Search

Soil processes in arid ecosystems are strongly controlled by resource scarcity. Grazing intensification can induce changes in ecosystem processes through multiple pathways, adding new constraints to those of local conditions. We focus on grazing-induced changes in litter traits and soil environment that may affect litter decomposition and N dynamics in temperate grass-shrub steppes. We performed three litterbag decomposition experiments to evaluate: i) the effect of litter traits in a common garden (ex situ), ii) the effect of soil environment using a foreign common litter substrate (in situ), and iii) the interactive effects of litter traits and soil environment (grazed vs. exclosure communities, reciprocal transplants in situ). Field experiments were replicated in three blocks with paired plots under a long-term exclosure (> 25 years) and under year-round sheep grazing. Local litter included mixtures of species of grasses and shrubs, separately. Grazing exclusion did not alter litter decomposition rates, either through changes in litter traits or in soil environment. Nevertheless, N released during grass litter decomposition was 286% higher in exclosures than in grazing communities. The difference was associated to changes in litter C:N ratio. The effects were maintained when results were integrated to the entire litter community. Our study suggests that litter decomposition rates in arid steppes are strongly controlled by local drivers. Ungulate grazing does not have an important influence on litter mass loss, but it can exert a strong control on N flux during decomposition, by changing grass litter traits.

Ecological modeling that includes plant population processes as a critical determinant of vegetation dynamics is useful for sustainable rangeland management. However, we know little about how long-term sheep grazing pressure drives the plant community structure through changes in different native grass species at both individual and population levels. In this study, we hypothesized that plant populations perform differently under different grazing management due to their specified preference by livestock animals. We also tested whether grazing-rest management, aimed at increasing long-term rangeland sustainability, improves the plant growth of forage grass species. We evaluated plant density, individual morphology and plant-size distribution of dominant grass populations in permanent exclosures and open fields under moderate and intensive grazing pressures in Patagonian steppes (South America). We also examined the effects of seasonal grazing-rest managements on the growth and tillering (asexual reproduction) of forage species plants, using temporary mobile exclosures. Grazing intensity changed population density and structure according to species. Compared to permanent exclosures, moderate grazing maintained the plant density of palatable species highly preferred by sheep, reduced the standing-dead biomass proportion of individual plants, and promoted the green biomass of tussocks. Conversely, intensive grazing (double stocking rates) decreased the plant density and individual size of species highly preferred by sheep, and increased the plant density of non-preferred species. Grazing-rest enhanced forage grass species growth and reproduction compared with year-round grazing management, especially during the growing season of a wet year. Our studies support that sheep can be managed to control the plant-size distribution of dominant grass species, their population dynamics, and thereby the overall forage availability at the community level. Both moderate grazing and grazing-rest management can improve the forage availability and preserve the dominant native grasses. We suggest applying a plant population dynamics perspective to facilitate sustainable management of global rangelands.

In general, there is more knowledge about aboveground structure and processes than about those belowground. It has been proposed that plants are belowground territorial and root segregation explains species coexistence. We explored this idea by studying root intermingling of perennial graminoid plants in an arid Patagonian steppe. We extracted soil samples under target species. Additionally, we buried root traps filled with sieved soil to quantify root colonization close to plants (zone of high influence), of five dominant graminoid species. We sorted root biomass by species from samples and traps using a root-trait taxonomic key with 95% accuracy. Both studies indicated that the zone of high influence is occupied by roots of target and neighbor species (1–4 species) but showed differences in relative biomass of species. After 4 months, species with high root RGR show dominance in traps. This soil pre-emption seems transitory because in the descriptive study dominant species predominated independently of target species. We propose that coexistence among active roots of several species could be explained by different soil occupation rates in the zone of high influence. More studies are needed in relation with resource use in order to comprehend relationship between above and belowground biomass distributions. Fil: Reyes, María Fernanda. 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

