Ana M. Cingolani

GRAZING EFFECTS ON RANGELAND DIVERSITY: A SYNTHESIS OF CONTEMPORARY MODELS

Two independent models concerning the effects of grazing on vegetation have gained wide acceptance in the last decade: Westoby et al.s state-and-transition (S– T) model, and Milchunas et al.s generalized model of the effects of grazing on plant community structure and diversity (MSL model). These two prevailing models, as they stand, are conceptually divergent. The MSL model implicitly assumes that, at a given site, for each grazing intensity there is a single equilibrium situation with a single diversity value. The S–T model suggests that rangeland dynamics include irreversible transitions and alternative equilibria. Here we propose a modification of the original MSL model, to encompass a wider range of real situations and to place it within the context of the S–T model. The four extreme cases proposed in the original MSL model are revisited, taking into account that (1) the “moisture” gradient can be generalized as a “productivity” gradient; (2) the selective pressure of herbivores on systems with lon...

Combined effects of environment and grazing on vegetation structure in Argentine granite grasslands

Abstract Effects of grazing and environment on vegetation structure have been widely acknowledged, but few studies have related both factors. We made 57 floristic samples in a highly variable landscape of mountain grasslands in central Argentina; 26 sample were in fence-lines with contrasting vegetation. For each sample, we recorded topographic and edaphic parameters, as well as grazing intensity indicators. Floristic gradients were analysed with DCA and relations with abiotic and grazing-related variables were detected with DCCA. Floristic axis 1 was explained by edaphic parameters associated to topography, ranging from communities in well drained soils on upper topographic positions to hydromorphic vegetation in poorly drained soils on lower topographic positions. Species richness decreased as soil moisture increased. Floristic axis 2 was associated with present and long-term grazing indicators, and reflected shifts in vegetation physiognomy and species evenness. Tall tussock grasslands, with low species evenness and evidences of low or null grazing intensity were located at one extreme. Tussocks were gradually replaced by short graminoids and forbs towards the centre of the gradient, as grazing increased, and evenness reached a maximum. In degraded sites with heavy long-term grazing intensities, short perennial species were replaced by an annual species, and evenness decreased. The magnitude of changes in floristic composition produced by grazing decreased with increasing soil moisture, and vegetation-environment relationships were stronger in moderate to highly grazed situations than in lightly or non grazed situations.

Efectos del fuego sobre un bosquecillo de Polylepis australis (Rosaceae) en las montañas de Córdoba, Argentina

El fuego es usado para transformar bosques en pastizales y estimular el rebrote de los pastos para aumentar su valor forrajero. Segun algunos autores, este uso del fuego ha provocado una retraccion de los bosques de Polylepis a sitios rocosos inaccesibles al fuego, mientras que otros autores postulan que su distribucion es debida al ambiente favorable en las rocas y no a la accion del fuego. Aqui evaluamos el efecto relativo del fuego y el ambiente sobre la supervivencia, produccion de semillas y crecimiento de Polylepis australis Bitter (Rosaceae) en un bosquecillo de las montanas de Cordoba. Realizamos un seguimiento de 74 individuos quemados y controles durante 3,5 anos. La presencia de rocas tuvo una fuerte influencia en el porcentaje en que se quemaron los individuos. A su vez, el porcentaje en que se quemaron se relaciono negativamente con la supervivencia, produccion de semillas y el crecimiento en altura. La produccion de biomasa fue mayor cuando se quemaron en un porcentaje intermedio. Exceptuando la proteccion al fuego, la roca no tuvo ningun efecto positivo sobre las variables medidas. Estos resultados sugieren que la distribucion actual de P. australis es al menos en parte debida al fuego. Para su conservacion sugerimos minimizar los fuegos, desarrollar actividades con alto riesgo de incendio en sitios con alta proporcion de roca y, si es posible, proteger los ejemplares expuestos al fuego con piedras.

Factors associated with woody alien species distribution in a newly invaded mountain system of central Argentina

To help determine the major factors associated with alien plant in a newly invaded mountain range; we analyzed the distribution patterns of woody alien species along the altitudinal gradient of the Córdoba mountains, in relation to biotic, abiotic and anthropogenic factors. We selected 303 plots using a Geographic Information System (GIS) covering all the variability of these factors. In the field we registered woody alien occurrence in these 303 and in 303 additional neighbor plots. We used 12 biotic, abiotic and anthropogenic variables from the GIS to predict the probability of presence of alien species through a logistic model. Then, we analyzed if neighbor alien occurrences could explain some additional variance. We created a probability map with 4 categories of alien occurrence which was then validated by new field sampling. Occurrence of woody aliens was highest in the eastern slope (with longer history of species introduction), at low altitudes, near sources of propagule pressure (human settlements, roads and neighbor sites with established alien plants), and associated to sheltered topographies. In the upper belt of the Córdoba Mountains woody invasion is incipient and thus in a transient stage. Accordingly, propagule pressure seems to be the major factor at play, while the relevance of disturbance and biotic interactions is less clear.

