Sap flow based transpiration estimates in species-rich secondary forests of different ages in central Panama during a wet-season drought

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 known land use in the 15 km² “Agua 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, and recorded data from heat-ratio sap flow sensors every 30 minutes starting in February 2015. All instrumented trees were identified to species and compared to local species distributions.

Basal area in the three forest types was 9.1, 10.8, and 50.2 m² ha^-1 for 8, ~25, and 80+ year old forests, respectively. Average daily transpiration was highly correlated to forest age, with highest rates in the oldest forest (3.0 to 18.2 mm ha^-1 day^-1), followed by intermediate (1.2 to 6.7 mm ha^-1 day^-1) and youngest forests (0.2 to 2.7 mm ha^-1 day^-1), suggesting roughly a doubling in transpiration from 8 to ~25 year old forests, despite similar basal area, and again from ~25 to 80+ year old forests. Flow rates in individual trees generally reflected the dry-to-wet season transition but behaved differently in response to the unprecedentedly dry conditions during the first half of 2015 in central Panama.