Differential and dynamic water regulation responses to El Niño for monospecific and mixed species planted forests

Abstract

AbstractMany tropical planted forests are being subjected to drier conditions due to climate change, but the interaction between drought and species diversity on species' response to drought is poorly understood. We took advantage of a historic El Niño drought in Central Panama to test drought responses of two species—Terminalia amazonia and Dalbergia retusa—in simplified, planted forests. We asked whether and how species adjust strategies when grown in monocultures and mixtures with reduced precipitation. We collected sap flux density, volumetric water content, litterfall, leaf physiology traits and leaf water potential before and during the drought. The main drivers of sap flux density (Js) in monocultures and mixtures changed from radiation, vapour pressure deficit (VPD) and air temperature to volumetric water content or precipitation towards the end of the drought. The exception was for D. retusa in the mixtures, where VPD was the main driver (0.73–0.77) of Js during the normal year and switched to precipitation during the drought year (0.47–0.72). Increasing VPD led to increases in Js until air temperature exceeded 32°C, at which point Js declined for D. retusa and stabilized for T. amazonia. T. amazonia's litterfall doubled during the drought, whereas D. retusa's litterfall peaked at the start of the dry seasons and was unaffected by the drought. D. retusa, however, delayed leaf flush until the drought ended, whereas T. amazonia transitioned through a series of water‐conserving strategies until prematurely shedding leaves prior to the peak of the drought.