Sap flow, canopy conductance and microclimate in a banana screenhouse

A field experiment was conducted to investigate the effect of a flat-roof screenhouse on banana transpiration (Tr) and microclimate during the summers and falls of 2005 and 2006 in Northern Israel. The clear polyethylene screen reduced radiation by between 8 and 25% depending on dust accumulation and aging. In the screenhouse, wind speed, global radiation and air temperature were reduced by more than 60, 20 and 1%, respectively, and relative humidity increased by 8% relative to an external meteorological station. Class A pan evaporation and reference crop evapotranspiration (ET0) were reduced by about 44 and 33% in the screenhouse, respectively. Inside, banana transpiration (Tr), measured with thermal dissipation probes, was about 90% of that outside the screenhouse. The relatively small reduction in Tr inside was caused by increased canopy conductance in the screenhouse during much of the day, which at mid-day was double that outside. Hourly average canopy conductance increased with increasing vapor pressure deficit (VPD) during much of the day and decreased late in the afternoon. Inside the screenhouse, leaves were large and whole with a high boundary layer resistance, but outside leaves were torn by the wind, which, we estimate, reduces the characteristic leaf dimension by an order of magnitude from 1.4 to 0.14 m, decreases boundary layer resistance and reduces the decoupling coefficient. The decoupling coefficient outside was up to 0.3 in the morning and declined to less than 0.1 in the afternoon when wind speed increased. Inside, the corresponding values were 0.8 and 0.5, respectively. This indicates that inside radiative factors dominate, while outside aerodynamic factors dominate during much of the day. A sensitivity analysis showed that the reduction in ET0 in the screenhouse is mainly due to the combined reductions of wind speed and global radiation. Inside, the screenhouse Tr was similar in two irrigation treatments (85 and 100% of class A pan evaporation) and also similar to outdoor pan evaporation.