Research linking functional traits to competitive ability of coexisting species has largely relied on rectilinear correlations, yielding inconsistent results. Based on concepts borrowed from natural selection theory, we propose that trait?competition relationships can generally correspond to three univariate selection modes: directional (a rectilinear relationship), stabilising (an n-shaped relationship), and disruptive (a u-shaped relationship). Moreover, correlational selection occurs when two traits interact in determining competitive ability and lead to an optimum trait combination (i.e., a bivariate nonlinear selection mode). We tested our ideas using two independent datasets, each one characterising a group of species according to (a) their competitive effect on a target species in a pot experiment and (b) species-level values of well-known functional traits extracted from existing databases. The first dataset comprised 10 annual plant species frequent in a summer-rainfall desert in Argentina, while the second consisted of 37 herbaceous species from cool temperate vegetation types in Canada. Both experiments had a replacement design where the identity of neighbours was manipulated holding total plant density in pots constant. We modelled the competitive ability of neighbours (i.e., the log inverse of target plant biomass) as a function of traits using normal multiple regression. Leaf dry matter content (LDMC) was consistently subjected to negative directional selection in both experiments as well as to stabilising selection among temperate species. Leaf size was subjected to stabilising selection among desert species while among temperate species, leaf size underwent correlational selection in combination with specific leaf area (SLA): selection on SLA was negative directional for large-leaved species, while it was slightly positive for small-leaved species. Synthesis. Multiple quadratic regression adds functional flexibility to trait-based community ecology while providing a standardised basis for comparison among traits and environments. Our analyses of two datasets from contrasting environmental conditions indicate (a) that leaf dry matter content can capture an important component of plant competitive ability not accounted for by widely used competitive traits, such as specific leaf area, leaf size, and plant height and (b) that optimum relationships (either univariate or bivariate) between competitive ability and plant traits may be more common than previously realised. Fil: Rolhauser, Andrés Guillermo. Universidad Nacional de San Juan; Argentina. 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: Nordenstahl, Marisa. Universidad Nacional de San Juan; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; 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: Pucheta, Eduardo Raúl. Universidad Nacional de San Juan; Argentina

Background and Aims Dioecious plants are of particular concern in view of global environmental changes because reproductive females are more sensitive to abiotic stresses, thus compromising population viability. Positive interactions with other plants may counteract the direct effects of any abiotic environmental stress, allowing them to thrive and maintain a viable population in suboptimal habitats, although this process has not been tested for dioecious species. Furthermore, almost no data are available on the outcome of such species interactions and their link with local spatial patterns and sex ratios. Methods We set up a field experiment with Poa ligularis, a dioecious native grass from the arid grasslands of South America. We studied the interaction of male and female plants with cushion shrubs of contrasting ecological strategies. We experimentally limited direct shrub-grass competition for soil moisture and transplanted plants to evaluate the amelioration of abiotic stress by shrub canopies (i.e. sun and wind) on grasses. We also studied the distribution of naturally established female and male plants to infer process-pattern relationships. Key Results Positive canopy effects as well as negative below-ground effects were more intense for females than for males. Deep-rooted Mulinum spinosum shrubs strongly facilitated survival, growth and reproduction of P. ligularis females. Naturally established female plants tended to distribute more closely to Mulinum than co-occurring males. Female growth suffered intensive negative root competition from the shallow-rooted Senecio filaginoides shrub. Conclusions Interactions with other plants may reduce or enhance the effect of abiotic stresses on the seemingly maladapted sex to arid environments. We found that these biased interactions are evident in the current organization of sexes in the field, confirming our experimental findings. Therefore, indirect effects of climate change on population sex ratios may be expected if benefactor species abundances are differentially affected. 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 Fil: Almeida, Rocio Jezabel. 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

