What the towers don’t see at night: nocturnal sap flow in trees and shrubs at two AmeriFlux sites in California

At the leaf scale a long held assumption is that stomata open to take up CO2 for photosynthesis in the presence of light and stomata close at night, causing both photosynthesis and transpiration are assumed to go to zero. Energy balance models and evapotranspiration equations often rely on net radiation as an upper bound, and some models reduce evapotranspiration to zero at night when there is no solar radiation. Emerging research is showing, however, that nocturnal transpiration can occur throughout the night in a variety of vegetation types and biomes. At the ecosystem scale, eddy covariance measurements have provided extensive data on latent heat flux for a multitude of ecosystem types globally. Nighttime eddy covariance measurements, however, generally are unreliable due to low turbulence. If nighttime water loss occurs significantly, then these eddy flux towers may be missing some key information on latent heat flux because some proportion is occurring at night. We installed and measured rates of sap flow using the heat ratio method (Burgess et al. 2001) at two AmeriFlux (part of FLUXNET) sites in California, USA. The heat ratio method is able to measure and quantify low rates of sap flow, including zero and negative (i.e., hydraulic lift) rates, unlike the widely used Granier method (Granier 1985), which requires an assumed zero sap flow to calibrate the measurements. We measured sap flow in five Pinus ponderosa trees, three Arctostaphylos manzanita and two Ceanothus cordulatus shrubs in the Sierra Nevada Mountains; and, five Quercus douglasii trees at an oak-savanna in the Central Valley of California. Nocturnal sap flow was observed in all species measured; significant nighttime water loss was observed in both species of trees. Vapor pressure deficit and air temperature were strong indicators of nighttime transpiration; the influence of wind speed on nighttime transpiration was insignificant at our sites. We differentiated refilling from water loss from data from 2005, and calculated the percentage of nighttime transpiration underestimated in the eddy covariance measurements at both sites. These data contribute to the emerging literature on nighttime transpiration for multiple species in two ecosystems at eddy flux sites