Journal of Environmental Management 80 (2006) 248–265 www.elsevier.com/locate/jenvman Assessment of the rangelands of southwestern Santiago del Estero, Argentina, for grazing management and research Carlos Kunst *, Eliseo Monti, He´ctor Pe´rez, Jose´ Godoy Instituto Nacional de Tecnologı´a Agropecuaria (INTA), Estacio´n Experimental Agropecuaria Santiago del Estero, Jujuy 850, 4200 Santiago del Estero, Argentina Received 30 October 2004; received in revised form 17 September 2005; accepted 10 October 2005 Available online 20 December 2005 Abstract Native rangelands of the southwest part of the province of Santiago del Estero, Argentina, are a key source of forage for cow–calf operations. The objectives of this study were to delineate the ecosystem units of the area, to describe the associated plant communities and to interpret the role that physical factors and disturbances such as fire and grazing have had in the changes of the structure of these plant communities. This information is needed for developing recommendations for grazing management, for prescribing appropriate improvement practices (e.g. shrub control, prescribed fire) and as guidelines for future research. The ecosystem was divided into smaller units using a hierarchical method, the categories of practical importance being ‘range unit’ and ‘range site’. They represent the catchment and hillslope scale of the water runoff–runon phenomenon, respectively. Vegetation was sampled using a block and cluster sampling design, registering tree, shrub, forb and grass species frequency, and the standing aerial biomass of the herbaceous layer in a sampling unitZ1 ha. Environmental data (topographic position, fire frequency, current and past use, and tree and shrub cover) were also registered for each sampling unit. Indirect ordination of sampling units classified according to range units and range sites, and correlation with environmental variables were performed using multidimensional scaling (MDS) as well as the vector fitting technique. Standing forage and stocking rate were estimated from biomass data. Results indicate that ‘range site’ is the ecosystem unit that should be considered for management purposes since it correlates well with plant communities: tall, hardwood forests are located on upland sites, woodlands are located on midland sites and savannas are located on lowland sites. Dense shrub thickets dominate in areas rated in poor condition, irrespective of range site. Disturbances such as fire and current and past use have a significant positive and negative correlation with range condition, respectively, suggesting that a state and transition model would explain vegetation dynamics better than the succession model. The estimated stocking rate in lowland sites in good condition was 2 ha UGK1, while in upland sites in poor condition the stocking rate was 90 ha UGK1. Active (fire, mechanical treatments) rather than passive (grazing management) methods should be used for range improvement in order to achieve the full potential of the ecosystem. q 2005 Elsevier Ltd. All rights reserved. Keywords: Chaco region; Range management; Range site 1. Introduction types, including woodlands, shrublands, savannas and forests (Bordo´n, 1983; Fumagalli et al., 1997). Sustainable grazing The province of Santiago del Estero is located in the management of these ‘rangelands’ requires both the generation ‘Chaco’ region, a vast plain that extends into northwestern of information leading to the proper management of animal Argentina and surrounding countries. Its climate ranges from numbers, such as forage yields, species composition, etc. and subhumid to arid and it is highly suitable for cow–calf the incorporation of new areas of complexity, such as spatial operations. In fact, the latter represent a significant source of variation and dynamics, into the management scheme (Walker, income for ranchers and farmers (Fumagalli et al., 1997). 1993). Ecological inventories should provide information on A key source of forage for cow–calf operations is the native physical landscape features as well as disturbance events and vegetation of the Chaco, that comprises several vegetation biotic processes (Maxwell et al., 1995). Several authors have described the landscape, plant communities, temporal dynamics and spatial variation of * Corresponding author. Tel.: C54 385 4224430. the native vegetation of the argentinian Chaco (Frenguelli, E-mail address: [email protected] (C. Kunst). 