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We built a grid-based spatial explicit stochastic model that simulates grazing events and basic processes like seedling establishment, growth or mortality of the dominant species in the grass steppes of Patagonia. After evaluating the model with field data, we performed simulation experiments aimed to explore the interaction of precipitation and grazing regimes on vegetation dynamics. Grazing generated a reduction in tussock density which results in a decline in aboveground net primary production (ANPP). Both response variables presented a non-linear behavior including high temporal variability and delay effects, which may prolong for decades. There was a clear threshold in the response of the variables to stock density, though changes become evident only when a highly selective grazing scenario was used. Under high stock density conditions, precipitation use efficiency (PUE) was 82% lower than the values for non-grazed runs. The inter-annual variability of precipitation was more important than the grazing regime in explaining differences in tussock density. Simulation results highlight important issues regarding rangeland management: grazing regime might be as important as stocking density as a degradation agent, temporal lags might obscure degradation processes for decades, the definition of monitoring variables need to consider their response time constants. Fil: Paruelo, José. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Pütz, S.. UFZ Centre for Environmental Research Leipzig-Halle; Alemania Fil: Weber, G.. UFZ Centre for Environmental Research Leipzig-Halle; Alemania Fil: Bertiller, Monica Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina Fil: Golluscio, Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Wiegand, T.. UFZ Centre for Environmental Research Leipzig-Halle; Alemania

Current climate change models for arid ecosystems challenge our ability to understand the effects of droughts on plant population dynamics. In a sandy soil plant community from the Patagonian steppe, we studied soil water dynamics and its effects on grass regeneration. Our hypothesis was that a decrease in precipitation would not change soil water dynamics of the top soil layer where most of the seeds and seedling roots are located. First, we simulated soil water dynamics with a balance model (DINAQUA) for different years and soil layers, in two different periods related to the emergence and survival of plants. Then, we performed a manipulative field experiment in which we generated a drought gradient by intercepting 0%, 25%, 50%, and 80% of the precipitation for two different microsites (vegetated and bare soil). We quantified drought effects on emergence, survival, and biomass of seedlings using a native grass ( Bromus pictus Hook). Soil water content in the top soil layer at the end of growing season was a 61% less than mid and bottom soil layers, independently of the climatic condition of the year. The simulations suggested that drought reduces the maximum depth of water infiltration, but available water for plants in the top layer where seedlings have their roots, was not modified, in agreement with our field experiment measurements. Consistently, the emergence (51%), survival (65%), and biomass (15 g/ind) of seedlings were not affected by drought levels. However, there is a positive microsite effect on biomass (36%), indicating a facilitative role of adult plants on grass seedlings. We interpreted these results as indicative that seed germination and seedling recruitment will not be heavily impacted by increasing water deficit in the Patagonian steppe.

Domestic grazing effects on primary productivity and community structure are controversial in rangeland ecology and frequently misunderstood. Although directly related with secondary production, biomass stock and biomass production at species level (biomass composition) has been relegated in field studies, especially in arid rangelands co-dominated by woody species. We estimated grazing effects on aboveground biomass in a temperate mixed grass-shrub steppe of Patagonia. We compared exclusion of sheep with two levels of continuous grazing: moderate (light) and intensive sheep grazing in an average precipitation year. Total green biomass (productivity) was twice as high in moderately grazed paddocks as in those without grazing and intensively grazed pastures, while standing dead grass biomass stock only decreased in intensive grazing. Shrub biomass was not modified by grazing management. In addition, grazing modified grass specific biomass composition, thus diminishing biomass quality in intensively grazed areas. This work provides evidence that in arid rangelands, continuous moderate grazing management could be an effective tool to increase productivity compared to grazing exclusion. Furthermore, moderate grazing would not cause major undesired changes in species composition. However, a potential risk of land use intensification exists because intensive grazing could decrease biomass production as well as promote negative composition changes. Fil: Oñatibia, Gastón Rafael. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina

