Partitioning energy and evapo-transpiration above and below a tropical palm canopy

For open canopies, the partitioning of energy and water fluxes between the upper canopy and the under-storey is key to understand the competition mechanisms between layers. We monitored stand evapo-transpiration by eddy-covariance (E), tree transpiration (T) by heat-dissipative sapflow, soil (G) and stand (J) heat storage during 3 years in a typical coconut palm row-plantation of the humid tropics, displaying a constant leaf area index (LAI = 3) and a grass under-storey. A two-level flux experiment yielded cross-validation of the evapo-transpiration of the under-storey (Eu) with E − T, through energy balance closure for E and sapflow calibration in the laboratory for T. On a yearly time step, E represented 40% of rainfall, the sensible heat flux (H) was 26% of net radiation and the Bowen ratio was 0.39, all indicating that water availability was close-to-optimum. T represented 68% of E, close to the 75% of soil coverage by palms. The inter-annual variability was low for potential evapo-transpiration (ETo), energy and water fluxes. The seasonal variability was more pronounced, driven by radiation and vapour pressure deficit (VPD). The canopy conductance of coconut palms appeared to be strongly controlled by VPD. Given its constant LAI, its continuous growth and its simple architecture, the coconut palm appears to be an ideal candidate for physiological work and agroforestry modelling.