Heat storage in forest biomass improves energy balance closure

Abstract

Abstract.Temperature measurements in trunks and branches in a mature ca. 100 years-old mixed pine and spruce forest in central Sweden were used to estimate the heat storage in the tree biomass. The estimated heat flux in the sample trees and data on biomass distributions were used to scale up to stand level biomass heat fluxes. The rate of change of sensible and latent heat storage in the air layer below the level of the flux measurements was estimated from air temperature and humidity profile measurements and soil heat flux was estimated from heat flux plates and soil temperature measurements. The fluxes of sensible and latent heat from the forest were measured with an eddy covariance system in a tower. The analysis was made for a two-month period in summer of 1995. The tree biomass heat flux was the largest of the estimated storage components and varied between 40 and −35 W m −2 on summer days with nice weather. Averaged over two months the diurnal 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 air were out of phase with the biomass flux and they reached maximum values that were about 75% of the maximum of the tree biomass heat storage. The energy balance closure improved significantly when the total heat storage was added to the turbulent fluxes. The slope of a regression line with sum of fluxes and storage as independent and net radiation as dependent variable, increased from 0.86 to 0.95 for half-hourly data and the scatter was also reduced. The most significant finding was, however, that during nights with strongly stable conditions when the sensible heat flux dropped to nearly zero, the total storage matched the net radiation very well. Another interesting result was that the mean energy imbalance started to increase when the Richardson number became more negative than ca. −0.1. In fact, the largest energy deficit occurred at Correspondence to: A. Lindroth (anders.lindroth@nateko.lu.se) maximum instability. Our conclusion is that eddy covariance measurements can function well during stable conditions but that the functioning under strong instabilities might be a so far unforeseen problem.

Remotely sensed land-surface energy fluxes at sub-field scale in heterogeneous agricultural landscape and coniferous plantation

“…Yet another reason for the larger errors over the forested site is the nature of the site and the flux tower setup. For example, due to the large size of the canopy a large amount of heat can be stored in the in-canopy air layer and in the tree biomass (Lindroth et al, 2010). In addition, at sites in which the EC equipment is mounted on a tall tower the 30 min averaging period might not be enough to capture all the contributing eddies (Finnigan et al, 2003).…”

Variations of Energy Fluxes and Ecosystem Evapotranspiration in a Young Secondary Dry Dipterocarp Forest in Western Thailand

“…On the other hand, we assumed that the effect of surface heterogeneity may partly explain the energy imbalance. Regeneration of this forest normally occurs through resprouting and is characterized by high stem density, lower basal areas, and shorter canopy height when compared to old-growth forests [36,37]. The difference in growth rate among tree species and consequently in tree height may affect surface heterogeneity.…”

“…Since the main characteristic of this forest ecosystem is its distinct seasonality, especially forest growth that occurs only during the wet season, the rest of residual energy was assumed to be stored in biomass. Lindroth et al [36] estimated that approximately 75% of residual energy may be found in biomass. Table 1).…”

“…Since the main characteristic of this forest ecosystem is its distinct seasonality, especially forest growth that occurs only during the wet season, the rest of residual energy was assumed to be stored in biomass. Lindroth et al [36] estimated that approximately 75% of residual energy may be found in biomass. The energy balance closure in this young secondary forest was relatively low (84-85%) when compared to that reported in boreal forests (94%) [22], and in evergreen broadleaf forests and savannas (91-94%) [32].…”

Extension of the Averaging Time in Eddy-Covariance Measurements and Its Effect on the Energy Balance Closure

“…Therefore, the stored energy in biomass can be neglected in our analyses. However, a forest's canopy heat storage is significant (Lindroth et al 2010) and needs to be included in the energy budget equation (Eq. 1).…”