Authors: Andrade, Laura Elena
Publication Date: 2016.
This thesis focused on the biotic factors that influence the abundance and spatial distribution of herbaceous plant populations in the central Monte desert, particularly grasses, including the spatial heterogeneity typical of the system: microsite and habitat scales. The spatial association between established grasses and tall shrubs, a two-phase mosaic structure of high- and low-cover patches characteristic of many other arid zones, was described for the first time in the typical habitats of the central Monte desert: algarrobales, open forests of Prosopis flexuosa trees in a shrub matrix dominated by Larrea divaricata, and jarillales, shrublands of Larrea cuneifolia. The heterogeneous spatial distribution of the soil seed bank, determined by abiotic factors acting on their primary and secondary dispersal, was identified as the main cause of the spatial pattern of adult herbs in the algarrobal. Survival of seedlings and young plants were not different enough among microsites to cause or revert that spatial pattern. Seed abundance limited the recruitment of new seedlings under benign environmental conditions. Postdispersal granivorous vertebrates removing seeds in autumn–winter limited the recruitment of the species they consume. Vertebrate herbivores reduced seedling survival rates and production of spikes in adult grasses. Domestic cattle altered some of these patterns and mechanisms. Cover of most grass species was lower in grazed areas even under extensive management with moderate stocking rates; some changes in species composition were also noted. Lower grass recruitment in the grazed area associated with diminished emergence rates, probably as direct consequence of trampling or as an the indirect effect through the lower plant cover in the habitat. Spike production decreased after tissue removal was lower in the grazed area, though a corresponding reduction in seed bank size was not detected. Jarillales, the most extensive habitat in the Monte but less frequent in the study area, showed less contrast between the two phases of its mosaic structure. Shrub patches were larger and bare patches smaller. Under the shrubs, forb recruitment was lower than in algarrobal despite their bigger seed bank, probably as the negative consequence of more abundant litter. Overall, these results provide evidence of the magnitude of vertebrate consumers’ effects limiting herbaceous plants recruitment in protected and grazed areas in the region. Domestic herbivores and postdispersal granivores consume mostly grasses, affecting their life cycle at different steps, altering their spatial patterns and diminishing their potential abundance. Still, these biotic interactions are subject to strong abiotic constraints, especially water availability in the soil as determined by frequency and magnitude of rain pulses during the growing season. This spatially and temporally variable interaction among biotic and abiotic factors ultimately determine the structure and preservation of herbaceous plant populations in the central Monte desert
Author affiliation: Andrade, Laura Elena. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Repository: Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales