Seasonally dry tropical forests are subjected to highly variable precipitation requiring adaptations to survive prolonged periods of limited water availability. Dry periods and seasonality are expected to be further amplified due to a changing climate, necessitating a mechanistic understanding of plant adaptations to mitigate drought stress. The 2015/16 El Niño–Southern Oscillation (ENSO) event ranks amongst the driest and hottest periods on record in Panama and thus provided an excellent opportunity to study seasonal water relations of common Panamanian tree species.
Using heat-ratio sap flow sensors, we measured sap flow of 76 trees between 2015 and 2017 in secondary forests aged approximately 8, 25, and 80 years in the 15 km2 Agua Salud study area, located in central Panama. Of those trees, 16 individuals representing three common species (Xylopia frutescens, Vismia macrophylla, Terminalia amazonia) were instrumented with additional sap flow sensors on three roots each. Raw heat ratio data were logged every 30 minutes and converted to sap velocities and sap flux using physical conversions specific to wood properties of that species. Additional environmental data were provided by in-situ soil moisture sensors and a nearby meteorological station.
Sap flow data of individual roots largely followed distinct seasonality of stem sap flow and was primarily driven by a combination of radiation and vapor pressure deficit. However, the roots of some smaller individuals of two species (Vismia macrophylla and Xylopia frutescens) exhibited inverted sap velocities, as expressed though decreasing flow in one root and simultaneously increasing flow in another root of the same individual. Interestingly, this behavior was only found during the dry season with low soil water content, but was not clearly related to root diameter, and resulted in a total volume of water flux that remained approximately similar throughout periods with inversions.
Our results provide evidence for distinct temporal partitioning among roots of trees growing in seasonally dry climates in response to limited soil moisture availability. Future research is needed to address the mechanisms that regulate observed flow inversions: stomatal control in the canopy or water potential limitations root-soil interface as a result of different rooting depths.
roots, sap flow, drought, el nińo, transpiration