Woody plant encroachment has been widespread in grassland and savanna ecosystems over the past century. This phenomenon jeopardizes grassland biodiversity and threatens the sustainability of pastoral, subsistence and commercial livestock grazing. As such, it may adversely impact-20% of the world's population. Although woody plant expansion has long been a concern of land managers
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in grassland and savanna regions, most research on this issue has focused on woody plant effects on grass production and the development methods to limit or reduce the abundance of trees and shrubs. Little is known of the rates and dynamics of the phenomenon or its impact on fundamental ecological processes related to energy flow and nutrient cycling. Grassland/savanna systems account for 30-35% of global terrestrial net primary production. Hence, when woody species increase in abundance and transform grasslands and savannas into shrublands and woodlands, the potential to alter land surface-atmosphere interactions and C and N sequestration and cycling at regional and global scales may be significant.
The La Copita case study documents the rate and magnitude of change in ecosystem biogeochemistry that can occur when a subtropical dryland landscape is transformed from savanna grassland to woodland. Linked succession-biogeochemistry models, confirmed with historical aerial photography and ground measurements, indicate that soil and plant G mass has increased 10% and 10-fold, respectively, with succession from presettlement savanna grassland to present-day savanna woodland. Ecosystem C storage will continue to increase as present-day woody vegetation communities mature and expand into remaining herbaceous areas. Accumulation of ecosystem C mass was accompanied by increases in soil N pools. Fluctuations in monthly woody plant root biomass in upper soil horizons exceeded foliar litter inputs by one to two orders of magnitude, suggesting that belowground inputs of organic matter drive changes in soil physical and chemical properties subsequent to woody plant establishment in grasslands. The deep root systems of woody plants have also increased C mass throughout the soil profile relative to that of grasslands. Increases in C and N pools have occurred in spite of increases in N mineralization, NO flux, soil respiration, and nonmethane hydrocarbon emissions.
These results are of potential global significance, given that large areas of Africa, South America, North America, and Australia have been undergoing similar land cover changes over the past century. The demonstrated capacity for carbon sequestration in this semiarid system suggests a need to reevaluate traditional perspectives on woody plants in rangelands as governments and industries seek ways to mitigate greenhouse gas emissions. However, sequestration of C by woody plants in drylands may come at the expense of elevated NOx, NMHC, and groundwater fluxes. Regional assessments of the potential consequences of global change are hampered by a lack of quantitative information on the geographic balance between woody plant expansion and clearing in the world's extensive and often remote drylands. Recent developments in linked remote-sensing ecosystem modeling approaches show promise for alleviating these monitoring constraints.
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