In many Northern Hemisphere salt marshes, recovery of disturbance-generated bare areas usually occurs within a few years. However, in some southwestern Atlantic marshes, bare patches may remain unchanged for several years. Here we investigated biological and physical factors that might affect bare patch colonization in a Mar Chiquita coastal lagoon salt marsh (Argentina). We evaluated natural seedling emergence of the pioneer pickleweed plant Sarcocornia perennis, considering distance to mature plants and the herbivory/bioturbation exerted by the crab Neohelice granulata (using exclosures). Almost no seedling emergence was observed in patches distant from mature plants, while higher emergence resulted in close patches. Crab exclusion did not affect seedling counts. Nevertheless, most of these seedlings did not survive high-temperature months. We analyzed if colonization was favored by increasing the availability of S. perennis seeds, and we found similar seedling emergence in patches both close and distant to mature plants. As in the previous experiment, most of them did not survive the high-temperature months, but some seedlings could establish within crab exclosures. Finally, we used a shade cloth to reduce solar radiation, but this did not increase seedling survival, which was only favored in crab exclosures. In summary, our results suggest that seed availability, followed by the presence of crabs (but not solar radiation), affect the rate of patch recovery. Additionally, our results show that life-history traits (seed ecology) of the colonizer can be a key and are a usually overlooked driver of salt marsh secondary succession. Fil: Kaminsky, Julieta. 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: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; 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 Fil: Iribarne, Oscar Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina

Question: Root biomass distribution and below-ground processes are the main drivers of plant community structure and dynamics in arid rangelands. However, despite their relevance, below-ground plant responses to disturbance regimes remain poorly understood. This knowledge is a pressing issue since most of our ideas derive from extrapolating above-ground ecological theory and data. Here, we asked how domestic grazing management changes (intensification and abandonment) impact on both below-ground species co-occurrence at microsite level (point-richness) in relation to above-ground patterns and below-ground biomass distribution of dominant graminoid species across different soil layers. Location: Patagonian steppe, Argentina. Methods: We sampled and compared ungrazed, moderately grazed and intensively grazed steppes. We evaluated below- and above-ground point-richness (microsite level: 6-cm diameter), as well as total and specific root biomass in the top 20 cm of soil along 50-m transects in commercial paddocks. Results: We found independence between fine-scale below- and above-ground responses. Below-ground point-richness was at least three times higher than above-ground point-richness. Besides, below-ground point-richness decreased from 4.2 species in ungrazed sites to 3.4 species in intensively grazed ones. Neither grazing intensification nor abandonment changed total root biomass, although intensification changed the root biomass composition, reducing preferred and increasing non-preferred grass species. Strikingly, below-ground point-richness and total root biomass did not differ between microsites covered and not covered by vegetation. This pattern was not affected by grazing. Conclusions: Our findings showed a higher degree of fine-scale below-ground species co-occurrence than above-ground in this Patagonian steppe. The intensification of grazing slightly decreased below-ground species co-occurrence and changed root composition (species abundance), although grazing management did not modify the high degree of independence between below- and above-ground structure. Even though intensive grazing markedly decreases above-ground biomass, total below-ground biomass does not vary along a wide range of grazing conditions. In general, most of our knowledge about vegetation responses to grazing is based on above-ground information, assuming that below-ground patterns and processes are analogous to those above-ground. This study suggests that below- and above-ground community structure at fine-scale are not strongly correlated and they can be differentially affected by grazing. 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: Reyes, María Fernanda. 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

Aims: In arid communities, it has been proposed that individual plants can extend their roots beyond their canopy exploring neighbouring bare ground areas. This becomes relevant in systems where the vegetation is distributed in patches surrounded by bare soil. However, whether roots of different species may be overlapping under bare ground areas is still controversial. The factors controlling root responses when no plants appear to be directly influencing the gap among patches are still unclear. The aim of our study was to detect perennial grasses responses to an N enrichment pulse. Methods: In a semi-arid steppe (Patagonia, Argentina), we buried root traps filled with sieved soil with and without N addition, under bare soil patches. Traps were harvested after 4 and 6 months. Trap neighbourhoods (30 cm in diameter) included at least three of the dominant tussock species. After harvests, we identified species in the traps by root traits, and quantified diversity, biomass and specific relative growth rates. Important Findings: Bare ground areas show simultaneous root growth of different species. Diversity of perennial grass roots was higher with N addition than without it in the first harvest (4 months), but this difference disappeared in the second harvest (6 months). Root biomass was maximal after 6 months in N addition traps. Species preferred by herbivores (Bromus pictus and Poa ligularis) showed rapid growth and responses to N addition. Differences between harvests may be an indicative that N pulses interact with rising temperatures and soil water content as growing season progress. Fil: Reyes, María Fernanda. 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

