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

solve equation 2 for Tm since the flow rat e
(F) is known . The values of Tm obtained b y
this direct method were compared to th e
values obtained by the three indirect method s
which were previously discussed .
The nominal mean residence time (Tm) ha s
been computed for several harvested field -
grown coniferous trees and the values compared
with those obtained from curve slopes ,
from first moment calculations and from peak
arrival times . The results show in general, that
Tm > TS for slope calculations and Tm <
Tm * for first moment calculations .
The nominal mean residence time agree s
most closely with Tp which was obtained b y
the midpeak or peak arrival time method .
2 0
I 0
o ► 1 I I_ I 1
0 10 20 30 40 50 6000 110 120 13 0
Tm(hr )
Figure 2 . Relationship between nominal mean residence
time (Tm) of plant water and mean residence
time obtained by peak arrival (Tp) fro m
activity-time curves .
Figure 2 shows the relationship between T m
and Tp for a group of harvested trees . Th e
slope of the linear least squares relationshi p
between the two estimates is 0 .916, and the
intercept is 7 .05 as compared with the expected
slope of one and expected intercept of
zero (1 :1 line, fig. 2). The coefficient o f
determination (r2 ) is 0.70 for the relationship.
The results suggest that Tp is an unbiased
estimator of Tm . This confirms that Tp
is the best estimate available for Tm since
other known possibilities have been examined
and found to be biased either above or belo w
the true values .
The scatter of data points on figure 2 indicates
that forest biomass determinations must
C
at present be approached statistically . Any
particular determination of mean residenc e
time of water may be substantially in error ;
however, a group of determinations appear s
to converge on the true Tm values of th e
group. Experimental error reduction is on e
objective of continuing studies in the Coniferous
Biome program .
Table 1 shows a comparison between tre e
biomass as computed by equation 2 an d
actual harvested weights. These data were obtained
in a uniform age plantation of red pin e
(Pinus resinosa) in 1970, and a similar stan d
of jack pine (Pinus banksiana) in 1971. Th e
results show individual examples of substantial
error; however, the estimates of mean tre e
biomass and mean forest biomass as measured
by the tritium method agree well with thos e
estimated by direct harvest . There is, of
course, no necessity for determining biomas s
of a group of trees on all individuals simultaneously
. Biomass can be determined at an y
time during which transpiration flow is takin g
place. The results suggest that the biomass of
these forests could have been reliably determined
by the tritium method alone .
Conclusions
Experiments designed to measure transpiration
rates in field-grown trees may also b e
used with little extra data collection for non -
destructively measuring tree biomass . The tritium
method can, in general, be used only for
determination of biomass which is actually
transmitting water . Flowers, fruits, bark, and
nonconducting heartwood are not included i n
the estimate. We have proposed a method for
including heartwood in the estimate but have
not yet proved it experimentally .
The most difficult parameter to obtain for
calculation of biomass is the nominal mea n
residence time of water in the plant . After
examining several alternatives for obtainin g
this quantity, we conclude from theory and
experiment that the time of arrival of pea k
tritium activity in tree foliage is the most reliable
estimate of nominal mean residence time .
Estimates of biomass of field-grown coniferous
trees show considerable statistical varia -
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