1940; Morello and Saravia Toledo, 1959; Sarmiento, 1963; 0301-4797/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. Morello and Ada´moli, 1974; Ada´moli et al., 1972; Ada´moli doi:10.1016/j.jenvman.2005.10.001 et al., 1990). These surveys present valuable information on C. Kunst et al. / Journal of Environmental Management 80 (2006) 248–265 249 the physical (climate, geomorphology, soils and fire regime) and biological (botanical composition) features of the Chaco ecosystems. However, differences in concept, objectives, level of perception, sampling methods of vegetation and interpretation sometimes preclude the application of this information in practical rangeland management. More recent information related to range condition and stocking rates is applicable only to a specific ranch or paddock (Kunst et al., 1987). The objectives of this study were: (a) to delineate and describe the ecosystem units of the southwestern part of the Santiago del Estero Province, Argentina; (b) to describe the plant communities (y rangelands) associated with the ecosystem units; (c) to interpret the relative role that physical site factors and disturbances such as fire and grazing have had in the changes of the structure of these plant communities; and (d) to estimate relative stocking rates from the peak forage produced by the herbaceous component of the rangelands. Inferences were also made about which of the current models of vegetation dynamics (e.g. succession and state-and- transition) (Briske et al., 2005) best explain the response of the plant communities to disturbances. Fig. 1. Range units of southwestern Santiago del Estero, with boundaries and major features. Adapted and modified from Angueira and Vargas Gil (1993). Detailed information on native rangeland is needed for developing recommendations for grazing management and prescribing appropriate improvement practices (e.g. shrub water infiltration, thus influencing local soil formation, control, prescribed fire) (Allen-Diaz and Bartolome, 1998; fertility, and growth conditions, as well as species composition Stringham et al., 2003); as guidelines for future research, and and yield of the plant communities (Teague and Smit, 1992; for appraisals of potential returns from economic investments Walker, 1993; Mauchamp et al., 1994; Ludwig and Tongway, (Danckwerts and Teague, 1989). 1995; Wondzell et al., 1996). We modified the Gasto´ method (Gasto´ et al., 1990, 1993) by introducing two ecosystem 2. Material and methods categories that represent two spatial scales of the water runoff– runon phenomenon: 2.1. Study area † the ‘range unit’ category (Schwartz and Walsh, 1991). It It is located in the southwestern part of the province of represents the ‘catchment’ scale of perception of the Santiago del Estero, Argentina, between 278 450–288 450 S process of the water runoff–runon phenomenon (Blo¨schl and 648 450–658 300 W. The study area is a square of and Sivapalan, 1995). A ‘range unit’ was outlined taking 50 km!50 km and comprises the Departments of Choya and into account landform at an approximate scale Guasaya´n of the same province, comprising approximately 1:100,000, soil series, soil associations, and soil texture 250,000 ha (Fig. 1). and depth. The distances involved could be expressed in units of 10 km each. 2.2. Ecosystem delineation and mapping † ‘range site’. It represents the ‘hillslope’ scale of perception of the process (Blo¨schl and Sivapalan, 1995). Range site We divided the ecosystem into homogeneous units using the was the basic ecosystem unit in this survey and was defined progressive and hierarchical disaggregation method developed by the relative position of a sampling unit (SU) in the by Gasto´ et al. (1990, 1993), which is similar to the concept of landscape. Three topographic positions were considered: ‘controlling factors’ described by Bailey (1996).Inthis lowland, intermediate or midland, and upland (Table 2): approach, the first controlling factor of the ecosystem is climate, they either receive extra water or lose water, according to acting at a broader scale, at the top of the hierarchy. On the other their position in the landscape. Distances involved are less hand, soil type and current use of the land act close to the ground, than 10 km, usually 1000–2000 m. at a greater scale. A detailed description of the hierarchy of controlling factors, the units into which the ecosystem was A map of range units and range sites with average scale divided, their conceptual definition and the scale of resolution y1:50,000 was produced by photointerpretation and photo- are presented in Table 1, which also has a list of the selected lecture of remote images (Landsat, aerial photographs and sources consulted. mosaics scale 1:50,000) and by gathering available geomor- In semiarid and arid regions of the world, the process of phological, climate and relief information, soil reports and water runoff–runon from source to sink areas controls soil maps at different scales (Table 1). 250 C. Kunst et al. / Journal of Environmental Management 80 (2006) 248–265 Table 1