Soil patchiness is a key feature of arid rangelands. As root proliferation contributes to soil exploration and resource uptake, it is ecologically relevant to understand how species respond to soil heterogeneity and coexist. Campbell et al.'s influential 1991 hypothesis proposes that dominant species deploy root systems (scale) that maximize soil volume explored. Instead, subordinate species show accurate root systems that exclusively proliferate in nutrient-rich patches (precision). After many experiments under controlled conditions, the generality of this hypothesis has been questioned but a field perspective is necessary to increase realism in the conceptual framework. We worked with a guild of perennial graminoid species inside a grazing exclosure in an arid Patagonian steppe, a model system for ecological studies in arid rangelands for four decades. We buried root traps in bare ground patches with sieved soil, with or without a pulse of nitrogen addition, to measure specific root biomass and precision at 6 and 18 months after burial. We also estimated scale (root density) in naturally established plants, and root decomposition in litter bags. Several species grew in root traps. Dominant species showed the highest root biomass (in both harvests) and scale. Subordinate species grew more frequently with nitrogen addition and showed lower biomass and scale. Similar total root biomass was found with and without nitrogen addition. Species differed in root decomposition, but correcting species biomass by decomposition did not change our conclusions. We did not find a relation between scale and precision, indicating that Campbell's hypothesis is probably not supported in this Patagonian steppe. Soil resource acquisition differences probably do not utterly explain the coexistence of dominant and subordinate species because the steppe is also affected by large herbivore grazing. We propose that root proliferation in this steppe is the result of the interaction between individual density in the community and specific root growth rates. Fil: Reyes, María Fernanda. 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

Aim: We studied the legacy effects of shrubs during the downgrade phase of high–cover patches. Specifically, are woody species able to modify environmental attributes at patch level to such an extent as to alter the colonization once they have vacated their original position? Methods: We monitored five environmental variables along an experimental four-stage downgrading gradient of high–cover patches during two years in cold- and warm-seasons, individual plant growth during three years, as well as the floristic composition of patches along the same gradient after 13 years. Results: The downgrade of high–cover patches reduces the aboveground protection due to the increase in wind speed (400–500%) and evaporation rate (43–160%) associated with shrub death and senescence. In addition, high–cover patches increase the total soil nitrogen (400–600%) and reduce the infiltration rate (44–73%) on the top layer. Leaf length and flower culms of grass tussocks were lower in bare soil patches (7.5 cm and 3) compared to whatever degradation stage of high–cover patches (9–10 cm and 18–32). Floristic composition after 13 years reveals that grass species occupied the patch stages differentially, with a disjunctive pattern among species within the Poa and Pappostipa genus. Conclusions: Legacy effects prompted by shrubs through changes in soil properties at the horizontal plane can conditioned the patch dynamics. The ability of different plant species to cope with the spatial heterogeneity at the horizontal plane should be included as a new criterion to define plant strategies from arid ecosystems according to the gap–phase dynamics and mosaic maintenance. Fil: Cipriotti, Pablo Ariel. 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. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Métodos Cuantitativos y Sistemas de Información; 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. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Recursos Naturales y Ambiente. Cátedra de Ecología; Argentina

Year-to-year variability in field seedling emergence in arid rangelands poses questions about the viability of perennial grasses threatened by overgrazing. There are practical difficulties in estimating long-term seedling emergence for perennial species with erratic behavior (high temporal variability). We present a novel approach combining seed germination studies, modeling, and field experiments to estimate long-term germination probability using Poa ligularis as a perennial grass model. Additional objectives were to evaluate: (i) germination requirements of different P. ligularis populations, (ii) field emergence behavior at two extreme locations within the distributional range in the southern region, and (iii) the influence of microsite or local soil on field emergence. Germination parameters were obtained from 8 populations collected across a 12° latitude transect. Germination parameters were combined with a soil water model to estimate long-term germination (used as a proxy of field emergence) in the southernmost location. Germination requirements differed across populations and correlated with environmental conditions of collection sites, demonstrating ecological significance. Field experiments showed very low emergence. Simulations for the southern location indicated erratic emergence. Fifty percent of the years had nil simulated field emergence and the magnitude of the events, when occurred, was small. The magnitude of these events depended on the population, demonstrating an additional genetic constraint. Erratic field emergence arises from the mismatch between environmental conditions and germination requirements. Knowing favorable climatic conditions triggering seed germination (and thus emergence) events are key for opportunistic management of species with this erratic behavior.

