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

method, using psychrometers or infra-red ga s
analyzers as the detecting instruments, is commonly
applied to plant material enclosed i n
assimilation chambers . The possible artifact s
of the systems pointed out for CO 2 assimilation
measurements (Jarvis et al . 1971) are
even more critical in the case of transpirational
assessments . This is true because of th e
role of energy balance and resulting lea f
temperature in determining the vapor pressur e
gradient from leaf to atmosphere . Also
adsorption and condensation problems ofte n
make measurements difficult and less accurat e
during cool moist periods either diurnal or seasonal,
although rates are characteristically low
under such conditions . The lysimeter technique
is suited to diurnal and seasonal studie s
without the artificiality of enclosing th e
foliage (Fritschen 1972) . Root disturbance
occurs in installation, but this should hav e
little effect on transpiration and passive up -
take of water . Meteorological methods, whic h
measure evapotranspiration, also maintai n
natural conditions around the foliage, bu t
necessitate sizable stands of homogenous
vegetation and uniform air conditions for bes t
measurements. The tritiated water metho d
(Kline et al . 1972) yields information on transpirational
activity only over periods of hour s
and days, but its advantage over the other
methods lies in its applicability to very larg e
trees, such as the 75-m-tall old-growt h
Douglas-firs in the H . J. Andrews Experimental
Forest. During certain periods o f
1972, all four of these methods of transpirational
measurement will be compared at th e
Thompson Research Center (Cedar River) .
The lysimeter study will be continuou s
throughout the 1972 spring through autum n
season, and the simultaneous measurement o f
transpired water in the gas stream from the
assimilation cuvettes will permit estimation of
leaf resistance as well as giving data on transpirational
rates .
Influence of Environmental Factors
General discussions of these factors an d
their influences have been given by Stalfelt
(1956), Kramer and Kozlowski (1960), an d
Kozlowski and Keller (1966) . Emphasis here
will be on more recent studies with conifers
and especially with species of special concern
in the Coniferous Biome .
The dominant influence of the vapor pressure
gradient from leaf to air on transpirational
rate when stomata are open has long
been recognized. This was verified by Ritchie
(1971) in a study with Abies amabilis and A .
procera of about 14-m height, using vapor
pressure gradient and also saturation deficit of
the atmosphere . On a statistical basis, th e
latter value accounted for 56 percent of th e
variation in transpirational rate on a seasona l
basis .
Depletion of soil moisture has also lon g
been recognized as a major factor in the control
of transpiration . This was verified by
Mullerstael (1968) using three species of pine ,
as well as other evergreens, and he als o
showed marked species differences . Hinckley
(1971) showed great reduction in transpiration
(sap velocity) in Abies amabilis and A .
procera with soil water depletion. Reed
(1972) developed a computer simulation o f
transpiration in the field . He showed that lo w
vapor pressure gradients limited transpiratio n
in the spring, but increased stomatal resistance
resulting from depleted soil moisture an d
water deficits limited transpiration during th e
summer. He also showed that the availabl e
soil moisture can vary greatly from year t o
year, which can cause considerable seasonal
and annual differences in total transpiration .
Although moderate air movement is commonly
recognized as a factor which increase s
transpiration because boundary layers are reduced,
few studies have been conducted with
high air velocities. Tranquillini (1969) varied
wind velocity from 0 .5 to 20 m sec-1 , and
observed that net assimilation of Pinu s
cembra increased somewhat up to about 4 m
sec-t , then declined . In Picea abies the decline
started at 1 .5 m sec-t . Caldwell (1970), working
with the same Pinus cembra plants ,
showed that stomatal aperture and transpiration
rate were only slightly reduced by hig h
wind speeds, but photosynthesis was reduce d
considerably because of changes in needle display
to the light. High wind would be expected
to be of importance in the Coniferou s
Biome only in extreme habitats .
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