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

1 5
FIGURE 2
1 0
18 0
FIGURE 3
u
160
ILO
-,
- 0
= 120
I00
SM
- 3 . 0
July 6 July 26
Aug 1 5
Time of year (1971 )
Sept L Sept 2 L
Figure 2 . Precipitation minus potential evaporation (P-PE) calculated by the Thornthwaite method for July ,
August, and September 1971 at the Thompson Research Center, Washington .
Figure 3 . Soil moisture (SM) in the top 90 cm of soil calculated by the Thornthwaite method and "predawn "
branch water potential (*1y ) ) for July, August, and September 1971 at the Thompson Research Center ,
Washington .
steady drying of the soil from field capacit y
to a water potential well below -1 .0 bar (fig .
3). The rain in early September recharged th e
soil to a point near field capacity . For the top
90 cm of this soil, field capacity is near 17 . 7
cm of water while the -1 .0 bar point is abou t
14.2 cm (Knutsen 1965) .
Since plants obtain water primarily fro m
the soil and lose water to the atmosphere ,
their internal water status will depend greatly
upon the SM supply and the atmospheric demand.
The relationship of plant water status
to SM is illustrated by examining "predawn "
branch water potential (*fib) periodically
throughout the study period . These measurements
were taken just prior to sunrise whe n
'b is usually the highest of the day and
should most accurately reflect soil water po -
tential (Slayter 1967, Klepper 1968, Waring
1969). Therefore, a continuous decrease in
*lPb should have occurred during the rainles s
period in July and August because of the
steadily decreasing SM (i .e ., decreasing soil and
water potential) during this period . Quite the
opposite was observed (fig . 3) . During the
period, "predawn" branch water potential decreased
steadily to a minimum on August 3
and then continually increased through
August . This was surprising as *fib seemed to
respond quickly to any change in SM, as illustrated
by the immediate decrease in I1b during
the temporary rainless period in September.
Also, maximal and minimal points withi n
this general trend proved to be significantl y
different at the 1-percent level (table 1) . The
apparent anomaly is probably explainable i f
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