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Theses Doctoral

Seeing the forest for the streams: A multiscale study of land-use change and stream ecosystems in the Amazon's agricultural frontier

Macedo, Marcia Nunes

Global demand for agricultural products is an increasingly important driver of deforestation in the Amazon Basin. This dissertation examines the consequences of agricultural expansion for stream ecosystems in the southern Amazon's agricultural frontier. At regional scales, the removal of watershed forest cover is known to change the energy balance and influence hydro-climatic cycling by altering stream flow, regional rainfall patterns, and land surface temperatures. At the landscape scale, these physical changes may be further exacerbated by land management practices that lead to the degradation of riparian forest buffers; decreases in connectivity; changes in the amount of light, nutrient, and sediment inputs; and decreases in water quality. Together, land use and management influence the quality and distribution of aquatic habitats within stream networks, potentially decreasing stream biotic integrity and resilience to further disturbances.
Brazil's Mato Grosso state is one of the most actively expanding agricultural frontiers in the world and represents an ideal case study for examining the linkages among tropical deforestation, agricultural expansion, and the conservation of freshwater ecosystems. Mato Grosso accounted for 40% of deforestation in the Brazilian Amazon during the early 2000s - primarily due to the expansion of soybeans and cattle ranching. Deforestation rates have since dropped throughout the Amazon, but there is a lack of spatially explicit information about the land use transitions accompanying this decline. To address this gap, I combined government data on deforestation and production with the MODIS satellite time series to quantify the spatial-temporal dynamics of land use change in the region. Although agricultural expansion during this period slowed with declining commodity prices, the decline in deforestation is partly explained by a shift from soybean expansion into forests (26% of expansion from 2001-2005) to expansion into already cleared pasture lands (9% of expansion form 2006-2010). Beyond documenting these trends, the resulting dataset is a critical first step in evaluating the influence of land use and land use history on freshwater ecosystems at multiple scales.
In the headwaters region of the Xingu River Basin, the proportion of small watersheds (microbasins) dominated by agriculture (>60% of area) increased from 20 to 40% from 2001 to 2010. At the same time, the stream network became increasingly fragmented by the removal of riparian forest buffers and installation of farm impoundments. I used high resolution satellite data (ASTER) to produce the first landscape-level documentation of farm impoundments in the region, mapping approximately 10,000 impoundments (one per 7.6 km of stream length) in 2007. At the catchment scale, I collected field data in 12 headwater streams to examine the effect of land management on instream water quality. Watershed forest cover (from MODIS), the density of impoundments (from ASTER), and the percent forest in upstream riparian buffers (from Landsat) were all associated with substantial increases in stream temperature. These increases in fragmentation and water temperature may have large cumulative effects on the stream network and reduce the ability of downstream protected areas to conserve freshwater resources. At the scale of the Amazon Basin, my analysis indicates that 30% of indigenous lands and protected areas are highly vulnerable to future reductions in hydrologic connectivity, simply because of their location within their watersheds. These impacts could be substantially mitigated through enforcement of existing legislation to protect riparian buffers and new regulations to limit the number of impoundments in emerging agricultural landscapes.

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More About This Work

Academic Units
Ecology, Evolution, and Environmental Biology
Thesis Advisors
DeFries, Ruth S.
Degree
Ph.D., Columbia University
Published Here
December 12, 2017
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