Rolando J.C. León

Grazing effects upon plant community structure in subhumid grasslands of Argentina

Changes in plant community structure are identified as a result of grazing in grasslands of the flooding pampa which evolved under supposedly light grazing conditions. The effect of excluding grazing upon total leaf area index was an increase of 30%. The largest response was observed in the distribution of leaves in the canopy. In the grazed areas, most of the green material was concentrated in the 0–5 cm layer while in the ungrazed treatments the largest portion of the leaf area was in the 10–30 cm layer. Grazing exclusion resulted in a small change in total basal area but a larger change in its distribution, from many small tussocks to less numerous large ones. The effect of grazing upon leaf area and basal area was accounted for by changes in vigor as well as by changes in species composition. The major effect of excluding grazing upon species composition was the disappearance of some native planophile species and most of the exotics. The species composition of grazed areas of both communities was very similar while there were large differences between the ungrazed areas and between the grazed and ungrazed areas of the same community. It is suggested that there is a group of species which responds to the coarse-grained ‘signal’ of grazing and its presence can cause dissimilar communities to converge under grazing conditions. The other group of species responded to the fine-grained ‘signal’ of the environmental conditions associated with topography.

Limits to tree species invasion in pampean grassland and forest plant communities

Factors limiting tree invasion in the Inland Pampas of Argentina were studied by monitoring the establishment of four alien tree species in remnant grassland and cultivated forest stands. We tested whether disturbances facilitated tree seedling recruitment and survival once seeds of invaders were made available by hand sowing. Seed addition to grassland failed to produce seedlings of two study species, Ligustrum lucidum and Ulmus pumila, but did result in abundant recruitment of Gleditsia triacanthos and Prosopis caldenia. While emergence was sparse in intact grassland, seedling densities were significantly increased by canopy and soil disturbances. Longer-term surveys showed that only Gleditsia became successfully established in disturbed grassland. These results support the hypothesis that interference from herbaceous vegetation may play a significant role in slowing down tree invasion, whereas disturbances create microsites that can be exploited by invasive woody plants. Seed sowing in a Ligustrum forest promoted the emergence of all four study species in understorey and treefall gap conditions. Litter removal had species-specific effects on emergence and early seedling growth, but had little impact on survivorship. Seedlings emerging under the closed forest canopy died within a few months. In the treefall gap, recruits of Gleditsia and Prosopis survived the first year, but did not survive in the longer term after natural gap closure. The forest community thus appeared less susceptible to colonization by alien trees than the grassland. We conclude that tree invasion in this system is strongly limited by the availability of recruitment microsites and biotic interactions, as well as by dispersal from existing propagule sources.

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.

Woody species invasion in the Rolling Pampa grasslands, Argentina.

Information on the geographical distribution of plant invasion has been recorded in detail in some areas of the world; however, in large regions such as South America there are a few, if any, records of the spread of alien plants and invasive species and even less information about their effects on ecosystems at different levels of organization. This study examines the extent to which woody species introduced during the last centuries are invading the Rolling Pampa (which is typical of the entire region of the Argentina pampas) and discusses whether this invasion is related to the species’ genetics or to environmental factors. All woody species were surveyed along landscape corridors (highways and intersecting secondary dirt roads and streams), as well as in farmed fields under three different tillage systems: zero tillage in the entire field for all crops in the rotation (where tillage was replaced by a presowing herbicide application), zero tillage for selected crops, and conventional tillage. Landscape corridors along the roads had been invaded by 40 woody species (mostly trees). On the farmed land, fields under the zero tillage farming system were invaded by seven woody species (three tree species and four shrubs). With zero tillage for select crops only, woody species richness was reduced to three (one tree and two shrub species). In the conventional tillage, there were only three invading species, all shrubs. In both the roadside and riparian corridors, the species with the highest constancy values were Gleditsia triacanthos L., Morus alba L., and Melia azedarach L. In both types of zero tillage farmed fields, M. alba was absent, but G. triacanthos and M. azedarach remained the species with the highest constancy values. Both genetic and ecological factors were important determinants for the invasion of the pampas by woody species. The woody invasion process has reached a point at which the pampean grasslands on the better-drained soils will no longer be restored to a grassland biome without human intervention.

Floristic changes induced by flooding on grazed and ungrazed lowland grasslands in Argentina.

Ch8nges in community composition of 2 gmssl8nd sites exposed to a flood of unusual intensity and duntion were investig8ted in the Flooding Puap8. These gr8sslurds ue subject almost 8nnurlly to floodings of lesser m8gnitudc. The study site8 were l dj8cent to e8ch other, urd differed in veget8tion structure urd composition. One hid been grazed continuously by cattle 8nd ~8s showing signs of intense deteriontion. The other h8d rem8ined ungr8xed during 15 years. Brul cover by species WM meuured in summer, before 8nd 8fter the flooding event. Compositionrrl difference between sites decreased with flooding from 68.9 to 39.1%. In the grued site the cover of alien forbs ~88 reduced by 4g%. After the flooding native gruninoids represented 99.7 8nd 86.7% of the cover, inside 8nd outside the exclosure respectively. Tot81 b8s8l cover ~8s not 8ffected but ~8s redistributed 8mong species 8lre8dy present before the flood. Floristic changes would h8ve led to 8n improvement of the forage source. We conclude that plant community response to the event ~8s influenced by the previous gruing history of the site. The large flood 8cted n 8n overriding environmental frctor which p8rti8lly reverted the effects of gr8xing upon gr8ssl8nd composition.