Summary 1 We use a grid- and simulation-based approach to extend point pattern analysis to deal with plants of finite size and irregular shape, and compare the results of our approach with that of the conventional point approximation. The plants are approximated by using an underlying grid and may occupy several adjacent grid cells depending on their size and shape. Null models correspond to that of point pattern analysis but need to be modified to account for the finite size and irregular shape of plants. 2 We use a mapped area of a grass-shrub steppe in semi-arid Patagonia, Argentina, to show that the shrub community is essentially randomly structured, but that shrubs facilitate grasses in their immediate neighbourhood. 3 The occurrence of this random spatial structure provides important new information on the biology of shrub populations. In general, previous data from semi-arid and arid ecosystems have shown that adult shrubs tend to show over-dispersed patterns, whereas juveniles are clumped. 4 We find that the point approximation may produce misleading results (i) if plant size varies greatly, (ii) if the scale of interest is of the same order of magnitude as the size of the plants, and (iii) if the plants of a given pattern are constrained through competition for space by the presence of other plants. The point approximation worked well in all other cases, but usually depicted weaker significant effects than when the size and shape of plants were taken into account. 5 Our approach to quantifying small-scale spatial patterns in plant communities has broad applications, including the study of facilitation and competition. Ecologists will be able to use the software available to take advantage of these methods.

Summary 1The recovery of rare, threatened plant populations can sometimes be achieved through modification of the disturbance regime. Accumulation of litter is likely to follow a reduction in grazing pressure and we therefore examine its effects on seedling recruitment of a threatened species in a temperate arid rangeland. 2We studied the effects of litter on seed longevity in the field and performed glasshouse and field experiments with natural and plastic litter to assess their physical and biological/chemical nature. Seeds were sown on the soil surface, buried or within the litter layer. Published data on spatial distribution of seeds were used to calculate the net effect of litter on seedling recruitment. 3Litter increased seed longevity. In the glasshouse, litter increased seedling emergence and growth for surface, but not for buried, seeds. Seeds within the litter layer (no seed-soil contact) showed reduced seedling emergence and growth. In the field, litter did not have a direct effect but emergence was promoted by burial. 4Integrating the effects of microsite quality and seed density showed that litter microsites recruited c. 50% of Bromus pictus seedlings, double that expected from its cover (25%). The positive effect of litter on seed density and on seed longevity outweighed the negative effect of litter acting as a mechanical barrier to burial. 5Inconsistent effects of litter on plant recruitment in arid environments may be due to responses being dependent on seed size and shape, and thus may represent indirect effects acting via the probability of burial. Alternatively, studies may report effects only on seed retention or emergence and growth rather than net effects on establishment.

Most arid and semiarid ecosystems around the world have been grazed by domestic herbivores. In many cases, grazing has degraded vegetation and soil. The possibility of restoring rangeland's good condition depends, partially, on the ability of remaining populations of desirable species to recover. In this work, we studied the exact spatial distribution of remaining palatable adult plants in fields with different grazing history (i.e., seed sources) and quantified the effect of interspecific competition with less palatable grasses on seedling emergence and survival (i.e., regeneration constraints). We worked in a Patagonian steppe composed of shrubs and perennial tussock grasses that has been grazed by sheep for >100 years. In order to evaluate the location of seed sources, we mapped the location of a palatable species (Bromus pictus Hook.) in paddocks with different long-term grazing intensity. In addition, we sowed seeds of B. pictus close to 2 dominant, less palatable grasses in 2 different years to evaluate the role of interspecific interactions on regeneration and the effects of climate variability. The proportion of B. pictus plants growing in protected places near less palatable species significantly increased with grazing intensity. Competition effects on emergence, survival, and growth depended on the year's moisture regime. During the dry year, competition with less palatable grasses reduced the emergence, survival, height, and number of leaves of palatable grass seedlings by 30%, 55%, 48%, and 40%, respectively. In the wet year, there were no effects of competition on emergence and height, and the effects on survival depended on the species of the less palatable neighbors. Our study supports the idea that management for recovering degraded rangelands in this ecosystem may benefit from considering the spatial distribution of remaining plants. It also indicates that the susceptibility of demographic processes to interspecific competition depends on the year and neighbor species.

Soil resource availability varies across seasons and soil layers. Both plant phenology, i.e. the sequence of growth and developmental changes along the year, and plant life form, i.e. the life cycle, distribution of roots, and longevity of leaves, determine the ability to use resources. The phenological heterogeneity within and among life forms of a single community may reveal key features of community structure, such as temporal niche segregation within life forms or convergence of phenological and life form patterns. We described the phenological patterns of most species of a Patagonian steppe composed of four life forms: perennial grasses, shrubs, annual herbs, and perennial herbs. By applying standard multivariate analysis techniques, we classified the species into three phenological groups and analysed the variation among groups and life forms. One phenological group, composed exclusively by grasses, had higher autumn-winter phenological activity than the other two groups. The non-grass groups differed in the date of beginning of vegetative growth and ending of the reproductive growth, one of the groups being earlier than the other. However, the three groups coincided at the phenophase of floral buds and open flowers, which occurred around the time of the maximum rate of phenological change and maximum fraction of photosynthetically active radiation being intercepted by the entire community. Phenological groups and life forms only partially overlapped: grasses and annual herbs belonged to individual phenological groups, whereas shrubs and perennial herbs spread across two groups. Thus, some life forms of the community were strictly related to a phenological behaviour, whereas other showed high temporal niche segregation. Simultaneously, diverse life forms converged into similar phenological patterns.

