D.V. Peláez

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.

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 .

Pinus halepensis invasion in mountain pampean grassland: Effects of feral horses grazing on seedling establishment

We evaluated the mechanisms that determine the increasing abundance of Pinus halepensis in mountain pampean grasslands in Argentina that is associated with the continuous presence of feral horses. We hypothesized that direct and indirect effects of feral horse grazing on grassland may affect the establishment of P. halepensis. We conducted a field experiment to evaluate this hypothesis, studying the response of seedling emergence, survival and growth to herbaceous vegetation defoliation and soil disturbance in sites with contrasting grazing histories. We also evaluated the composition and structure of plant communities of each experimental site. Direct effects such as heavily defoliated perennial grasses and indirect effects such as reduced specific and functional richness, diversity and evenness enhanced the emergence, survival and early growth of the seedlings. High bare ground percentage and low grass biomass also positively affected seedling establishment. The experimental evidence demonstrates that the combination of invasional meltdown hypothesis and weakened biotic resistance are the mechanisms involved in P. halepensis invasion.

Plant water potential for shrubs in Argentina.

grasses in southeastern Arizona (1970-1982). J. Range Manage. 36:649-652. Frasier, G.W. 1985. Technical Note: A precision planter for small plots. J. Range Manage. 38:187-190. Frasier, G.W., J.R. Cox, and D.A. Woolhiser. 1985. Seedling survival response of seven grasses to initial wet-dry sequences. J. Range Manage. 38:372-377. Frasier, G.W., D.A. Woolhiser, and J.R. Cox. 1984. Emergence and seedling survival of 2 warm-season grasses as influenced by the timing of precipitation: A greenhouse study. J. Range Manage. 37:7-11. Gelderman, F.W. 1970. Soil survey of Walnut Gulch Experimental Watershed, Arizona: A special report. USDA Soil Conserv. Serv. Agr. Res. Serv., Oregon. Schreiber, H.A., and N.G. Sutter. 1972. Available soil water: Timedistribution in a warm season rangeland. J. Hydrol. 15:285-300. Smith, R.E., and H.A. Schreiber. 1973. Point processes of seasonal thunderstorm rainfall. 1. Distribution of rainfall events. Water Resources Res. 9:871-884. Wilson, A.M., and D.D. Briske. 1979. Drought and temperature effects on the establishment of blue grama seedlings. p. 359-361. In: Proc. Ist Internat. Range. Congr., Denver, Colo. Aug. 1978 Soc. Range Manage.

Functional Responses of Woody Prosopis caldenia Seedlings to Drought and Livestock Grazing in Semiarid Rangelands of Argentina

In the semiarid rangelands of Central Argentina overgrazing and drought alter the competitive balance between woody and herbaceous vegetation, favoring an increase in Prosopis caldenia. The objective of this study was to evaluate the effect of competition in perennial grasses and water stress on the relative growth rate (RGR) and related functional attributes of P. caldenia seedlings in sites with different grazing histories: a site exposed to long term (40 or more years) continuous grazing by cattle (grazed experimental site), and a long-term (around 30 years) exclosure (ungrazed experimental site). The total average RGR of the seedlings that grew in the grazed site was significantly higher, whereas the additional water supply favored those seedlings that grew in the ungrazed site. Significant relationships existed between the RGR and all the attributes measured, except in the ungrazed site where only those attributes related to leaf traits were correlated with the RGR. The above- and below-ground biomass of perennial grasses negatively affected the RGR of the P. caldenia seedlings in both experimental sites and soil moisture favored the RGR of the seedlings in the ungrazed site. Our work reveals the ability of seedlings of P. caldenia to take advantage of changes in environmental conditions, such as the release of resources due to overgrazing and to increase its RGR, which ensures its establishment.

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.