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

I
- 5
-l o
-1 5
HPV
1
10 . 0
I . 6
E
U
7 . 5
5 . 0
1 . 2
0 . 8
2 . 5
.
0 . 4
3 0
Ta
VPO
I 1 L
3 0
2 0
10
1 0
- I
0800 1200 1600 0800 1200 1600 0800 1200 600
Time of Day (PST )
Figure 1 . Daylight patterns of branch water potential (lib), heat pulse velocity (HPV), leaf resistance (RL), ai r
temperature (Ta), and atmospheric vapor pressure deficit (VPD) for July 21, August 22, and September 22 ,
1971, at the Thompson Research Center, Washington .
tological records of the U .S . Weather Bureau
Station at Landsburg, Washington, located
about 3 km west of the Research Center a t
approximately the same elevation .
Results and Discussion
Water status and environment data examined
in this paper represent typical data collected
on clear days during the summer of
1971 . The days presented as examples are
July 21, August 24, and September 22 (fig .
1) . An expected overall pattern for plant
water status on these days is discernible in the
results. During the early morning "predawn,"
the air was cool and fairly humid ; RL was
high suggesting that stomata were relatively
closed ; 'b was high while HPV was quite low .
As the day progressed, Ta increased, thus in -
creasing VPD and intensifying the evaporative
demand . As stomata began to open, probabl y
in response to increasing light levels, R L
values began to decrease . Since VPD had in -
creased and stomata had begun to open, th e
tree began to transpire causing an increase i n
HPV readings. Continued water loss resulted
in a decrease in t/b until about midday . In th e
afternoon, RL increased because of stomatal
closure probably due to decreasing lib . With
this came a corresponding decrease in HP V
and an increase in branch water potential .
The summer was marked by an abnormall y
wet June, with more than 10 cm of precipitation
having fallen. A rainless period followe d
during most of July and August ; only on 1
day, August 21, did P surpass PE (fig . 2). Th e
drought ended with a rainy period during th e
first 2 weeks of September . This was followed
by 2 weeks without rain before another stor m
occurred near the end of September . The dry
period during July and August produced a
26 7