One of the potential mechanisms for the impact of herbivores on nutrient cycling is the effect of selective grazing on litter quality through changes in species composition. However, the scarce evidence collected on this mechanism is controversial and seemingly influenced by site-specific variables. In this paper, we explored the consequences of grazing-induced changes in species composition on litter quality and nitrogen cycling with a regional perspective. Along a 900-mm of mean annual rainfall gradient, we selected species promoted and diminished by grazing from three natural rangelands of Argentina, analyzed their litter quality, and determined their decomposition and nutrient release kinetics under common greenhouse conditions. Litter quality and decomposition rates were strongly associated with plant response to grazing. However, the magnitude and direction of these differences depended on the ecosystem considered. In the wettest site, the species promoted by grazing (forbs) had higher nitrogen and phosphorus contents, faster decomposition rates, and higher release of nitrogen to the soil than species diminished by grazing (C3 and C4 grasses). In the intermediate and dry sites, species promoted by grazing had lower nitrogen and phosphorus contents, and slower decomposition rates than those diminished by grazing (C3 grasses in both cases). Decomposition of the entire group of species was not correlated with mean annual rainfall, but when litter of the species diminished by grazing was analyzed, it was negatively correlated with precipitation. Nitrogen was immobilized more often than mineralized, even after one year of incubation. Immobilization was negatively correlated with precipitation. All these results indicate that grazing may significantly alter nutrient cycling by affecting litter quality through changes in species composition. These effects seem to be larger when species replacements induced by grazing either involve functional groups, as it was the case in our wettest site, or change root to shoot ratios. Therefore, the functional groups involved in the replacement of species as well as shifts between belowground and aboveground allocation should play a key role in grazing-induced changes on nitrogen cycling. Fil: Semmartin, María Gisela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Distel, Roberto Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina Fil: Moretto, Alicia Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Ghersa, Claudio Marco. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Summary 1 Anthropogenic changes in the global climate are shifting the potential ranges of many plant species. 2 Changing climates will allow some species the opportunity to expand their range, others may experience a contraction in their potential range, while the current and future ranges of some species may not overlap. Our capacity to generalize about the threat these range shifts pose to plant diversity is limited by many sources of uncertainty. 3 In this paper we summarize sources of uncertainty for migration forecasts and suggest a research protocol for making forecasts in the context of uncertainty.

Drylands cover about 41% of Earth's land surface, and 65% of their area supports domestic livestock that depends on the above-ground net primary productivity (ANPP) of natural vegetation. Thus, understanding how biotic and abiotic factors control ANPP and related ecosystem functions can largely help to create more sustainable land-use practices in rangelands, particularly in the context of ongoing global environmental change. We used 311 sites across a broad natural gradient in Patagonian rangelands to evaluate the relative importance of climate (temperature and precipitation) and vegetation structure (grass and shrub cover, species richness) as drivers of ANPP, precipitation-use efficiency (PUE) and precipitation marginal response (PMR). Climatic variables explained 60%, 52% and 12% of the variation in grass cover, shrub cover and species richness, respectively. Shrub cover increased in areas with warmer, drier and winter rainfall climates, while the response observed for both grass cover and species richness was the opposite. Climate and vegetation structure explained 70%, 60% and 29% of the variation in ANPP, PUE and PMR, respectively. These three variables increased with increasing vegetation cover, particularly grass cover. Species richness also increased with ANPP, PUE and PMR. ANPP increased, and PUE decreased with increasing mean annual precipitation, whereas PMR increased with the proportion of precipitation falling in spring–summer. Temperature had a strong negative effect on ANPP and PUE, and a positive direct effect on PMR. Standardized total effects from structural equation modelling showed that vegetation structure and climate had similar strengths as drivers of ecosystem functioning. Grass cover had the highest total effect on ANPP (0.58), PUE (0.55) and PMR (0.41). Among the climatic variables, mean annual precipitation had the strongest total effect on ANPP (0.51) and PUE (−0.41), and the proportion of the precipitation falling in spring–summer was the most influential on PMR (0.36). Synthesis. Vegetation structure is as important as climate in shaping ecosystem functioning Patagonian rangelands. Maintaining and enhancing vegetation cover and species richness, particularly in grasses, could reduce the adverse effects of climate change on ecosystem functioning in these ecosystems. Fil: Gaitan, Juan J.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Oliva, Gabriel Esteban. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Bran, Donaldo E.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Maestre, Fernando T.. Universidad Rey Juan Carlos. Escuela Superior de Ciencias Experimentales y Tecnolog ía. Area de Biodiversidad y Conservación. Departamento de Biolog ía y Geolog ía; España Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Jobbagy Gampel, Esteban Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Matemática Aplicada de San Luis; Argentina Fil: Buono, Gustavo G.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Ferrante, Daniela. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Nakamatsu, Viviana B.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Ciari, Georgina. Instituto Nacional de Tecnologia Agropecuaria. Estación Experimental Agroforestal Esquel; Argentina Fil: Salomone, Jorge M.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Massara, Virginia. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina

Drought is an increasingly common phenomenon in drylands as a consequence of climate change. We used 311 sites across a broad range of environmental conditions in Patagonian rangelands to evaluate how drought severity and temperature (abiotic factors) and vegetation structure (biotic factors) modulate the impact of a drought event on the annual integral of normalized difference vegetation index (NDVI-I), our surrogate of ecosystem functioning. We found that NDVI-I decreases were larger with both increasing drought severity and temperature. Plant species richness (SR) and shrub cover (SC) attenuated the effects of drought on NDVI-I. Grass cover did not affect the impacts of drought on NDVI-I. Our results suggest that warming and species loss, two important imprints of global environmental change, could increase the vulnerability of Patagonian ecosystems to drought. Therefore, maintaining SR through appropriate grazing management can attenuate the adverse effects of climate change on ecosystem functioning. Fil: Gaitán, Juan J.. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Bran, Donaldo. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Oliva, Gabriel Esteban. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Maestre, Fernando T.. Universidad Rey Juan Carlos. Escuela Superior de Ciencias Experimentales y Tecnología. Departamento de Biología y Geología, Física y Química Inorgánica. Area de Biodiversidad y Conservación ; España Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Jobbagy Gampel, Esteban Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Matemática Aplicada de San Luis; Argentina Fil: Buono, Gustavo. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Ferrante, Daniela. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Nakamatsu, Viviana. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Ciari, Georgina. Instituto Nacional de Tecnologia Agropecuaria. Estación Experimental Agroforestal Esquel; Argentina Fil: Salomone, Jorge. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina Fil: Massara, Virginia. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Sur. Estación Experimental Agropecuaria Chubut; Argentina

Summary 1 Bouteloua gracilis and Buchloe dactyloides are dominant species in the shortgrass steppe. Previous studies have suggested that these species have similar resource requirements and, as there is no temporal segregation in growth, we suggest that competition for below-ground resources should be intense. Classical competition theory predicts that, under stable conditions, coexistence of species with similar requirements can occur if intraspecific competition is more intense than interspecific competition. We therefore compared the competitive abilities of the two species under both inter- and intraspecific conditions, another determinant of coexistence in plant communities. 2 A 3-year experiment was conducted in a small homogeneous area and a separate 2-year experiment was conducted in a larger area. In both experiments, a hexagonal planting design was used to achieve all possible combinations of species and to control plant size and neighbourhood asymmetries at the start of the experiment. Half the target plants were grown within steel cylinders to reduce below-ground competition. 3 We found similar results in the two experiments. Plant biomass and seed production were always higher in plants growing with reduced competition, although seed production was quite variable. Relative competition intensity did not differ between intraspecific and interspecific competition. 4 Our results suggest that competition between these coexisting shortgrasses is intense, but do not support the prediction of a difference between intra- and interspecific competition. We discuss alternative explanations.

The shortgrass steppe is co-dominated by two C4perennial grasses, Bouteloua gracilis andBuchloe dactyloides. At our site and throughouteastern Colorado Bouteloua gracilis has higher cover andbiomass than Buchloe dactyloides. We hypothesizedthatthe interaction between seedling recruitment differences and disturbance regimeand tolerance to drought conditions were the most likely causes of the observeddifferences in relative abundances. We used a simulation model to investigatethe relative importance of the three factors. We studied plant biomass of thesetwo species in 18 simulated treatments that resulted from a factorialcombination of seedling recruitment differences, disturbance regime, andtolerance to drought conditions. Analysis of the simulation outputs with ANOVAindicated that biomass of each species responded positively to increases inrecruitment probability. Increasing disturbance frequency favoredBuchloe dactyloides over Boutelouagracills, whereas the susceptibility Buchloedactyloides to drought favored Boutelouagracilis. The results indicated that differences in droughttoleranceand seedling recruitment probabilities along with their interactions withdisturbance regimes exert a major control on the biomass of the species. In theabsence of or with intermediate disturbance, a higher recruitment probabilityand greater tolerance to drought of Bouteloua gracilisthanBuchloe dactyloides yielded patterns of relativebiomass similar to the patterns observed in the shortgrass steppe.