Avian responses to tourism in the biogeographically isolated high Córdoba Mountains, Argentina

Species do not respond identically to the presence of humans, and this may have consequences at higher-levels of ecological organization. We established bird transects on and off recreational trails in the high Córdoba Mountains of Argentina, a biogeographic island characterized by high levels of endemism, to examine the effect of human visitation at three different levels: (a) community (avian species richness and diversity), (b) guild (relative density of carnivores, granivores, insectivores, and omnivores), and (c) population (relative density of individual bird species). Human presence in the high Córdoba Mountains decreased avian species richness and diversity, and reduced insectivorous relative density, but we did not detect significant effects on granivores, omnivores, and carnivores. At the population level, 6 of 28 species were negatively affected by human visitation; four of these species are of conservation concern. Our results show negative responses to recreationists at multiple levels (e.g., reductions in density, displacement of species from highly visited areas), which may be related to spatial and temporal access to suitable resources, physical disturbance or species-specific tolerance thresholds. Our study area had lower levels of human visitation relative to other protected areas in the Northern Hemisphere, which raises the issue of whether this kind of biogeographically isolated habitat may be too fragile to sustain increasing levels of tourism.

The steppes of Tierra del Fuego: floristic and growthform patterns controlled by soil fertility and moisture

Northern Tierra del Fuego is the insular extreme of the Magellanic steppe, a unique example of a cold-temperate oceanic grassland in South America. We surveyed this poorly known region with a landscape-based procedure to classify plant communities and investigate environmental gradients. Sampling sites were selected with the aid of aerial photos, and floristic composition, soil, topography and landform type were recorded in each site. Classification was achieved with Two-Way Indicator Species Analysis and gradients were analyzed with Detrended Canonical Correspondence Analysis. Two broad groups of communities were separated by the presence of typical acidophilous dwarf shrubs: acidophilous/mesotrophic and neutrophilous vegetation. Along the main floristic gradient, ericoid dwarf shrubs were replaced by forbs and grasses as soil acidity decreased. The acidic habitats were on Pleistocene glacial and alluvial deposits, and had coarse texture, poorly decomposed organic matter, and low base status. The eutrophic habitats were on Tertiary rocks of medium to fine texture and had well-decomposed organic matter and high base status. A second axis of variation was associated to latitudinal position and slope aspect, revealing a precipitation/evaporation gradient. Scrubs dominated by midheight shrubs, with species from the southern forests, were at the humid extreme (higher latitudes/south aspects); and short grass steppes, with taxa commonly found further North in the continent, at the xeric end (lower latitudes/north aspects). Although fertility and moisture seem to control the overall floristic gradient, the extreme vegetation types of the fertility gradient, both having a very simple structure, could be attributed to grazing disturbance. Floristics and ecology of the Fuegian steppe resemble to a certain degree some boreal oceanic systems.

High-Andean vegetation and environmental gradients in northwestern Patagonia, Argentina

We analysed the heterogeneity of high-elevation vegetation on three mountains along a west-east transect at 41 ?S lat. in the Andes of northwestern Patagonia, Argentina. In this area, high-Andean vegetation occurs as islands on mountain tops above Nothofagus pumilio forests - with the timberline at ca. 1700 m a.s.l. We recorded floristic, topo- graphic and substrate data in 166 sites stratified according to longitude, altitude, slope and aspect. Vegetation data were classified with TWINSPAN and ordinated with Detrended Correspondence Analysis. The relationship between environ- mental and floristic variation was analysed using Canonical Correspondence Analysis. In order of importance, geographi- cal longitude, altitude and aspect were the major determinants of vegetation variation, whereas substrate texture and slope appeared less important. The combination of these factors resulted in two main vegetation gradients. The first gradient is related to a moisture availability gradient, primarily deter- mined by longitude and secondarily to variation in wind exposure (east vs. west aspect). The second vegetation gradi- ent is related to variation in temperature, primarily deter- mined by altitude, and secondarily by variation in insolation related to the contrast between north and south aspects. The four communities obtained with TWINSPAN are therefore associated with four characteristic habitat types: moist-cold, moist-warm, dry-cold and dry-warm. The community of warm and dry environments is the richest and has elements in com- mon with dry steppe communities situated at lower elevations to the east, while the vegetation of the cold-moist habitat type has unique elements that are typical of the southern Andes. Although current climatic factors appear to be the major determinants of high-Andean vegetation gradients, historical events of Pleistocene times probably also affected the vegeta- tion patterns we see today.

