Sap Flow Based Transpiration Estimates in Species-Rich Secondary Forests of Different Ages in Central Panama During a Wet-Season Drought

Tuesday, 7 June 2016
Mario Bretfeld1, Brent E Ewers2, Jefferson S Hall1 and Fred L Ogden3, (1)Smithsonian Tropical Research Institute, Balboa, Panama, (2)University of Wyoming, Botany, Laramie, WY, United States, (3)Univ. of Wyoming - Dept 3295, Laramie, WY, United States
Abstract:
Many landscapes that were previously covered by mature tropical forests in central Panama today comprise of a mosaic of mature forest fragments, pastures and agricultural land, and regrowing secondary forests. An increasing demand for water due to urbanization and the expansion of the Panama Canal, along with a predicted transition into a dryer climatic period necessitate a better understanding regarding the effects of land use and land use history on hydrological processes. Such knowledge, including water storage, residence times, and fluxes is essential to develop effective land management strategies and propose incentives to alter land use practices to enhance hydrological services.

To quantify transpiration rates at different stages of secondary forest succession, we measured sap flow in forests growing for 8, ~25, and 80+ years since last land use in the 15 km2Agua Salud study area, located in central Panama. In each forest, we selected a subset of at least 15 individuals representing the local tree size distribution. All instrumented trees were identified to species and compared to local species distributions. Sap flow was measured using heat-ratio sensors every 30 minutes, starting in February 2015, and upscaled to stand-scale transpiration rates.

Basal area in the three forest types was 9.4, 16.1, and 31.5 m2/ha for 8, ~25, and 80+ year old forests, respectively. Average daily transpiration was 1.6 mm (8 years), 5.2 mm (~25 years), and 5.5 mm (80+ year). Primary external drivers of transpiration were vapor pressure deficit and solar radiation, while soil moisture changes were not reflected in rates of transpiration. Our results suggest that secondary forests in central Panama reach transpiration rates similar to those of older forest by ~25 years age and are not water limited even in an extremely dry year, with important implications for reforestation efforts and hydrological models.

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