Arid ecosystems present a two-phase mosaic structure of high- and low-cover patches. Vegetation patches differ among ecosystems in size and shape. However, recent studies indicate striking similarities in patch dynamics and in mechanisms explaining their origin and maintenance. Two major types of system, banded and spotted vegetation, which are characterized by patch shape, both originate from common mechanisms, although each is dominated by a different driver. Banded vegetation occurs when water is the dominant driver of the redistribution of materials and propagules, whereas spotted vegetation results when wind is the major redistribution driver. Model analysis indicates that patchy vegetation structure enhances primary production.

We present a conceptual model in which plant-soil interactions in grasslands are characterized by the extent to which water is limiting. Plant-soil interactions in dry grasslands, those dominated by water limitation (‘belowground-dominance’), are fundamentally different from plant-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light (‘indeterminate dominance’). In the belowground-dominance grasslands, the strong limitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous aboveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influenced by plant functional type (lifespan, growth form, root:shoot ratio, photosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal time scales, but may be reduced to the level of bare ground following death of an individual plant in as little as 3 years. These resource islands may have a great deal of significance as an index of recovery from disturbance, an indicator of ecosystem stability or harbinger of desertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is relatively continuous, and thus the major force in plant-soil interactions is related to the feedbacks among plant biomass production, litter quality and nutrient availability. With increasing precipitation, the over-riding importance of water as a limiting factor diminishes, and four other factors become important in determining plant community and ecosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, several different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by different plant traits, for example root:shoot allocation, height and photosynthetic pathway type (C3 vs. C4) and nitrogen fixation, each of which has a different influence on litter quality and thus nutrient availability. Recent work has indicated that there are strong feedbacks between plant community structure, diversity, and soil attributes including nitrogen availability and carbon storage. Across both types of grasslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or changes in grazing or fire regime, alters the pattern, quantity, and quality of soil organic matter in grassland ecosystems. The reverse influence of soils on plant communities is also strong; in turn, alterations of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.

The Patagonian steppe is formed by tussock grasses and shrubs in a bare-soil matrix (50% cover), and as in other arid and semiarid systems, vegetation is arranged in patches. Although there is a good understanding of the probabilities of seedling establishment of Bromus pictus in relation to location within these patches, these probabilities account for only a portion of the spatial dynamics of the community. The objective of this paper was to assess the pattern and dynamics of Bromus pictus seed availability in this community, which represent the other portion of the recruitment equation. We first evaluated its seed bank along transects. Plant- or litter-covered areas had 85% of the sampled seeds; however, they accounted for 55% of the area. Bare-soil areas had seeds only if they were located close (≤10 cm) to a plant or litter microsite. In a second study, we analyzed the movement of seeds using pitfall traps. Traps located near plants captured seed amounts similar to those from traps located in bare-ground areas (far from plants). The contrast between the high number of seeds in transit and the low number of seeds retained by bare-ground microsites illustrates the importance of lateral secondary movement of seeds. We used this information on seed distribution and previous data about establishment probabilities to estimate the spatial pattern of recruitment. Microsites that are vegetated or close to individual plants are expected to recruit the greatest number of individuals. Vegetation patches play an important role in modifying ecological processes in arid and semiarid communities. Our results highlight the importance of seed distribution in the formation and maintenance of these patches.

Geographical variation in genetically based traits helps to elucidate the effect of distinct ecological and evolutionary processes on widespread plants. Whereas abundant information exists on genetic patterns of woody species in western humid Andes, such information is scarce for the neighbouring dry Patagonian steppe. We examined genecological traits of two native forage species vulnerable to overgrazing (Bromus pictus and Poa ligularis) in dry Occidental Phytogeographical District. We compared within-population genetic diversity and among-population (n = 6) divergence by using isozyme electrophoresis. We also cultivated plants under common garden to compare genetically based morphology (plant height, number of tillers by plant and weight per tiller). Analysis showed that 8 and 13 loci were polymorphic of 9 and 19 resolved loci in at least one population for Bromus and Poa, respectively. In general, plant traits decreased from north to south in both species. Genetic and quantitative results (FST/QST index) showed evidence of local adaptation in populations of both species. Genetic divergence among populations was significant. We detected two different geographical groups divided at the same latitude (42–43°S) in both species, supporting the hypothesis of a past vicariance event. Sustainable management of these forage species to cope with land-use and climate change will be enriched by the inclusion of genecological knowledge. Fil: Leva, Paula Edit. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Premoli Il'grande, Andrea Cecilia. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio de Ecotono; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina

Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.