Can livestock and fires convert the sub-tropical mountain rangelands of central Argentina into a rocky desert?

Soil erosion, as a result of livestock grazing, has been widely reported for arid and semiarid ecosystems, but information is lacking in more mesic ecosystems where erosion is generally studied in relation to agriculture. To test the hypothesis that, in the high-mountain rangelands of Cordoba (Argentina), grazing by livestock can drive the system into a rocky desert, 200 4 � 4m plots under different livestock stocking rates and timings of grazing were monitored for 5 years. Fourindicatorsof soilerosion: changerate of rocksurface andof totalbare surface,advance rateof erosionedges, andtheir activity persistence were estimated for each plot. Erosion edges are steps with a vertical bare soil surface, whose advance usuallyleavesbehindanexposedrockarea.Foreachplot,theaverageannualstockingrateforthe5-yearperiod,andanindex ofseasonality,werecalculated.Multipleregressionswereusedtoanalysethedata.Underhighstockingrates,rockandbare surface increased, edges advanced faster and persisted more actively, while under low or nil stocking rates, rock and bare surfacedecreasedandedgestendedtostabilise.Fromtheseresults,itwasestimatedthatunderhighstockingrates,18%ofthe wholeareacouldbetransformedintorockysurfacein400years.As fireisausualtoolforthisrangelandmanagement,surface soillossduring1yearin77burnedandunburnedplots,withandwithoutpost-firelivestockgrazing,werecompared.Burned plots lost 0.6cm of surface soil when grazed, and 0.4cm when ungrazed, while unburned plots lost less than 0.05cm when grazed, and gained 0.07cm when ungrazed. It was concluded that the present-day combination of livestock and fire management has the potential to convert this rangeland into a rocky desert. It is suggested that commercial livestock production,asitiscarriedonatpresent,isnotsustainable,andsomesuggestionsonchangesnecessaryforafuturesustainable grazing industry are made. Additional keywords: desertification, land degradation, overgrazing, restoration, soil erosion.

Landscape heterogeneity and long-term animal production in Tierra del Fuego.

Grasslands of northern Tierra del Fuego sustain 1 sheep/ha and are very extensively managed, with flocks roaming freely in large paddocks (2,000-4,000 ha). This system requires knowledge of landscape-level constraints and influences upon production for decision making. On a typical sheep ranch we checked upland floristic gradients against 30-years records of animal production. Community types and landscape units were surveyed and mapped. Using gradient analysis techniques we obtained animal production differences at the landscape scale that were strongly related to a vegetation gradient associated with soil fertility. Extensive and strongly variable lithological mantles allowed expression of the fertility gradient at that scale. Landscapes with fertile soils and neutrophilous community types were best for sheep breeding. These landscapes produced a mean of 37% more lambs ha-1 yr-1 than lands with soils of intermediate fertility and slightly acidophilous community types, and 116% more lambs ha-1 yr-1 than lands with highly infertile soils and highly acidophilous vegetation. Contrarily, the soil moisture gradient, being mainly expressed at the topographic scale, was not related with sheep production records. A forage gradient which was identified behind the fertility gradient supported our findings. Poa spp., the main item in sheep diets, and other important forage species attained the highest covers in neutrophilous community types. With the range in proportion of lowlands present in this ranch (12 to 30%), no relationship was found between the percentage of hygrophitic vegetation in the landscape and animal production.

A landscape-scale model for optimal management of sheep grazing in the Magellanic steppe

Abstract Effective management of rangelands requires the development of landscape-scale models for predicting spatial and temporal variability of forage. In the Magellanic tussock steppes, as in other cold-temperate regions, grazing capacity is dependent on the winter season. To develop a management tool for the region, we analysed links between winter forage availability, weather, stocking rate and vegetation structure. We studied four paddocks over five years with a range of stocking rates from 0 to 1.53 sheep.ha–1. We sampled forb and non-tussock graminoid biomass, vegetation structure and faecal pellet abundance at the end of each summer. Daily temperature and rainfall data were also recorded. A regression model explained the amount of winter forage as a positive function of graminoid cover, spring minimum temperature, annual precipitation and a negative function of dwarf shrub canopy, bare soil and stocking rate (R2 = 0.59). Interactions of structural variables with precipitation and stocking rate were detected, indicating strong fluctuations of forage availability in lawn communities dominated by short graminoids. The most probable causes of this response would be higher utilisation and lack of canopy structure. Our results illustrate how maps of vegetation structure, obtainable from satellite images, with weather and stocking rate data could be used for predicting optimal stocking rates in large, heterogeneous sheep paddocks. Nomenclature: Correa (1969–1984); Moore (1983). Abbreviation: EE = Ewe equivalent.