Effects of El Niño on tree water use responses and water stress: do complementary interactions in mixed species tree plantations enhance resistance to drought over monocultures?

Severe and prolonged droughts are becoming increasingly common, yet the effects of drought on specific species and species combinations are poorly understood. We took advantage of the 2015-2016 El Niño Southern Oscillation (ENSO) event in central Panama to test differences in drought response in monocultures and mixtures of Terminalia amazonia and Dalbergia retusa, comparing the response to pre-ENSO conditions of 2014. Mean daily sap flux density (J_s, gm^-2s^-1) declined significantly during the 2015 ENSO drought year for all trees in monocultures and mixtures. All treatments increased J_s from 2015 to 2016 except for D. retusa in the monocultures, where J_s significantly declined. During 2014, leaf water potential (Ψ_L, MPa) was significantly more negative for T. amazonia than D. retusa, but no differences in Ψ_L were observed between monocultures and mixtures. Toward the end of the drought in 2016, Ψ_L of T. amazonia in monocultures was significantly more negative than T. amazonia in the mixtures. In 2015, the Ψ_L of D. retusa in the mixtures was more negative than in the monocultures, but this was not significant. VPD and radiation were the main drivers of J_s before the ENSO event and transitioned to VWC at the end of the ENSO conditions. Our results suggest that complementary interactions in mixed species plantations affect water regulation and moisture stress of trees in ways that enhance resistance to drought compared to monocultures. These findings have implications for designing plantations for climate change adaptation and reduced susceptibility to drought.