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

Pharis et al. 1967). Again Helms (1970) ha s
studied CO 2 assimilation in ponderosa pine
during the summer in the natural environment,
and envisions extending this study t o
all seasons of the year. Intensive work on CO 2
assimilation of all of the other species named
above from the standpoints of environmenta l
factors and the other influences on their behavior
will be needed for realistic modeling o f
their photosynthetic production .
Water Relationships
Although water deficit was taken up in th e
previous section as a potential limiting facto r
in CO 2 assimilation and respiration, the general
subject of water relationships is of concern
in water balance, nutrient cycling, an d
other soil-plant-atmosphere relations, as wel l
as in its effects on plant metabolism . Also th e
balance between water absorption and transpiration
determines any water deficit in th e
plant. Water relationships of woody plants
were thoroughly reviewed by Polster (1967a) .
Water Absorptio n
The factors affecting water uptake in
woody plants were discussed in detail i n
Kramer and Kozlowski (1960) . In the usuall y
well-drained and well-aerated soils of the
western forests, temperature of the soil an d
roots and the level of soil moisture are th e
principal factors affecting water uptake in th e
root zone. These factors are being regularl y
monitored at the intensive study sites of th e
Biome . Their influence will be of most concern
in the magnitude of water deficits . Sinc e
water uptake is wholly passive during times o f
high water use, increased resistance to uptake
in the root zone can be expected to enhanc e
water deficits . Further, rate of water uptake
and conduction may exert some effect o n
mineral uptake (Slatyer 1967) . Investigation s
of water uptake will probably not be a part o f
the Biome studies, because of the difficultie s
of measuring this quantity in large soil-roote d
plants, and the near equivalence of absorptio n
rates to transpiration rates .
Water Conduction
For woody plants, this aspect has bee n
thoroughly reviewed, including special features
of conifers, by several authors (Hube r
1956, Kramer and Kozlowski 1960, Zimmermann
and Brown 1971) . With reference to th e
objectives of the Coniferous Biome, interes t
in conduction pertains to the magnitude o f
negative sap pressures (as a measure of water
deficit), to its use as an indicator of transpirational
fluctuations where actual transpirational
measurements can not be made readily ,
to intra-tree water adjustments includin g
potential storage in stems and branches, an d
to its influence on rate of translocation of
mineral nutrients . Nonetheless, rates of conduction
will be inferred of necessity for ou r
models from transpirational rates . However ,
some indication of the nature of potential
storage of water in stems or branches may b e
attained from measurements using sap velocity
flow meters and by study of intra-crown
variation in the sap pressure (Scholander technique)
(Waring and Cleary 1967) of stems an d
needles (Hinckley 1971, Ritchie 1971) .
Transpiratio n
The predominant interest in water relation -
ships is with transpiration both from a hydro -
logic viewpoint concerned with total plan t
use, and from its influence on water deficit s
and on resistance to water vapor and CO 2
transfer. This major interest has resulted i n
substantial work on the transpiration of conifers,
and considerable attention to species o f
interest in the Coniferous Biome . Thes e
studies will be considered below under th e
headings of measurement, influence of environmental
factors, effects of leaf temperature
and resistances, and the significance of
water deficits .
Measuremen t
Although there are various methods fo r
measurement of transpiration, in the fiel d
with large plants the air-flow method using
leaf enclosures, lysimeter systems, meteorological
techniques, and tritiated water
methods are most feasible. The air-flo w
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