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Objective 10 |
Objective 10. Determining Biomass
Accumulation Rate. Figures 11 and 12 show the results
of a global-scale analysis of forest biomass accumulation rates (Johnson et
al., in review). Across a wide range of conditions, accumulated
temperature during the growing season is a good predictor of stand biomass.
We are certain that at least one of the stands we propose to excavate is
even-aged, and that its age can be accurately determined from ring counts. It
appears to be about 20-25 y old. At least one of the other three stands
appears to be "ageable"-- that is, the earliest cohort of trees
have the same age, and the live biomass of the stand accumulated over the
interval represented by the oldest cohort. This floodplain forest is nearly
monospecific, with many large Metasequoia stems of similar diameter
(ca. 70 cm) and age (ca. 90 y) which makes it reasonable to do the
calculations shown in Figures 11 and 12. We will attempt
to use the isotope-derived estimates for light- and light/dark-season
temperatures (assuming they are reliable) and the age of the stands
determined from longitudinal sections and stumps of the oldest cohort in
even-aged stands to determine accumulated temperatures (growing season degree
years, x-axis in Figures 11 and 12). Wood volume
will be estimated from stump diameters and the allometric equations
determined from trees on each of the four sites. Eocene wood density will be
assumed to be the same as in the NLR's. Foliar and branch biomass (<10%
the total forest biomass) will be estimated from allometric relationships
derived from modern Metasequoia, Glyptostrobus, etc. Above
ground biomass (wood + foliage) can then be compared with the data in Figures
11 and 12 to determine how
quickly those polar forests accumulated biomass under light and CO2
conditions that differ markedly from those experienced by any modern forests.
The high density of large-diameter stems at this locale (ca. 1000 stems/ha)
and their rapid radial growth leads us to speculate that the rate of biomass
accumulation in this Eocene forest exceeds that of any of the forests in
Figures 11-12. One problem to be resolved is that it is
difficult to determine how many of the existing stumps represented dead trees
vs. live trees. Note that Figures 11 and 12 show measures of
live aboveground biomass. There are constraints on the density of live stems
and their size to which virtually all modern monospecific stands conform, and
this can be used to constrain the percentage of live trees in the stands we
sample. This is known as the "negative 3/2's power law" (sensu
Perry 1994). Reasonably well-constrained biomass accumulation
rates from the High Arctic region of the CO2-enriched Eocene would
be a starting point for physiological investigations aimed at providing
insights into how forests respond (or don't respond) to this light regime and
an atmosphere enriched in CO2. |