The main objective of an ecosystem sustainable management is to preserve its capacity to respond and adapt to current disturbances and/or future changes, and maintain the provision of environmental goods and services. Two very important properties linked to this objective are the ecosystem resilience and resistance to disturbance factors. The Structural–Functional State and Transition Model (SFSTM) is a conceptual framework that allows evaluating the ecosystem resilience and resistance based on structural and functional attributes. On the other hand, the Landscape Functional Analysis (LFA) presents a method to assess the rangeland “health” based on structural vegetation and soil indicators, creating indexes to evaluate the ecosystem functional integrity. The aim of this study is to integrate LFA and SFSTM as an approach to help validate indicators and indexes associated with the resistance, resilience, and stability of a temperate rangeland ecosystem. States and transitions model for a shrubby-grasses steppe of the Western Patagonian District was used as a reference system. Changes in vegetation structure, soil surface, and loss of soil due to erosion were determined in sites with different grazing histories. Based on the SFSTM, we assessed the relationships between ecosystem structural changes with the recruitment process of the plant community and ecosystem integrity indexes (sensu LFA). Our results indicate that the decrease in the recruitment process, related to different grazing histories, was associated with a loss of ecosystem functional integrity. This was associated to a decrease in the ability to retain, store, and use rain water, and also in nutrient cycling. This suggests that the integration of the LFA methodology to the SFSTM can be used for indexes validation, which in turn allows the identification of critical thresholds associated with ecosystem resilience loss. Finally, throughout the integration of indicators of LFA into SFSTM, we established relationships between ecosystem resistance, resilience, and stability in response to a disturbance factor (e.g. overgrazing). Thus, we used this information to define states in stable, unstable, mixed-unstable, and indifferent-stable dynamic equilibriums. Our proposed approach provides a tool for ecosystem assessment regarding the identification of states that can be restored and those that might be more susceptible to degradation. Such information might help in the prevention of crossing a critical threshold and be used for sustainable management programs in rangelands. Fil: López, Dardo Rubén. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Brizuela, Miguel Angel. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina Fil: Willems, Priscila. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomia; Argentina Fil: Siffredi, Guillermo. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Bran, Donaldo Mauricio. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina

Since many arid ecosystems are overstocked with domestic herbivores, biotic stress could have a stronger influence in modulating the balance of species interactions than expected from the stress gradient hypothesis (SGH). Here we tested a priori predictions about the effect of grazing on species interactions and fine scale spatial structure of grasses in water‐limited ecosystems. We used detailed vegetation mapping and spatial analysis, and performed a field experiment where the direct and indirect components of positive interactions were disentangled to provide evidence of links between process and pattern. We found associational resistance (biotic refuge) to be the dominant process in grazing situations, while competition, instead of direct facilitation, seemed to govern grass spatial patterns when herbivore pressure was relaxed. These results suggest that facilitation between grasses in arid communities may be related to herbivory rather than nurse plant effects. Associational resistance tends to have the strongest effect on spatial aggregation of species at intermediate grazing pressure. Results suggest that contrary to SGH, this physical clustering of species decreased when grazing pressure reached their maximum levels. Positive associations remained significant only when palatability differences between neighbours is large, suggesting that managing stocking rate is a key factor determining the persistence of herbivory refuges. These refuges are potential foci to initiate population recovery of high quality forage species in arid degraded areas. Fil: Graff, Barbara Pamela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Aguiar, Martin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

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.

Different ways of estimating soil evaporation losses were compared in an arid region of Patagonia. Microlysimeters were used as a reference estimate and tested against approaches based on (1) surface energy balance and (2) meteorological data. Cumulative losses during a 12-day drying cycle, calculated from microlysimeter data and from the model that uses meteorological data, were very similar. The energy balance method yielded estimates higher than the model based on meteorological data and overestimated the evaporation rate, especially at low rates of evaporation. This feature could be related to the plausibility of an assumption made by the method.

