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Objective 5 |
Objective 5. Determine the Evolution of
Stand Architecture. Longitudinal sections of tree stems
contain important evidence about the growth, dynamics, architecture and
history of injury (Duff and Nolan 1953, 1957; Farrar 1961; Myers 1963; Larson
1963, 1965; Fayle and Bentley 1989; Leblanc 1990). The stubs of pruned
branches buried in newer wood (e.g. embedded knots) record the architecture
and branching pattern of the tree as it grew, so the evolving architecture of
a stand can be qualitatively determined. Using allometric equations developed
from modern Metasequoia, branch length, branch weight and foliar
weight per branch can be determined as a function of the diameter of the
branch where it joins the stem (Vann et al. 1998), so it is possible
to reconstruct how the sampled trees looked at different stages of their life
history, and make interpretations of the nature of the stand at different
times during its development. One large Metasequoia log (70 cm
diameter, 5 m of excavated length) was sectioned to determine how much information
was preserved in the stem (Figure. 8, Figure 9). It had
branches along the length of the stem which were later self-pruned resulting
in embedded knots (Fig.
9). This is characteristic of even-aged stand development, where the
earliest cohort grows in a dense stand of bushy saplings, which thins through
competition. As the canopy closes, lower branches are too shaded to have a
positive carbon balance, and they are "pruned" by the tree. Smaller
Metasequoia stems in the same level (<20 cm) do not have branches
along their full length which suggests that Metasequoia was
reproducing in the stand, and that it was shade tolerant. As already noted, the preservation of
stems and branches of the upper crown may not be sufficient to gain a full
understanding of the architecture of the upper canopy, but useful
longitudinal sections of the stem should be obtainable to diameters of ca. 10
cm, which is likely to be >80% of the height of the larger trees. For now, the specific hypotheses guiding
our efforts need to be rather general, as we are in the initial stage of
exploring a large and variable ecosystem. Based on our experience to date, we
expect the stand reconstructions will show that: Hypothesis 1. Swamps and floodplains were occupied by mature
forests which are closed canopy, Metasequoia-dominated, >25 m in
height, with basal areas (at breast height) >100 m2 ha-1.
These will be high-biomass forests by modern standards. We expect that at
least some of the mature forest studied will be even aged. That is, the
oldest and largest trees will be the same age. Younger trees which did not
have any portion of their crowns in the upper canopy will be the same species
as the older trees, indicating shade tolerance.
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