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Shrub encroachment is frequent in grazing lands in the Chaco region, Argentina. Fire is used by cattlemen to reduce the dominance of the shrub stands, improve forage standing biomass and quality, and enhance grazing accessibility. In this research, we assessed the combined effect of roller chopping þ fire on the plant canopy of three native shrub species, Acacia gilliessi, Celtis ehrenbergiana, and Schinus bumelioides. We used a randomized design, with three factors, year, burn dates, and fine fuel load. Before the burn, the area received a roller-chopping treatment. Canopy volume reduction was assessed using, DifV ¼ −1*(V i − V a ) and DifS ¼ −1*(S i − S a ), respectively, where V ¼ plant volume and S ¼ sprout number. Suffixes i and a represent measurements taken before the roller-chopping treatment and one growth season after the fire, respectively. Fire intensity, headfire residence time, woody residues, and initial plant volume were evaluated as covariates. Results indicate a negative fire effect on canopy volume and sprouting. A. gilliesi was more susceptible to fire than the other two species. High fine fuel load and high fire intensity generated a severe effect on sprout number. The same trend was observed for coarse fuels. The canopy volume reduction was greater in A. gilliesi than the other species, reaffirming its high susceptibility to fire. Head fires were faster in 2009 than in 2008 and in the late than in the early burn dates. These results suggest that mechanical þ fire treatments can be useful management tools for controlling shrub encroachment in the western Chaco.

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The dispersal of Opuntia imbricata (cholla) on rangeland may be directly or indirectly affected by the presence of livestock and wildlife. Overgrazing creates gaps or open spaces for seeds or stems that fall to the ground to become established (passive dispersal). In addition, livestock or wildlife may cause stems and fruits to fall to the ground (active dispersal). (2) A theoretical model is formulated for the dispersal of plants based on the diffusion equation which assumes one or more mechanisms of dispersal (e.g. passive and active dispersal). The x2 goodness-of-fit test is used to determine whether the data may be explained by one or by two mechanisms of dispersal. Methods for estimating the model parameters are presented based on the x2 method and the method of moments. (3) The model and analytical techniques are applied to field data on the spatial distribution of Opuntia imbricata in four different pastures. (4) The results indicate that dispersal of cholla can be attributed to two dispersal mechanisms. Passive dispersal alone cannot account for the current spread of cholla on some of the pastures.

Our objective was to assess the current fire regime of a 600 ha savanna dominated by the grass species Elionorus muticus Spreng., located in Santiago del Estero Province, north-western Chaco region, Argentina. The degree of tolerance of some native woody species to fire, the fire mean fire frequency (FF), and Weibull median probability (WMPI) were evaluated. Sampling sites were located in the ecotone between the savanna and the surrounding forests. A database was developed from fire scars found in cross sections of native tree and shrub species, cut at different heights above ground; that covered the recent 70 years of fire history (1925–1996). Results indicate that the savanna has a mean FF of 0.179 fires year–1 and an FI = 3 years. The mean height of fire scars found in trees and shrubs which indicate medium to high fireline intensities with flame lengths larger than 1 m are frequent in the savanna. Native species have different degrees of tolerance: Aspidosperma quebracho blanco (tree) and Schinopsis quebracho colorado (tree) are more tolerant to fire than Acacia furcatispina (shrub) and A. aroma (shrub). Bark thickness of the tree species (1–1.5 cm in mature individuals) allows them to withstand the frequent, high intensity fires of the savanna.

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