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

Table 1 .- Tukey's w Multiple Comparison Test for seasonal "predawn" branc h
water potentials (*fib) showing significant differences at th e
1-percent level. Values underlined are not significantly different.
Date 7/15 9/15 8/24 8/31 8/17 8/22 7/21 7/27 8/ 3
*>!ib 0.8 0.8 1.2 1.0 1 .4 1 .4 1 .4 1 .7 2 . 2
certain plant and environmental changes
which occurred through the summer an d
which decreased daily transpirational wate r
losses are taken into account .
The differences in daily plant water status
which occurred on clear days during the stud y
period would help explain the *0b trend observed
(fig. 1). These differences seemed primarily
due to a general decrease in the atmospheric
demand through the study period (fig .
2) . Generally, higher average Ta's and VPD' s
and higher PE's occurred earlier in the summer,
despite higher SM levels . A higher evaporative
demand produced higher average HPV' s
and greater water loss, thus developing lowe r
branch water potentials . The later part of
August was marked by relatively lower Ta's ,
VPD's and PE's, even though SM had de -
creased. Hence, lower transpirational losse s
occurred and higher 4/b developed . Durin g
this period, a general increase in diffusion resistance
was also evident .
It is also possible that changes in leaf morphology
during the summer could have helpe d
produce a trend of increasing *fib during a
period of continually decreasing SM . Chlorosi s
in leaves, leading to necrosis before abscission ,
has been shown to be induced by drought ,
vascular disease, and normal senescence in
woody and herbaceous plants (Kramer and
Kozlowski 1960, Talboys 1968) . Such manifestations,
characteristic of "drought hardening"
in plants, can produce decreased transpiration
and photosynthesis rates due to persistent
stomatal closure even while leaves are fully
turgid (Talboys 1968, Turner 1969) . By the
middle of August, the leaves of the vine mapl e
clump investigated had developed numerou s
chlorotic spots; necrosis became evident during
September. Therefore, increasing *1Pb during
August could have been influenced by th e
systematic reduction in transpiration in portions
of the leaves .
Conclusion
The diurnal patterns of water status in vin e
maple were examined for clear days during
the summer of 1971 . As expected, the seasonal
pattern of plant water status was de -
pendent upon a combination of certain soil ,
plant, and atmospheric factors which affecte d
water uptake and loss by the plants . Soi l
water potential, which steadily decreased during
the rainless periods, should have progressively
limited water availability and resulte d
in the development of lower "predawn" plan t
water potentials. However, during a continuous
drying cycle, changes in certain plant an d
atmospheric factors seemed to influenc e
water loss greatly . Early in the summer ,
greater water losses occurred due to highe r
evaporative demands, and the plants were
unable to recharge fully during the night eve n
though adequate soil moisture was probably
available . The development of chlorotic spots
suggested that during a prolonged drought, o r
as the leaves approached abscission, the ability
of the entire clump to transpire decreased .
Also, as the atmospheric demand lessened
later in the summer, less water loss occurre d
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