Energy balance partitioning and closure within the canopy of a mountain pine beetle attacked lodgepole pine stand

Disturbance to forest ecosystems due to insect attacks may impact not only regional and global climate but also the microclimate within forest canopies. Recently the mountain pine beetle (MPB) epidemic in Western Canada resulted in large-scale impacts on climate and stand structure. The killing of trees changes the vertical distribution of net radiation (Q*) and the partitioning of latent (Q_E) and sensible (Q_H) heat flux in the different layers of an attacked forest canopy. This affects the growth conditions of secondary structure and in turn influences the recovery time of attacked forest stands. In this study, we analyzed the vertical distribution of the energy balance components and determined the energy balance closure for various levels within the canopy.

This study presents results from measurements in an open, MPB-attacked lodgepole pine stand with considerable secondary structure (stand height, z_h = 17 m, leaf area index = 0.55 m² m^-2) near Prince George, BC. Data from an intensive observation period in July and August 2010 were analyzed to quantify the vertical variations of all energy balance components. Eddy covariance (EC) systems consisting of 3-dimensional sonic anemometers (Campbell Scientific Inc., model CSAT3), open-path infrared gas (CO₂ and water vapour) analyzers (LI-COR Inc., model LI-7500) and fine-wire thermocouples were operated at 7 heights simultaneously over 36 days on a 30-m-tall tower from z/z_h = 0.07 to 1.4. This arrangement provided a vertical profile of directly measured divergence of Q_E and Q_H inside the forest canopy. These EC measurements were complemented by Q* measurements at six heights within and above the forest, which together enabled the analysis of the forest canopy's energy balance in 7 layers. Changes in energy storage in the soil and boles were measured using heat flux plates and bole thermocouples, respectively.

The relatively low stand density resulted in approximately 55% of the average shortwave irradiance reaching the ground. Similarly the 24-h averaged Q* at the ground was about 55% of that measured above the canopy. Q_H dominated Q_E throughout the canopy. While flux divergence calculations indicated relatively strong sources of Q_E at the ground where the secondary structure was located, only very weak sources of Q_E were found in the upper part of the canopy, which was mainly occupied by dead lodgepole pine trees. Q_H sources varied little within the canopy, but were smaller than at the ground. Soil heat flux (Q_G) accounted for approximately 3% of Q*. Furthermore, sensible (ΔQ_H) and latent (DQ_E) heat storage in the air, as well as biochemical energy storage through CO₂ assimilation (ΔQ_C) and sensible heat storage in biomass (ΔQ_B) were also calculated. ΔQ_H was the largest of the energy balance storage components followed by ΔQ_C, ΔQ_E and ΔQ_B. We calculated energy balance closure for each measurement level. At ground level and in the upper canopy Q_H, Q_E, Q_G and all storage components added up to 92 to 98% of Q*; however, at the height of the secondary structure a residual of 20 to 30% was found. This was possibly due to the difficulties of representatively sampling turbulent fluxes and radiation, and the overall low turbulence in the lowest part of the canopy.

Overall, the open stand structure allowed solar radiation to penetrate efficiently through the canopy, which resulted in a relatively constant Q_H source throughout the canopy. All trees (dead pine trees and secondary structure) contributed to Q_H sources whereas Q_E originated mainly from the ground and the secondary structure. Energy balance closure above 0.15 z_h was exceptionally good (within 8%). However, poorer closure was found in the lower canopy layer (<0.15 z_h) where the secondary structure was located.