No abstract

Arbuscular mycorrhizal fungi (AMF) are vital for maintaining ecosystem structure and functioning and can be affected by complex interactions between plants and herbivores. Information found in the literature about how ungulate grazing affects AMF is in general contradictory but might be caused by differences in grazing intensities (GIs) among studies. Hence we studied how different GIs affect the composition, diversity, and abundance of AMF communities in a semiarid steppe of Patagonia. We predicted that 1) total AMF spore abundance (TSA) and diversity would decrease only under intense-grazing levels and 2) AMF species spore abundance would depend on their life-history strategies and on the GI. To test our predictions, we compared AMF communities among nongrazed (NG), moderately grazed (MG, 0.1-0.3 sheep ha 1), and intensely grazed sites (IG, N 0.3 sheep ha 1). GI was the most important factor driving changes in TSA and diversity, regardless of host plant identity. TSA, diversity, and evenness significantly decreased in IG sites but were not affected by MG. AMF species spore abundance varied depending on their life-history strategies and GI. Families with high growth rates like Glomeraceae and probably Pacisporaceae showed the highest spore abundance in all sites but decreased under IG. Species with higher carbon demands like Gigasporaceae showed low spore abundance and frequency in NG and MG sites and were absent in IG sites. In contrast, species with low growth rates, but efficient carbon usage, like Acaulosporaceae, showed low spore abundance in all sites but increased in IG sites compared with NG or MG sites. We conclude that intensification of grazing reduces AMF diversity and abundance, with the likely loss of AMF benefits for plants, such as improved nutrient and water uptake and soil aggregation. Therefore, sustainable grazing systems should be designed to improve or restore AMF communities, particularly in degraded rangelands, like the Patagonian steppes.

Abstract.Fire is a critical disturbance in the structuring and functioning of most Mediterranean ecosystems. In northwestern Patagonia, vegetation patterns are strongly influenced by fire and environmental heterogeneity. Dendroecology, together with satellite imagery and GIS, have been demonstrated to be useful tools in studies that relate to fire effects with patches, patterns and species dynamics at landscape scale. Such studies can be approached from landscape ecology, which has evolved in the last years supported by the development of remote sensing and GIS technologies. This study evaluates the spatial dynamic of F. imbricata in response to fire using remote sensing, GIS and dendrochronology techniques, at landscape scale. Two sites were evaluated and one of them was affected by fire in the year 1999. The digital processing images (using the NBR spectral index) and the dendroecological analysis verified this. A fire, occurring in 1978, was also detected by the analysis of F. imbricata growth rings. The relation between F. imbricata shrubland dynamics and spatial configuration with fire, land topography and hydrography was established in the study area.

One strategy of plant survival during post‐fire succession is to persist and regenerate by recruiting new individuals from a fire‐resistant seed bank. The heat, smoke, and charcoal released during plant combustion may act (individually or in combination) as a cue for post‐fire seed germination. Fabiana imbricata is a shrub that forms persistent seed banks in the northwestern Patagonian grasslands and shows a high recruitment from seeds during post‐fire succession. Mathematical models showed that this species is advancing over the grasslands in response to fires. To corroborate these findings, we studied the role of fire on F. imbricata seed germination. In order to achieve this, a factorial experiment was designed in laboratory conditions to study the effect of heat, charcoal, smoke, scarification, and their interactions on F. imbricata seed germination. Seeds treated with the higher temperatures required a longer period of time to germinate, thus, significantly affecting the mean germination time. Total germination percentages in F. imbricata were significantly enhanced by smoke and scarification, but the interaction of heat, smoke, and scarification was more important than the effect of each fire factor alone. The positive response to fire cues exhibited by F. imbricata indicates that this species would have an adaptive advantage to colonize these grasslands if fire frequency increased, as predicted for this environment. Hence, fire will contribute to the grassland encroachment by this species and, therefore, to the loss of biodiversity and productivity of northwestern Patagonian grasslands.