PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Proceedings-Research on Coniferous Forest Ecosystems-A symposium.
Bellingham, Washington-March 23-24, 197 2
Energy flux studies in a
coniferous forest ecosystem
Abstract
Lloyd W. Ga y
Associate Professor of Forest Climatolog y
Department of Forest Engineerin g
Oregon State University
Corvallis, Orego n
The fluxes of thermal energy between the atmosphere and a young Douglas-fir forest were measured durin g
two contrasting summer days, one cloudless and one overcast. The energy budget components were evaluated
by the Bowen ratio method, with ceramic-wick psychrometers at the 26 .16 m, 28.16 m, or 31 .16 m levels. The
maximum height of the tallest trees was 28 m, and the general level at the top of the closed canopy was abou t
22 m. Daily totals of the energy budget components (cal/cm 2 ) under cloudless skies on July 29, 1971, were :
solar radiation, 584; net radiation, 410 ; change in storage, 5 ; convection, -135; and latent energy, -280. The
albedo was 0.09 on both the clear and the overcast day . Analysis of the overcast conditions of July 31, 1971 ,
yielded the following values: solar radiation, 171 ; net radiation, 134 ; change in storage, 6 ; convection, -39; and
latent energy, -102.
Problems of measurement and analysis are discussed. These include the storage term in the biomass, and th e
small gradients of potential temperature and vapor pressure above the canopy . Clear day gradients at noon, for
example, were in the order of -0.03°C m-' and -0.03 mb m-' . Techniques are presented for minimizing measurement
errors.
Introduction
The level of biological activity at the surface
of the earth is closely associated wit h
cycles of energy and mass . The cycles of mas s
and of energy are virtually interchangeabl e
concepts. Indeed, the transpiration an d
photosynthetic components of the mass cycl e
can be studied through examination of th e
cycle of energy, with the energy required t o
change the phase of H2 0 and CO 2 serving as
the connecting link .
The magnitudes and phase relationships of
the mass and energy cycles are affected by th e
characteristics of the surface, and by the stat e
of the atmosphere . The properties of vegetation,
particularly of low, cultivated ecosystems,
have been investigated thoroughly i n
a variety of studies that have clearly demonstrated
many advantages for energy budge t
evaluations of transpiration and photosynthesis
(Baumgartner 1965) . The advantage s
include sensitivity, mobility, and the benefit s
to be gained by use of a nondestructive technique
. The application of these techniques t o
the forest ecosystem appears feasible and useful.
A number of studies have already been
reported, but in general the effects of forest s
upon the cycles are not yet well known
(Baumgartner 1971, Tajchman 1971) . Coniferous
forests, as a class, are good absorbers of
solar radiation. The roughness of coniferou s
crowns also appears to effectively enhanc e
mixing in the atmosphere near the top of th e
canopy. These factors, combined with the
large surface area of canopies, make forests
into very efficient exchange surfaces for water
vapor, carbon dioxide, and energy .
Studies of the fundamental cycles of energ y
and mass have begun at the Cedar River site i n
the Coniferous Forest Biome. A variety of
interrelated studies are planned in coopera -
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