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Continued
from:
Estimated
Ages of Some Large Giant Sequoias:
General Sherman Keeps Getting Younger
Nathan
L. Stephenson, Ph.D
Methods
and data
Choice of individual giant sequoias for analysis
The primary criteria for choosing individual sequoias for analysis were (1)
the sequoias were among the largest known, and (2) the cores and other data
needed for age estimation were already available (that is, no sequoia was
to be cored solely for the purpose of this study). Specifically, for a given
sequoia to be included, original increment cores or the necessary measurements
from those cores had to be available, along with measurements of the tree's
bark thickness and diameter at the height at which the cores were taken. These
data requirements limited the pool of sequoias available for analysis. While
many large sequoias have been cored for studies of human impacts, ring-width
chronology development, climatic reconstructions, forest dynamics studies,
and fire history reconstruction, only a limited subset of those sequoias have
associated records of diameter at core height. Diameter at core height is
essential for age estimation, and cannot be estimated readily from published
diameters at breast height (DBH: the standard for diamater measurements)
of individual sequoias. Cores are rarely taken exactly at breast height, and
sequoia bole diameter usually changes rapidly with increasing distance from
breast height.
The following seven large sequoias were selected for analysis (Table 1). The General Sherman, Washington, and General Grant trees are the world's three largest trees, with the General Sherman and General Grant trees being among the most heavily visited of all sequoias. The Boole tree is the seventh largest, and is well-known as being the largest sequoia on lands managed by the U.S. Forest Service. The Grizzly Giant is heavily visited because of its craggy appearance and status as one of the two largest sequoias in Yosemite National Park, whereas the Cleveland tree is a lesser-known and seldom-visited tree in Sequoia National Park. Finally, the Sentinel tree is a well-known sequoia beside the road at the southern entrance to Giant Forest in Sequoia National Park.
The General Sherman,
Washington, General Grant, Grizzly Giant, and Cleveland trees all were cored
by R. J. Hartesveldt and his colleagues for various studies during the late
1950s and early 1960s. All cores and data sheets for these trees are archived
at Sequoia National Park, except I was unable to locate the original core
for the Washington tree, and therefore relied exclusively on Hartesveldt's
ring measurements for that tree. The Boole tree was cored by researchers from
the University of Arizona in 1992. Finally, the Sentinel tree was cored by
V. G. Pile and me in 1998 at the request of National Park Service staff, who
wished to have an age estimate for displays near the tree.
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President
Theodore Roosevelt's party at the Grizzly Giant, Mariposa Big Tree Grove,
1903. Front center are Roosevelt and John Muir. |
Estimating
tree ages
I estimated ages of these seven sequoias using an equation
(refer to link for a detailed description) that combines knowledge of tree
size with information gained from partial increment cores. When tested on
231 sequoia stumps up to 3200 years old and 6.5 m in diameter, this approach
gave age estimates that were within 10% of actual age 62% of the time, and
within 25% of actual age 98% of the time, assuming that two 60-cm increment
cores are available for analysis; fewer or shorter cores gave less precise
estimates. This level of precision is a substantial improvement over that
of previously published methods, which estimated tree age from diameter alone,
by assuming that basal area increment is constant through time, or by linear
extrapolation of growth rates from the innermost portion of an increment core.
Confidence
intervals
Previous work of mine showed that as both the number and length of increment
cores increase, confidence in sequoia age estimates also increases (Table
2). However, the numbers and lengths of cores used did not always fall neatly
into the categories in Table 2. To determine confidence intervals, core lengths
were therefore rounded to the nearest category shown in Table 2 (either 30
or 60 cm). In two cases (the General Sherman and General Grant trees), three
cores rather than two were used. However, since confidence is improved relatively
little by increasing core number (it is improved more by increasing core length;
Table 2), confidence intervals for only two cores were used.
The number of years elapsed between the year in which a tree was cored and the year 2000 was then added to the endpoints of the tree's confidence intervals, as was the estimated number of years it took each sequoia to grow to the height at which it was cored. Admittedly, the latter step does not change a sequoia's age confidence intervals to reflect the uncertainty associated with estimating the number of years it took a sequoia to grow to the height cored. However, uncertainty added at this stage is small compared to the uncertainty of estimating the tree's age at core height.
Statistics
on the longest-lived sequoia known
As a yardstick for interpreting results, I used the age and size of the longest-lived
sequoia known -- a cut stump in Converse Basin, Giant Sequoia National Monument,
designated CBR26 by its discoverers (R. Touchan and E. Wright of the University
of Arizona's Laboratory of Tree-Ring Research). Touchan has precisely crossdated
3207 rings on the stump. It is missing much of its sapwood, so the outermost
ring dates to 1834. However, the extensive logging of Converse Basin Grove
occurred between 1893 and 1908. Thus, at least 59 years of sapwood are missing,
and the tree therefore was at least 3266 years old when it was cut. (It is
unlikely that the tree exceeded 3290 years old, including the time it took
the tree to grow to the height sampled by Touchan and Wright.) The stump is
relatively small: 5.8 m in diameter near ground level and 4.3 m in diameter
at the cut surface 2.2 m above ground level. Even with sapwood and bark intact,
the tree's diameter at 2.2 m above ground level was probably less than 5 m
when it was cut, much smaller than any of the trees analyzed here (Table 3).
While we will never know the volume of the living CBR26, it is clear that
many hundreds of sequoias alive today (probably well over one thousand) are
larger than CBR26 was before it was cut.
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