Heat Storage and Energy Closure in Two Topical Montane Forests in Hawaiʻi

Abstract

Eddy covariance data collected above land surfaces provide direct measurements of evapotranspiration (ET), the sum of all evaporative processes over a particular land cover including transpiration, soil evaporation, and wet canopy evaporation, and a limited number of assessments of these data in tropical forests have been carried out. Evaluation of eddy covariance data is regularly conducted by assessing the energy balance, including radiation exchange measured above the vegetation and heat storage in the vegetation, soil, and the column of air below the eddy covariance sensors. In a native Hawaiian tropical montane forest (Nahuku) and a nearby forest (ʻŌlaʻa) that has been partially invaded by strawberry guava (Psidium cattleianum), vertical and radial distribution of all biomass components were evaluated from detailed stand surveys, biomass samples, allometric relationships, wood density, fresh to dry weight ratios of plant materials, and temperature measurements of stem biomass. Eddy covariance data were analyzed at the two sites in Hawaiʻi Volcanoes National Park, Hawaiʻi, for a 34 month period to evaluate the importance of biomass and air heat storage to the energy balance and determine site specific energy closure characteristics. Total fresh biomass was estimated to be 69.8 ± 11.7 kg m-2 and 75.9 ± 16.6 kg m-2 at Nahuku and ʻŌlaʻa, respectively, and the contribution of separate biomass components to energy closure were evaluated in detail. Despite statistically similar fresh biomass between stands, energy storage was found to be significantly greater at the forest site with P. cattleianum tree invasion (ʻŌlaʻa) than at the native Metrosideros polymorpha forest stand (Nahuku). The difference was attributed to a higher proportion of smaller stems at ʻŌlaʻa, absorbing and releasing more heat for a given mass. Inclusion of biomass and air heat storage in the energy balance improved the relative energy closure (Ω), the slope of the linear regression (forced through the origin) of the sum of latent and sensible heat fluxes measured above the canopies for each 30-minute period from 0.767 to 0.805 at Nahuku and from 0.918 to 0.997 at ʻŌlaʻa. The mean absolute energy imbalance (EIABS), the mean of the differences between the available energy and the sum of latent and sensible heat fluxes for each 30 minute interval for a binned group of values, was also reduced for most parts of the diurnal cycle. These results indicate that it is necessary to include heat storage in energy balance investigations to reduce error in energy balance adjustments of ET. However, it was found that the relative energy closure is not constant over all environmental conditions and has complex relationships with friction velocity, atmospheric stability, and time of day. Therefore, energy closure adjustments to ET estimates should consider environmentally controlled variation in the relative and absolute energy closure in order to reduce error in estimates of land-atmosphere gas exchange. Furthermore, including all significant heat storage terms does not close the energy balance at the native forest site, which is likely due to additional site specific factors influencing the characteristics of turbulent flows over the surface.