GRAZING AND THE PHENOLOGY OF FLOWERING AND FRUITING IN A MONTANE GRASSLAND IN ARGENTINA - A NICHE APPROACH

We compared flowering and fruiting patterns of plants showing different pollination modes, to assess whether consistent differences emerged between grazed and ungrazed herbaceous communities. We also analysed the relationships among their phenologies, pollination modes and growth forms. We analysed a site protected from large herbivores for 12 years and a grazed site. In both of them we recorded flowering and fruiting events for the 20 most frequent species in different climatic periods during one year. We calculated their niche breadth, niche overlap and the dispersion of maximum peaks of flowering and fruiting. Grazing was not associated with significant changes in species composition or with invasion by exotics. All the recorded species were native and most of them were present in grazed and ungrazed sites, albeit with different relative frequencies. The pollination spectrum was dominated by wind-pollinated and unspecialized insect-pollinated species, with only two species pollinated by hovering moths. Both flowering and fruiting were highly concentrated in a short period during the summer. Interruption of grazing did not promote significant changes in niche breadth or overlap. Flowering and fruiting peak dispersions within pollination guilds were indistinguishable from a random distribution. There was no evidence to suggest the existence of a systematic ecological force leading to divergence in phenologies within pollination guilds. Grazing influence on times of flowering and fruiting was inconsequential. There were groups of species whose members had similar growth forms, floral syndromes, and temporal patterns of flowering and fruiting, and thus were highly overlapping along those three niche axes. Climatic constraints leading to ecological convergence seemed to be sufficiently strong to prevent the expression of segregated patterns within guilds of the most frequent species.