A solid background in population ecology is valuable for anyone concerned with natural resources management. However, without the opportunity of field experience, it may be difficult for students to become familiar with some of the principles of population biology. Microcomputers and programmable pocket calculators allow the construction of simple simulation models, making it possible to carry out some ‘numerical experiments’ in the classroom which may play the same role in the learning process as in vivo experimentation This work introduces a mathematical model based on the Lotka—Volterra equations and a computer program developed for a programmable calculator. They can be used to assess the influence of environmental heterogeneity and disturbances on the results of competitive relationships between two plant populations. As an example, the interaction between two tussock grass species of the Patagonian steppe, Stipa speciosa Trin. et Rupr. and Festuca pallescens (St Yves) Parodi (the second being less x...

Isolating the single effects and net balance of negative and positive species effects in complex interaction networks is a necessary step for understanding community dynamics. Facilitation and competition have both been found to operate in harsh environments, but their relative strength may be predicted to change along gradients of herbivory. Moreover, facilitation effects through habitat amelioration and protection from herbivory may act together determining the outcome of neighborhood plant-plant interactions. We tested the hypothesis that grazing pressure alters the balance of positive and negative interactions between palatable and unpalatable species by increasing the strength of positive indirect effects mediated by associational resistance to herbivory. We conducted a two-year factorial experiment in which distance (i.e., spatial association) from the nearest unpalatable neighbor (Stipa speciosa) and root competition were manipulated for two palatable grasses (Poa ligularis and Bromus pictus), at three levels of sheep grazing (none, moderate, and high) in a Patagonian steppe community. We found that grazing shifted the effect of Stipa on both palatable grasses, from negative (competition) in the absence of grazing to positive (facilitation) under increasing herbivore pressure. In ungrazed sites, belowground competition was the dominant interaction, as shown by a significant reduction in performance of palatable grasses transplanted near to Stipa tussocks. In grazed sites, biomass of palatable plants was greater near than far from Stipa regardless of competition treatment. Proximity to Stipa reduced the amount of herbivory suffered by palatable grasses, an indirect effect that was stronger under moderate than under intense grazing. Our results demonstrate that facilitation, resulting mainly from protection against herbivory, is the overriding effect produced by unpalatable neighbors on palatable grasses in this rangeland community. This finding challenges the common view that abiotic stress amelioration should be the predominant type of facilitation in arid environments and highlights the role of herbivory in modulating complex neighborhood plant interactions in grazing systems.

Bajo condiciones actuales, extensas areas de las zonas templadas del oeste de America del Norte y del sur de America del Sur tienen regimenes climaticos aridos a subhumedos, que son vulnerables a cambios climaticos inducidos por actividades humanas. Predicciones obtenidas a partir de modelos de circulacion global bajo una duplicacion del CO2 atmosferico sugieren grandes cambios en temperatura media anual, y cambios pequenos o nulos en la precipitacion media anual y la proporcion de precipitacion estival. Nuestro objetivo fue evaluar como las predicciones de cambio climatico obtenidas de modelos de circulacion global influiran sobre los patrones climaticos, e inferir a partir de ello la distribucion de los ecosistemas de las zonas templadas de America del Norte y del Sur. Calculos de deficit hidrico anual sugieren que, debido al cambio climatico, se duplicara el area afectada por condiciones muy secas. Esta expansion ocurrira en las cercanias de las zonas aridas actuales. Calculos mensuales de deficit hidrico sugieren que aproximadamente la mitad de la zona templada de cada continente se ve afectada por al menos un mes de deficit. Bajo un clima con doble CO2, estas areas se expandirian y cubririan hasta 77 % de las areas templadas de America del Norte y hasta 80 % de America del Sur. Los cambios en la distribucion de ecosistemas resultantes probablemente seran debidos a la expansion de los desiertos a expensas de los pastizales en America del Norte y del Sur, y la expansion de los pastizales a expensas de los bosques deciduos y boreales en America del Norte. Nuestros analisis asumen que los cambios climaticos futuros estaran abarcados por las predicciones de los tres escenarios de duplicacion de CO2 que utilizamos. La situacion mas probable es que los cambios reales, si es que ocurren, seran distintos a nuestros escenarios. Por lo tanto, nuestros analisis deberan interpretarse como indicaciones de la sensibilidad de partes de las zonas templadas de America del Norte y del Sur a aumentos de temperatura. La principal conclusion de nuestros analisis es que cualquier aumento de temperatura debido a cambios climaticos resultara en una expansion de las porciones mas aridas de ambos continentes

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).

