatively high homogeneity in the diets, 2 well-defined seasonal trends were detected, one in the fall-winter period and another in the spring. Cattle diet diversity was minimum during the coldest months of the year.

Botanical composition of cattle and vizcacha diets in central Argentina.

Cattle (Bos taurus) and vizcacha (Lagostomus maximus) diets were examined monthly in the semiarid Caldenal in central Argentina. Cow-calf operations are the most important economic activities within the region. In spite of a widespread distribution of the vizcacha in Argentina, comparative studies of the diet of cattle and vizcacha are scarce. The objective of this work was to analyze the botanical composition, seasonal trends, and possible dietary overlap between cattle and vizcacha. Diets were determined by microscopic analysis of cattle and vizcacha feces collected from November 1994 through December 1995 in a shrubland community of the southern Caldenal. Grasses were the bulk of the diet for both herbivores. Piptochaetium napostaense (Speg.) Hack. was the most abundant grass in vizcacha (53%) and cattle (40%) diets. Prosopis Caldenia Burk. pods partially (34%) replaced this grass in cattle diets during late summer and fall. Consumption of P. napostaense was generally higher (13%) in vizcachas than in cattle, especially during the dry period of the study (21%). During the drier months, cattle consumed more of the less preferred grasses (48%). Forbs were poorly represented in the diets perhaps because of scarce rains and low availability. Classification and ordination techniques revealed seasonal trends and overlapping diets. A greater overlap (75%) was found during the wet period due to simultaneous consumption of P. napostaense by both herbivores. Trends in diet diversity were similar with indices generally higher for cattle than for vizcachas, especially during the dry period.

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

Abstract 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 P . napostaense , S . tenuis , and S . gynerioides were similar or greater ( p 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 .

Response of grass species to different fire frequencies in semi-arid rangelands of central Argentina

The study was undertaken toquantify theeffect of different controlled firefrequencies onfoliar cover,density, individualbasal area, and mortality of the mostcommon perennial grass species inthe semi-arid rangelands of thesouthern Caldenal in central Argentina over a 20-year period. Cover of bare soil was also assessed. The study comprised three fire treatments: (i) high fire frequency (controlled burns every 3-5 years; HFF); (ii) low fire frequency (controlled burns every 8years;LFF);and(iii)unburnedcontrol(C).Firetreatments,regardlessoffrequency,inducedanincreaseinfoliarcoverand densityindesirablegrasses,nochangesinintermediategrasses,andadecreaseinundesirablegrasses.Individualbasalarea tendedtobehigherfordesirablegrassesandlowerforintermediateandundesirablegrasseswhensubjectto fire.Mostofthe speciesunderstudyexhibitedhighermortalityratesintheHFFtreatmentthanintheLFFandCtreatments.Theresultsofthe studysuggestthatrecurrentcontrolledburnsofmoderateintensitymayfavourtheherbageproductionofdesirableperennial grasses. This, in turn, assuming appropriate grazing management, may have a beneficial impact on livestock production. Nevertheless,giventheeffectsof fireonthecoverofbaresoilandmortalityofgrasses,furtherresearchisneededinorderto determine the appropriate fire frequency in terms of rangeland sustainability.

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 < 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 < 0.05) than plants exposed to TC. The highest (P < 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.

Effects of firebreaks on ant density (Hymenoptera, Formicidae) in a semiarid region, Argentina

In arid and semiarid regions, the presence of roads or firebreaks can affect microclimatic variables that influence the abundance of soil nesting ants. We studied ant nest density in environments with different soil types (loose and compacted soil), and vegetation cover (shrubland, grassland and bare soil) south of Caldenal, La Pampa, Argentina. We selected three areas with woody cover (shrubland), herbaceous cover (grass), and 80% of bare soil (firebreaks) within a 12 ha study area where large herbivores were excluded. We recorded soil surface temperature, humidity, pH and degree of soil compaction in each area. The density of nests was assessed by randomly placing three transects (80 m x 5 m) in each experimental unit. Soil temperature was higher in firebreaks and soil compaction was higher in the shrubland and the grassland. No differences in ant assemblage were found regarding nest density among environments. However, Acromyrmex striatus (Roger, 1863) was found mostly in firebreaks where loose soil with greater porosity allows more gas exchange and water infiltration. Our findings revealed that the construction of firebreaks favors the establishment of leaf-cutting ants, which due to their competitive advantage, could negatively affect ant and plant composition in the community.