Heat storage in forest biomass improves energy balance closure

[摘要] Temperature measurements in trunks and branches in a mature ca. 100 years-old mixedpine and spruce forest in central Sweden were used toestimate the heat storage in the tree biomass. The estimated heat flux inthe sample trees and data on biomass distributions were used to scale up tostand level biomass heat fluxes. The rate of change of sensible and latentheat storage in the air layer below the level of the flux measurements wasestimated from air temperature and humidity profile measurements and soilheat flux was estimated from heat flux plates and soil temperaturemeasurements. The fluxes of sensible and latent heat from the forest weremeasured with an eddy covariance system in a tower. The analysis was madefor a two-month period in summer of 1995. The tree biomass heat flux was thelargest of the estimated storage components and varied between 40 and −35 W m⁻²on summer days with nice weather. Averaged over two months thediurnal maximum of total heat storage was 45 W m⁻² and the minimum was−35 W m⁻². The soil heat flux and the sensible heat storage in airwere out of phase with the biomass flux and they reached maximum values thatwere about 75% of the maximum of the tree biomass heat storage. Theenergy balance closure improved significantly when the total heat storagewas added to the turbulent fluxes. The slope of a regression line with sumof fluxes and storage as independent and net radiation as dependentvariable, increased from 0.86 to 0.95 for half-hourly data and the scatterwas also reduced. The most significant finding was, however, that duringnights with strongly stable conditions when the sensible heat flux droppedto nearly zero, the total storage matched the net radiation very well.Another interesting result was that the mean energy imbalance started toincrease when the Richardson number became more negative than ca. −0.1. Infact, the largest energy deficit occurred at maximum instability. Ourconclusion is that eddy covariance measurements can function well duringstable conditions but that the functioning under strong instabilities mightbe a so far unforeseen problem.

[学科分类] 地球化学与岩石