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.

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.

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.

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.

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.