Juan Anchorena

Spatial micro-patterns in the steppe of Tierra del Fuego induced by sheep grazing

In the Festuca gracillima tussock steppe of north- ern Tierra del Fuego there is a physiognomic pattern at the community scale composed of a matrix of tussocks, scrub patches of Chiliotrichum diffusum and lawn patches. In this paper we compared floristic composition, soils, microclimate and sheep use of these three types. Species composition greatly differed among physiognomic types. Lawn patches showed the highest diversity and maximum cover of exotics, grazing escape growth forms and species of high-fertility habitats. Soil profiles of tussock steppe and lawn were similar but the A- horizon of the latter contained more clay and cations. Radia- tion and temperature during daylight at the soil surface in summer were highest in the lawn patches. Soil impedance, a good indicator of trampling, was highest on the lawn patches. According to dung pellet density, lawn patches received much heavier use than the tussock matrix. Our results suggest that lawn patches supported the highest grazing impact and un- dergo a eutrophication process favoured by high mineral in- puts, high mineralization rates in the soil during the warm season and restricted leaching through the profile. They also support the idea that a feedback mechanism between vegeta- tion heterogeneity and sheep grazing behaviour maintains the patches. Although grazing behaviour suggests that the lawn patches favour animal production, their expansion would re- sult in a system less protected against the impact of water deficits and temperature variations and more vulnerable to erosion and to the loss of important species.

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.

A soil nutrient gradient in Magellanic Empetrum heathlands

The relationship of environmental and soil-nutrient variables with the floristic composition of Empetrum rubrum heathlands and related communities from northern Tierra del Fuego were studied by means of ordination techniques. A floristic gradient was found which had its axis of major variation closely related to a gradient in soil trophic status. Increases in the densities of Empetrum and other cushion plants (Bolax gummifera, Azorella lycopodioides, Pernettya pumila) along the floristic gradient were: (a) associated with a lower number of species and total cover, and greater soil erosion; (b) positively correlated with the C/N ratio and aluminium content in the soil; and (c) negatively correlated with pH, calcium content and base saturation. Climate, lithology, habitat exposure, and the history of human activity would be key factors in developing the oligotrophic conditions that favoured the establishment of different Empetrum heathlands.

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.

Effects of liming and fertilization on forage yield and vegetation in dry heath soils from Tierra del Fuego

Abstract The effects of increasing nutrient availability and liming on the growth of Dactylis glomerata and its neighboring vegetation were studied in dry heath communities dominated by Empetrum rubrum from northern Tierra del Fuego (Argentina). We carried out three pot experiments and a field experiment using acid heath soils collected from communities dominated by different degrees of cover by E. rubrum. All studies were consistent in showing high responses of D. glomerata to phosphorus (P) additions. Lime amendments depressed plant growth. This was attributed to two effects, increased sorption of added P by lime and decreased desorption of native P. The results also showed that the growth of D. glomerata on soil containing a high proportion of E. rubrum leaf litter was severely depressed. In the field experiment, additions of P increased the growth of D. glomerata and other grass species. This suggests that increasing nutrient availability may change the competitive relationship between shrub species a...

Wetlands of the Magellanic Steppe (Tierra Del Fuego, Argentina)

Our main objective was to classify vegetation and soils of wetlands in northern Tierra del Fuego (Argentina) along a latitudinal precipitation gradient within the Magellanic Steppe Zone. We presented the first detailed ecological characterization of these wetlands by relating floristic composition to local site conditions, bedrock and climate. The survey consisted of 125 phytosociological censuses and 52 soil profile descriptions. Soils were classified according to FAO, and vegetation samples were explored by applying numeric methods such as cluster analysis and indirect ordination that included a post-hoc correlation with environmental variables. Floristic composition of freshwater communities was strongly related to a latitudinal gradient, and to soil pH, base cations and C/N ratio. Most eutrophic marshes were found on organic soils situated in springs on Tertiary sediments at mid-latitudes. Most acidic marshes occurred within Pleistocene catchment areas at higher latitudes (higher precipitation). Wet grasslands on mineral soils of low C/N ratio within formerly glaciated areas at lower latitudes (lower precipitation) represented the dry end of the moisture gradient covered by this study. Saltwater communities predominated on playas of Tertiary sediments and in estuarine systems. They were characterized by alkaline soils and high Na concentrations. Magellanic wetlands form a floristic continuum from the semiarid north to the subarid south with composition not necessarily related to landscape position and soil units, but strongly related to soil variables of secondary taxonomic level. We conclude that at least at the resolution level of our study, floristic composition along the observed gradient is strongly influenced by regional climate.

Grazing and Grazing Exclusion Along a Resource Gradient in Magellanic Meadows of Tierra del Fuego

Abstract Although Magellanic meadows have been subject to ungulate herbivory for a long time; they have suffered severe grazing by domestic sheep since the end of the 19th century. We hypothesized that, if protected from grazing, the present dominant dwarf forms would be replaced by taller competitive forms, mainly grasses. We then monitored floristic composition along a gradient of soils over periods of 5 to 16 yr on seven meadow sites under grazing and, in five, also on exclosures. Initial and final floristic samples were classified with TWINSPAN and ordinated with Detrended Correspondence Analysis (DCA). We also tracked cover changes in life-forms (grasses, dwarf sedges/rushes, rhizomatous helophytes, and cushion shrubs). At the end of the study, we assessed floristic similarity between paired grazed and exclosed treatments, and compared their life-form cover, diversity, biomass, and soil compaction with Students t and Mann-Whitneys tests. Temporal displacements of composition in ordination space showed the effect of a long drought (DCA axis 1) and of grazing removal (DCA axis 2). Grazing exclusion had a much greater impact in wet meadows–where grasses became the dominant life-form–than in dry meadows. The effect of drought was shown in 1) the expansion of the cushion shrub Azorella trifurcata (Gaertn.) Pers. in both grazed and nongrazed treatments of the drier wetlands; 2) the decrease in Caltha sagittata Cav. in riparian meadows, and its encroachment under grazing in the wettest spring meadow; and 3) the large increase in Poa pratensis L. in exclosed wet meadows. Despite this strong response to environmental conditions, overall results supported the idea that these wetlands are equilibrium systems. Although the drier meadows have crossed an irreversible threshold, the Caltha-encroached wet meadows returned to a grass state in about 4 yr when protected from grazing. Their resilience suggests that they could be partly restored with rest-rotation grazing.