Grasses and grazers in arid rangelands: Impact of sheep management on forage and non-forage grass populations.

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.

Females engage in stronger relationships: positive and negative effects of shrubs are more intense for Poa ligularis females than for males.

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.

Do species' strategies and type of stress predict net positive effects in an arid ecosystem?

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.

Plant species richness and shrub cover attenuate drought effects on ecosystem functioning across Patagonian rangelands.

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.

Shifts in positive and negative plant interactions along a grazing intensity gradient.

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.