Abstract
Components of the radiation and energy balances were measured over a clear-cut area and a mature, mixed forest during the summer of 1981 at the Petawawa National Forestry Institute, Chalk River, Ontario. The work concentrated on the clear-cut site which supported a canopy layer composed primarily of bracken fern and logging remnants.
Forty days of radiation data were collected at the clear-cut site. After the first four weeks of measurements (the ‘green’ season), most of the ferns quickly died, and their foliage changed appearance from a green to brownish colour (the ‘brown’ season). The daily mean reflection coefficient of solar radiation determined over the ‘green’ season was 0.20 and decreased to 0.13 for the ‘brown’ season. The corresponding value for the forest was 0.13, based on a limited amount of data. The clear-cut site received 11% and 21% less net radiation than the forest on a 24-hr and daylight-hours basis, respectively, as a consequence of the higher reflection coefficient and larger daytime longwave radiation emission.
A reversing temperature difference measurement system (RTDMS), incorporating ten-junction thermopiles was employed at each site in order to determine Bowen ratios (β) via differential psychrometry. Both systems performed well, especially the RTDMS over the forest which was capable of resolving very small differences of temperature, typically less than 0.2 °C over a height of 3 m. The mean hourly Bowen ratio, calculated from values from 0800 to 1600 hr, varied from 0.2 to 1.0 for the forest and from 0.4 to 0.8 for the clear-cut site in the ‘green’ season.
A significant canopy heat storage component of the energy balance, Q S , was found at the clear-cut site. In the early morning, a portion of the available energy was used to heat the biomass materials and air within the canopy layer. The stored heat within the canopy was released later in the day, increasing the available energy total.
The daily mean value of the Priestley-Taylor coefficient α (Priestley and Taylor, 1972) for the ‘green’ season at the clear-cut site was 1.14, and individual values tended to increase during wet surface conditions and decrease when the surface dried. The daylight mean α value during dry canopy conditions at the forest was 1.05, and much higher values occurred when the canopy was wet. The enhancement of α for the wet forest was a result of the evaporation of intercepted rain (which is not limited by stomatal resistance) and the concomitant transfer of sensible heat to the forest.
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McCaughey, J.H. A radiation and energy balance study of mature forest and clear-cut sites. Boundary-Layer Meteorol 32, 1–24 (1985). https://doi.org/10.1007/BF00120711
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DOI: https://doi.org/10.1007/BF00120711