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

net assimilation in excised branches fro m
24-year-old Douglas-fir trees which had been
fertilized with N . However, the increases wer e
noted only at light intensities of 2,000 ft- c
and above .
Diurnal and Seasonal Pattern s
(including adaptation )
Diurnal cycles or fluctuations in light, temperature,
water vapor pressure gradients, an d
other external factors are interactive with
internal factors such as endogenous rhythm o f
stomatal behavior, recovery from water deficit ,
and translocation. Therefore the response of a
tree day by day is very complex .
Gentle (1963) and Helms (1965) studie d
diurnal responses of 38-year-old Douglas-fir i n
detail over a 4-year period . Superimposed o n
the daily responses to light and darkness were
short-term and longer term fluctuations attributable
to varying temperature, radiation and
cloud cover, relative humidity, water deficit,
and undetermined influences . Midday depressions
were common in the summer but also
occurred in cool autumn weather . Similar detailed
studies of diurnal behavior of the other
species important in the Biome are lacking .
The diurnal endogenous changes in stomatal
aperture have been studied in several
species. Fry (1965) noted that saplings grow n
from seed gathered at Pack Demonstratio n
Forest, La Grande, Washington, did no t
exhibit any closing of the stomata during th e
day or night if water status was favorable .
This was largely confirmed by Reed (1968) ,
although he observed a slight closing tendenc y
at night. Phillips (1967) studied 38-year-old
Douglas-fir trees at Pack Demonstration Forest
and found that the stomata of leaves i n
the lower crown closed at night. A geographic
or provenance difference in stomatal response
of Douglas-fir was demonstrated in the field
in southern Oregon, where stomata were open
at night in the spring and early summer, but
closed at night from July through September
(Reed 1972). Drew, Drew, and Fritts (see
footnote 4) and Lopushinsky (1969) independently
observed that the stomata of Pinus
ponderosa are closed at night . Abies amabilis
growing in the field was observed to hav e
more open stomata in early morning than did
Abies procera, indicating that the stomata of
A. amabilis were probably more open durin g
the night (Hinckley 1971). The stomatal
behavior of western hemlock is unknown an d
needs attention .
The influence of season of the year ha s
already been touched on in connection with
winter depression of assimilation . In a more
general context, seasonal patterns of CO 2
assimilation were studied by Gentle (1963)
and by Helms (1964, 1965) in the 38-year-ol d
Douglas-fir stand mentioned above . As would
be expected, winter assimilation was low and
quite variable, depending on the light an d
temperature prevailing. Spring and autum n
performances were good, although somewha t
below the summer . Higher temperatures presumably
made respiration a good deal highe r
in the summer than in spring and autumn and .
particularly than in the winter . Also Wood -
man (1968, 1.971) studied net assimilation in
one of the trees of this lame stand during th e
growing season of 1967. A somewhat different
pattern can be expected in conifers
growing in regions with very dry summers ,
with much Of the yearly total of photosynthesis
occurring during the winter and spring .
Such studies of seasonal patterns are imperative
for adequate models of production in
forest stands . This is particularly true becaus e
a considerable number of variables in addition
to those of the physical environment have
marked influences (Kozlowski and, Kelle r
1966). These include morphological variatio n
(sun and shade foliage, stomatal distribution ,
cuticular thickness, and others), adaptatio n
with respect to light intensity and temperature
in par icular, interspecific and intraspecific
variations, shading, dormancy, age of
foliage, position in the crown, and the natur e
of past seasons . Some information on these
features is available for Douglas-fir. However
very limited knowledge, gained mostly fro m
studies with seedlings, is available in this connection
for the other species of special concern
in the Coniferous Biome programwestern
hemlock, ponderosa pine, and one of
the true firs l Hodges and Scott 1968, Kruege r
and Ruth 1969, Ludlow and Jarvis 1971b t
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