In most arid ecosystems, the vegetation is organized into two-phase mosaics, where high-cover vegetation patches are interspersed in a matrix of low plant cover. We studied the role of the biotic interaction balance (competition/facilitation) between shrubs and grasses as a driver of patch dynamics and maintenance of two-phase vegetation mosaics. Following Watt's seminal model, we conducted two field experiments in which we manipulated different vegetation patches to obtain the different stages along the building and degradation dynamics of high-cover patches. In addition we applied two possible belowground competition treatments (natural and experimentally reduced). We measured plant variables (emergence, survival, height, flower culms) on grass seedlings and transplants. We integrated all plant measurements into a single positive and negative component, to calculate the net balance along three stages of the patch dynamics proposed. The net biotic interaction balance was negative during the early and mature stages of high-cover patches because the average standardized effect from the negative component was below −0.44, while the positive component was not different from zero. However, the net biotic interaction balance was positive during the degraded stages of high-cover patches because the negative average component was −0.37, while the positive component reached 0.58. The negative net effects during early and mature stages of high-cover patches can be explained by the occurrence of wet years, because high rainfalls hide the aboveground facilitation. Our findings point out the importance of complementary mechanisms to the interaction balance in the mosaic maintenance (e.g. trapping of seeds by shrubs) according to the inter-annual rainfall variability.

Fil: Cipriotti, Pablo Ariel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Parque Centenario. Instituto de Investigaciones Fisiologicas y Ecologicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomia. Departamento de Metodos Cuantitativos y Sistemas de Informacion; Argentina

Summary 1. Dioecious plants frequently face sex-related resource allocation trade-offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. 2. Gender differences in traits have been considered to be inherently linked to sex-biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. 3. We linked the results of process-based field experiments with fine-scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slowgrowing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. 4. In large-herbivore exclosures, we found that sex-biased spatial patterns at the microsite scale can be predicted from gender-specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade-off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. 5. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female-biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female- to male-bias with increasing domestic grazing intensity. 6. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male-biased sex ratios with grazing intensity could not be predicted solely by gender-specific traits.

A proposed refinement to the stress-gradient hypothesis requires consideration of the strategies of the interacting species and the characteristics of the stress factors. While the strength and direction of these interactions can be predicted for different ecosystems, this idea remains largely untested in the field. We performed a manipulative field experiment complemented with a descriptive study to test the predictions in a natural setting that represents the extreme end of a precipitation gradient. There, wind driven desiccation and water availability are the main stressors (non-resource and resource-based stresses, respectively). We evaluated the interaction between the shrub and grasses that are dominant in the Patagonian steppe. The species had differences in morpho-functional traits and drought tolerance that fit into the C-S axis of Grime's strategies. We experimentally separated root zones to limit direct competition for soil moisture and reduce the resource-based stress on grasses. We also manipulated the distance to shrubs to evaluate non-resource stress amelioration by canopies (e.g., sun and wind) on grasses. Finally, we evaluated the distribution of naturally established C and S grasses in the neighborhood of C and S shrubs to infer process-pattern relationships. Our growth data coincide to a large degree to the predictions. We found positive effects on the growth of beneficiaries when stress was non-resource based and when strategies differed (i.e., Cshrub-Sgrass and Sshrub-Cgrass). We also found strong negative effects when the abiotic stress was driven by water, particularly on C grasses. Additionally, shrubs only increased the survival of grasses when strategies differed (i.e., Cshrub-Sgrass and Sshrub-Cgrass). Our manipulative and descriptive study supported previous results that showed that stress-tolerant species are important for the persistence of competitive species at high stress. While the applicability and generality of these predictions remains to be tested with more field experiments, some ecological factors, such as stress types and species traits, can explain much of the variation in how dominant shrubs and grasses interact in this extreme arid environment. Moreover, this framework could be extended to specifically test the importance of facilitation under different levels of stress. 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