Question: What is the effect of vertical seed distribution (i.e. seeds on soil surface or buried) and the relative importance of vertical and horizontal seed distribution (i.e. seeds near established vegetation or near bare soil) for seedling emergence? Location: Patagonian arid steppe. Methods: To evaluate the effect of vertical seed distribution on seedling emergence, four sowing treatments were used in field and greenhouse experiments. Vertical vs. horizontal seed distribution and the occurrence of seedlings were analysed in the field. Effects of trampling and mechanical drilling on seedling emergence were investigated experimentally in the field. Results: In field and glasshouse experiments buried seeds showed a higher emergence and larger seedling size than seeds lying on the soil surface. The observational field study indicated that natural emergence was not associated with particular microsites or plant cover (horizontal distribution), but that most seedlings emerged from buried positions (vertical distribution). Buried seeds represented less than 10% of the sampled seed cohort but account for almost 60% of the seedlings. The trampling/drilling experiment showed that sheep trampling and mechanical drilling increased seedling emergence compared to non-treated controls. Conclusions: The population dynamics of Bromus pictus is strongly constrained by the vertical position of seeds, which largely affects emergence, and seed horizontal distribution, which largely affects seedling survival. Our results showed that sheep trampling after seed dispersal may be a low-input technique for increasing grass recruitment.

We performed an analysis of the climatic patterns of the temperate zones in North and South America using a global database of monthly precipitation and temperature. Three synthetic variables, identified by a principal component analysis (PCA) of the monthly data, were used: mean annual precipitation, mean annual temperature and the proportion of the precipitation falling during summer. We displayed the spatial gradient of the three variables by constructing a com- posite colour raster image. We used a parallelepiped classification algorithm to locate areas in both continents that are climatically similar to five North American Long Term Ecological Research sites and to two South American long- term ecological research sites. The same algorithm was used to identify areas in South America which are climatically similar to some of the major grassland and shrubland types of North America. There is substantial overlap between the climates of North and South America. Most of the climatic patterns found in South America are well represented in North America. How- ever, there are certain climates in North America that are not found in South America. An example is a climate with relatively low mean annual temperature and high summer precipitation. The climatic signatures of three North American LTER sites (Cedar Creek, CPER and Sevilleta) were not found in South America. The climatic signatures of two LTER sites (Konza and Jornada) had some representation in South America. Two South American research sites (Rio Mayo and Las Chilcas) were well represented climatically in North America. The climates of six out of seven selected North American grassland and shrubland types were represented in South America. The northern mixed prairie type was not represented climatically in South America. Our analysis sug- gests that comparisons of North and South America can provide a powerful test of climatic control over vegetation.

The Pataxonian steppe is composed mainly of shrubs and tussock grasses organized in two types of patches: scattered grass tussocks in a matrix of bare soil and shrubs, each surrounded by a dense ring of grass tussocks. We analyzed the variation of competition, facilitation, and seed distribution through space and time as major driving forces in the development of this patch structure. Emergence and survival of grass seedlings increased with distance from shrubs when the ring of grasses was left intact. On the contrary, when the ring of grasses was experimentally removed, seedling survival decreased with distance from the shrub. Differences in root density, soil water potential, and evaporation accounted for these patterns (...)

The Patagonian steppe consists of shrubs encircled by relatively dense stands of grasses; the areas between shrubs have scattered tussocks, interspersed with bare soil. This study investigates the seed bank and the establishment of a perennial grass, Bromus pictus, around shrubs and in the scattered-tussock patches, with special attention to root competition. About 20 times more seeds of B. pictus were found in the soil of the dense grass zone around shrubs than in the scattered-tussock patches. B. pictus seeds were placed in both types of patches, either with the natural level of below-ground competition present, or experimentally reduced by a fine mesh. Seedling survival and growth were reduced by root competition. Where root competition was experimentally reduced, the aerial protection afforded by the shrub increased the growth of seedlings. We conclude that root competition between adult plants and seedlings is mainly for soil water, and is greater near shrubs than in the scattered-tussock patches. Root competition from established plants is greater than the aerial protection afforded by shrubs, in the area close to shrubs.

A model for estimating volumetric water content as a function of soil water potential and gravel content was developed. In the system studied gravel can withhold up to 67% of the amount held by the fine material. Water content at field capacity decreases 50% when gravel content (>11 mm) increases from 0% to 40% of total soil weight. For those values of gravel content, penetration depth of a rainfall of 20 mm increases from 38 cm to 59 cm when the initial water content is 60% of the field capacity.