Vegetation heterogeneity and diversity in flat and mountain landscapes of Patagonia (Argentina)

We studied floristic and diversity patterns and their environmental controls in two landscapes of contrasting to- pography in the Patagonian steppe. The analyses were focused on the effects of water availability gradients and landscape configuration on plant species distribution and coexistence. Floristic variation was investigated using Correspondence Analysis. The relationship between floristic and environmen- tal variation was analyzed using Canonical Correspondence Analysis and correlation tests. We explored diversity patterns by relating spatial distance to floristic dissimilarities. The floristic gradient was determined by shrub and grass species and was related to precipitation in the flat area, and to precipi- tation, elevation and potential radiation in the mountain area. Site species richness increased with water availability in both areas. Mean site species richness and species turnover in space was higher in the mountain than in the flat area. Landscape species richness and floristic gradients were more concen- trated in the mountain than in the flat area. In contrast to shrubs

The role of a native tussock grass (Paspalum quadrifarium Lam.) in structuring plant communities in the Flooding Pampa grasslands, Argentina

The patches of Paspalum quadrifarium-dominated grasslands found atpresent could be remnants of the vegetation that defined the Pampas landscape inthe past. The mere presence of such physical structures should lead to systemsin which many other species are dependent upon both the autogenic creation ofsurface area for living space and the autogenic and allogenic modulation ofresources controlled by these structures. We carried out amensurative–comparative study in naturally occurring sites dominated byP. quadrifarium that occupy different positions within thelandscape in the Flooding Pampa. We found different species assemblages in thesestructurally homogeneous stands, related to the edaphic and topographicgradients. The comparison with sites from similar positions in the landscapewhere this native tussock-grass was absent showed that the dominance ofP. quadrifarium is a minor determinant of floristicdifferences, which are mainly controlled by abiotic factors. We assigned thesedominated grassland stands to previously delimited vegetation units for theFlooding Pampa grasslands and we compared them with the vegetation unitsassigned, in terms of composite properties of the community that may beindicators of ecosystem processes, such as biological invasions andproductivity. Our results show that this native tussock grass is associated withhigher floristic richness, higher representation of perennials and grasses andlower presence of alien plants. This homogeneous pattern of variation across thevegetation units would suggest that, despite the absence of dramatic changes incommunity composition, the dominance of this tussock grass plays an importantrole in structuring species diversity patterns.

Old-field successional dynamics on the Inland Pampa, Argentina

. The first 10 yr of old-field successional dynamics on the Argentine Inland Pampa were studied on a series of adjacent plots established consecutively between 1978 and 1989. We examined differences in species abundance patterns among plots in order to detect the spatial and temporal variability of succession. Perennial grasses steadily increased in cover and replaced the dominant annual species after 5 yr. Pioneer dicots persisted in older seral stages with 20 — 23 species/plot. Overall, exotic species (mostly the grasses Lolium multiflorum and Cynodon dactylon) contributed much to the plant cover in these communities. Native grasses comprised 45 % of total cover at years 7 — 10 of succession, but occurred with less than 7 species/plot. Substantial variation was found in the successional pathway, which reflected the particular sequence from annual forbs to short-lived and perennial grasses in the various plots. The course of succession was apparently influenced by a 2-yr period of unusually high rainfall. Deyeuxia viridiflavescens, a native perennial grass virtually absent before the wet period, spread over the study area and dominated seral communities for 3 yr, irrespective of plot age. Climatic conditions thus affected the successional turnover of life forms by increasing the rate of colonization by perennial grasses. We further point out the constraints imposed on secondary succession by the life histories of ‘available’ species.

Environmental controls of NDVI dynamics in Patagonia based on NOAA-AVHRR satellite data

. Variation in vegetation in extra-Andean Patagonia (Argentina) was analyzed using spectral data derived from AVHRR/NOAA satellite. The study of seasonal dynamics of the Normalized Difference Vegetation Index (NDVI, i.e. a combined index of the reflection in the red and infrared bands) highlighted similarities in functional aspects between regional vegetation units which are dissimilar in a geographical, physiognomical and/or floristical way, and also suggested that gross primary production is correlated with mean annual rainfall. The first axis in a Principal Component Analysis of NDVI data was correlated (r2 = 0.90) with NDVI as integrated for the study period. The second axis was correlated (r2 = 0.50) with the differences in NDVI during the growing season, reflecting seasonality. Mean annual rainfall accounted for 60% of integrated NDVI variability among vegetation units. Much of the residual variance (62%) was accounted for by the inverse of the distance to the Atlantic Ocean, which is interpreted as an ocean effect on vegetation functioning in the extra-Andean Patagonia.

The Use of Satellite Imagery in Quantitative Phytogeography: A Case Study of Patagonia (Argentina)

Ecological surveys of arid zones, both with basic and applied phytogeographical objectives, are very difficult because of their magnitude and the low density of their renewable natural resources. The lack of adequate knowledge of the heterogeneity and the functioning of the vegetation is common in these regions. This leads to increasing damage of the environment as a consequence of the inbalance between supply and demand in the use of their natural resources (Soriano 1986).