We revisited a growth-form removal experiment after 12 years with the aim to compare the long-term population responses of three shrub species. Specifically, we were interested to know if the resource partitioning and plant interactions act as complementary mechanisms of the shrub-grass coexistence. In 1983 different plots from grass-shrub Patagonian steppes were submitted to three treatments: control, grass-, and shrub-removal during three consecutive years. In 1997 we studied nine plots from the original experiment and recorded all shrubs to compare shrub density, population size structure, vitality, spatial patterns and neighbourhood interferences of three native and co-dominant shrub species: Mulinum spinosum, Senecio filaginoides, and Adesmia volckmanni. After 12 years of recovery, shrubs as a growth-form group, fully re-established in plots where they were removed and attained 75% higher density in grass-removal plots than in control plots. However, long-term population responses of Mulinum, Senecio, and Adesmia to removals were distinctive. On the other hand, negative and positive interferences among shrub species and between shrubs and grasses generated a complex network. Morphological and functional differences in shrub and grass species and their interactions at population level and long-term could be a key to achieve a better comprehension of shrub-grass coexistence from semi-arid ecosystems. © 2010 Elsevier Ltd. Fil: Cipriotti, Pablo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. 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

Past research has shown that changes in grazing-resistance traits may be associated with genetic changes in plant populations. Little is known about spatial genetic relationships within plant populations (spatial genetic structure) and any grazing effects on these relationships. Here we present observations of the fine-scale spatial genetic structure in three grass species in semiarid environments (Arizona, Mexico, and Argentina). In each environment, populations of a dominant grass species were sampled from two sites with contrasting livestock grazing histories. Plant genotypes were described with the use of amplified fragment length polymorphism markers. In Arizona, populations of sideoats grama (Bouteloua curtipendula var. caespitosa Gould and Kapadia) differed in that one has never experienced livestock grazing, whereas cattle have grazed the other. In the other two environments, populations exposed to long-term heavy grazing were examined, along with those that experienced either moderate grazing (Mexico, blue grama [Bouteloua gracilis {Willd. ex Kunt} Lag. ex Griffiths]) or extended exclusion of livestock (Argentina [Poa ligularis Nees ex Steud.]). Based on independent analysis of each population, we observed no differences in average gene diversity between populations of each species. With the use of analysis of molecular variance we found slight but significant genetic differentiation between populations with different grazing histories in Arizona and Argentina. Significant genetic structure was present in all populations and indicated an inverse relationship between spatial and genetic distance. Interestingly, this relationship was most pronounced in the cattle-free sideoats grama population, suggesting larger genetic neighborhood areas in the absence of livestock. Less distinct differences in spatial genetic structure associated with grazing history were evident in the other two species. We hypothesize that livestock grazing may lead to increased homogeneity in genetic structure at the landscape scale. Effectively examining this hypothesis presents many experimental challenges.

There is limited knowledge about root morphological differences at the species level. Consequently, plant ecological groups are largely defined on aboveground traits. Our hypothesis was that roots of graminoid species differ in morphological and functional attributes. In field and greenhouse studies, we measured morphological and functional root traits of eight dominant graminoids. Multivariate analyses of root traits arranged species into two groups. Species were unequivocally classified by either discriminant analysis or a taxonomic key. Traits that contributed most to identification were diameter, colour, and branching. Species from one group had large root diameters, more branching, and lower tensile strength, specific length, rate of new root generation, and RGR than the other group. The grouping by root traits matched previous classifications: one group had been described as more xerophytic, less preferred by livestock, and more delayed in phenology than the second group. Our study shows that (1) a set of root morphological traits may be reliably used to recognize species, and (2) root traits reflect the major ecological grouping of species, even when they all belong to the same growth form. For Patagonia and similar sites, our work will open the way to more detailed, specific-level studies on community underground organization.

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.

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

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.

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

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

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.

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.

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.