2005. Effects of grazing on plant patterns in arid ecosystems of Patagonian Monte. Á/ Ecography 28: 561 Á/572.Our objective was to assess the relationship between the spatial patterning of perennial grasses (total, grazed, and non-grazed) and shrub patches in rangelands under different grazing pressures of the Patagonian Monte. We selected three grazed paddocks with the usual stocking rate for the area, where previous studies showed that a piosphere formation is common. At each paddock, we analysed the grain of heterogeneity at sites located at two distances from the single watering point (near, far), using high-resolution aerial photographs. At these sites, we also assessed in the field the density, size, cover, and spatial patterning of grazed and non-grazed perennial grasses and shrub patches. The grain of heterogeneity of shrub patches was coarser in sites near the watering point than in those distant from it, as a consequence of the increase in size of both, bare soil and shrub patches. Field sampling showed that a coarse grain of heterogeneity relative to fine-grained sites resulted from changes in species composition, increased bare soil areas and reduced perennial grass cover. In coarse-grained sites, lower perennial grass cover resulted from lower density and/or smaller size of grass bunches than in finegrained sites. We did not find significant differences among sites in the proportion of perennial grazed grasses. Since the density and cover of perennial grasses was higher in fine-than in coarse-grained sites, we suggested that fine-grained sites are more important as feeding stations than coarse-grained sites. The consequences of this differential use could lead to degradation of fine-grained sites and to higher homogeneity in spatial plant structure and floristic composition within paddocks with respect to the condition observed at present, increasing the size of the highly degraded zone within the piosphere. At the patch level, we found that at about one third of the sampled transects, both total and non-grazed perennial grasses were spatially aggregated with shrub patches. However, in most transects grazed perennial grasses were indifferently distributed in relation with shrub patches, showing that grazers display high selectivity of foraging sites at macro level (i.e. high and low grazing pressure sites at the paddock level), but random occupancy of vegetation units (randomness in the distribution of grazed perennial grasses at the patch level). The intensity of the positive association between non-grazed grasses and shrub patches was higher in fine-grained than in coarse-grained sites and may be attributed to higher protection against herbivores associated to denser shrub patches in fine-relative to coarse-grained sites. We concluded that a feedback exists between the spatial distribution of species preferred by grazers and the spatial patterning of use of these species. A. J. Bisigato

No abstract

Summary1. Diaspore abscission determines many aspects of seed dispersal by wind. While there is yet no complete mechanistic framework for understanding abscission by wind, empirical studies to date have suggested that abscission generally (i) occurs above some threshold wind speed and (ii) depends on the drag force generated by the wind. 2. We revisit these findings and formulate two alternative hypotheses for abscission mechanisms based on a simple model of a forced harmonic oscillator: large diaspore displacement [through a maximum deflection threshold, (MDT)] and material fatigue [through a maximum cumulative stress threshold (MCST)]. We use simulations of abscission events based on these hypotheses and experiments on diaspore abscission of three Patagonian grasses and a cosmopolitan herb to test the performance of two abscission functions differing in whether they have a threshold wind speed for abscission. We also quantify the effects of non-random diaspore abscission on dispersal distances using a well-tested model for seed dispersal by wind. 3. Both the MDT and MCST hypotheses appear realistic and indicate that while the instantaneous wind speed determines abscission, the history of wind speeds experienced prior to the detachment from the plant also plays a role.4. An evaluation of abscission functions against simulated and experimental abscission data shows that while the presence of a threshold wind speed in theory appears unrealistic, in practice a threshold may appear to exist in high-wind-speed environments where all diaspores are blown off the plant before the abscission layer can develop sufficiently to break during lower wind speeds. 5. Under non-random diaspore abscission, high-wind-speed events during otherwise calm periods increase long-distance dispersal (LDD), thereby decreasing differences in dispersal distances between low-and high-wind-speed environments. 6. Synthesis. We formulated two realistic mechanisms of diaspore abscission by applying concepts from materials science: large diaspore displacement and material fatigue. These reveal that the ambient wind speed 'history' experienced by a diaspore plays an important role in the timing of abscission and in the distance travelled, without any thresholds, and that the effect of the diaspore-wind interaction on LDD varies between environments with different wind speed regimes.

No abstract

We compared the size and spatial pattern of the germinable soil seed bank (GSB) of the three dominant perennial-grass species in the arid Patagonian Monte of Argentina. These species differ in plant functional traits, seed morphology, and in their preference by grazers. During three consecutive years, we sampled the soil seed bank at 1 m intervals and the intercepted diameter of perennial-grass species and shrub patches along 50-m linear transects at three sites with different grazing pressure. Soil samples were incubated at field capacity during 12 weeks and emerged seedlings counted. The largest-seeded and most xerophytic Stipa speciosa did not form a GSB. The other two grass species with more mesophytic traits and smaller seeds than S. speciosa formed different GSB. Poa ligularis formed a smaller and more heterogeneous GSB than Stipa tenuis. Grazing had not effect on GSB size of P. ligularis but increased the proportion of seeds under dense shrub canopies. In contrast, grazing reduced the size and also increased the proportion of the GSB of S. tenuis under shrub canopies. We conclude that the size and the spatial pattern of the GSB of the dominant perennial-grass species in the arid Patagonian Monte were related not only to seed and plant traits but also to the spatial distribution of grass plants and their exposure to grazers.