Vertical Distribution of Radiation and Energy Balance Partitioning Within and Above a Lodgepole Pine Stand Recovering from a Recent Insect Attack

Abstract

The current outbreak of mountain pine beetle (MPB) that started in the late 1990s in British Columbia, Canada, is the largest ever recorded in the north American native habitat of the beetle. The killing of trees is expected to change the vertical distribution of net radiation (\(Q^*\)) and the partitioning of latent (\(Q_\mathrm{E}\)) and sensible (\(Q_\mathrm{H}\)) heat fluxes in the different layers of an attacked forest canopy. During an intensive observation period in the summer of 2010, eddy-covariance flux and radiation measurements were made at seven heights from ground level up to 1.34 times the canopy height in an MPB-attacked open-canopy forest stand \((\hbox {leaf area index} = 0.55~\mathrm{{m}}^{2}\ \mathrm{{m}}^{-2})\) in the interior of British Columbia, Canada. The lodgepole pine dominated stand with a rich secondary structure (trees and understorey not killed by the beetle) was first attacked by the MPB in 2003 and received no management. In this study, the vertical distribution of the energy balance components and their sources and sinks were analyzed and energy balance closure (EBC) was determined for various levels within the canopy. The low stand density resulted in approximately 60 % of the shortwave irradiance and 50 % of the daily total \(Q^*\) reaching the ground. Flux divergence calculations indicated relatively strong sources of latent heat at the ground and where the secondary structure was located. Only very weak sources of latent heat were found in the upper part of the canopy, which was mainly occupied by dead lodgepole pine trees. \(Q_\mathrm{H}\) was the dominant term throughout the canopy, and the Bowen ratio (\(Q_\mathrm{H}/Q_\mathrm{E}\)) increased with height in the canopy. Soil heat flux (\(Q_\mathrm{G}\)) accounted for approximately 4 % of \(Q^*\). Sensible heat storage in the air (\(\Delta Q_\mathrm{S,H}\)) was the largest of the energy balance storage components in the upper canopy during daytime, while in the lower canopy sensible heat storage in the boles (\(\Delta Q_\mathrm{S,B}\)) and biochemical energy storage (\(\Delta Q_\mathrm{S,C}\)) were the largest terms. \(\Delta Q_\mathrm{S,H}\) was almost constant from the bottom to above the canopy. \(\Delta Q_\mathrm{S,C}\), \(\Delta Q_\mathrm{S,B}\) and latent heat storage in the air (\(\Delta Q_\mathrm{S,E}\)) varied more than \(\Delta Q_\mathrm{S,H}\) throughout the canopy. During daytime, energy balance closure was high in and above the upper canopy, and in the lowest canopy level. However, where the secondary structure was most abundant, \({\textit{EBC}} \le 66\,\%\). During nighttime, the storage terms together with \(Q_\mathrm{G}\) made up the largest part of the energy balance, while \(Q_\mathrm{H}\) and \(Q_\mathrm{E}\) were relatively small. These radiation and energy balance measurements in an insect-attacked forest highlight the role of secondary structure in the recovery of attacked stands.