Grassland Creation in a Montane Tropical Rainforest and its Effects on Soil-Vegetation Nutrient Pools and Nutrient Cycles: A Case Study in the Gran Pajonal of Eastern Peru

Scott, Geoffrey Arthur James
Street, John M.
Geography and Environment
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[Honolulu] : [University of Hawaii at Manoa], [May 1974]
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Field studies were carried out in the Gran Pajonal in Amazonian Peru to determine relationships between swidden agriculture followed by burning and; vegetation succession, soil changes, nutrient inventories, nutrient cycles. Phytosociological study results indicate that the small grasslands, which constitute three per cent of Gran Pajonal cover, are anthropogenic in origin. Felling forest and burning slash provide the Campa Indians with swidden soil conditions suitable for growing manioc and maize. Within only a few years swidden soils undergo adverse physical and chemical changes, and crop nutrient uptake and production decrease. Rapid succession to secondary forest follows complete swidden abandonment. If vegetation invading abandoned swiddens is regularly burned over, a community dominated by Pteridium aquilinum replaces succession to secondary forest. With annual burning and soil changes resulting from exposure following burning, fern is replaced by Imperata brasiliensis. Eventually Andropogon and Cyperus species dominate the grassland fire-sub-climax association. In primary and mature secondary forest stands nutrient inventories within the biomass are very large, internal soil-to-vegetation-to-soil nutrient cycling is more important than nutrient flows into and out of the local ecosystem, and the major nutrient pool is in the living biomass as opposed to the soil. If forest is felled for swidden and then converted to derived grassland, nutrient inventories within the living biomass are greatly reduced, external nutrient cycling has greater relative importance than under forest, the major nutrient pool lies in the soil, and there occurs a very sizable net loss of nutrients. Biomass for three, ten and 25-30-year-old secondary forest stands are 50, 90 and 220 mt/ha respectively, while values of over 600 mt/ha are found in primary forest. The regular burning of abandoned swidden areas give rise to fern communities with 20-30 mt/ha biomass. Fern is replaced by grassland fire-sub-climax communities having biomass values of 4-7 mt/ha. Small watershed settling basin experiments indicate that the removal of vegetation cover through burning promotes mineral particle and nutrient losses through sheet erosion, while unburned vegetation covers reduce such losses to a minimum. In particular, during the first wet season following swidden formation, very sizable amounts of dissolved nutrients are lost from ash-strewn swidden surfaces. Evidence is also found to indicate that old grassland will return to secondary forest if burning ceases.
PhD University of Hawaii at Manoa 1974
Includes bibliographical references (leaves 322–332).
grassland ecology, Peru, rain forests, soil fertility, plant ecology, Gran Pajonal, shifting cultivation, Campa Indians agriculture, soils, plant-soil relationships
xvii, 332 leaves : illustrations (some color), maps
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Geography.
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