Consequences of agroforestry management on understory vegetation in Uruguay
Six, Laura Jane
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Uruguay, part of the Campos ecoregion, which has a long history of grazing. Recently, afforestation has also become a common land-use change across the Campos. As a novel ecosystem develops, the effects of these multiple disturbances are largely unknown. In this dissertation, I present results of field experiments to provide some understanding of the interactive effects of grazing and afforestation in the Campos. First, I described vegetation change across a Eucalyptus grandis agroforestry management cycle by characterizing richness and composition of understory vegetation across five phases of a plantation cycle (Grassland, Young Forest, Mid-stage Forest, Old Forest, and Post-Harvest). I found that mid-stage forests were most different in richness and composition from other phases, especially Grasslands, and that sites exhibited potential for recovery after harvest, as richness increased in Old Forests and Post-Harvest sites. Second, I characterized soil physical and chemical properties across the same Eucalyptus grandis cycle. I found that soils appeared largely unaffected by afforestation (instead differing primarily with sample depth), which may be a result of limitations in sample size, as well as already reduced fertility from long-term grazing. Third, I used a manipulative approach to quantify the individual and interactive effects of grazing and afforestation on vegetation at multiple spatiotemporal scales. From species area curves, I found that small-scale richness was greater in grasslands, but a greater rate of species accumulation in forests resulted similar richness at larger scales. At the site-level, while seasonal changes in vegetation were evident in grazed areas, the removal of grazing resulted differing richness by habitat: richness decreased in grasslands with time since exclosure, but increased in forests. My results demonstrated a complex response of vegetation to disturbance, that varied by the nature of the disturbance mechanism. Finally, I quantified pine seedling germination and establishment in plantations and adjacent grasslands to examine potential spread of this exotic species. I found that seedling density in grasslands was minimal compared to plantations, and that the mechanisms controlling encroachment differed between grazed and ungrazed areas: in ungrazed grasslands, the dense vegetation cover prevents establishment, whereas in grazed grasslands, intensive livestock grazing prevents tree establishment.
- Forestry