Objective 8

Objective 8. Determine from wood anatomy, additional constraints on tree, stand, and climate characteristics.

From the isotope studies and stand reconstructions, we can likely assemble a fairly detailed understanding of climate and the major characteristics of the stand. Better yet, the unusual preservation of the Eocene plant remains allows detailed measurements of anatomical characteristics which can further constrain some of our interpretations, and which may yield interesting insights when compared with the anatomy of NLR's. We suggest that the following are useful measurements:

These are indices of how much of the stem of the tree supports branches with live foliage. As such, they provide an independent check on the reconstructions made from field samples. Fibril angle (as an indicator of juvenile vs. mature wood) of longitudinal tracheids is a measure of the closeness of the live crown to cambial initials (Romberger et al. 1993, Panshin and de Zeeuw 1980). Large fibril angles for outer growth rings near the base of a tree indicate a long live crown and wide tree spacing, or moderate tree spacing and extreme shade tolerance. When combined with the morphology of dissected stems to determine the distribution of embedded knots, surface knots, and the spacing of stems in the plots, inferences about shade tolerance can be made. It seems possible that uninterrupted light might change this relationship (continuous light might stimulate greater auxin production leading to higher levels at the base of the tree), so comparison to modern Metasequoia would be useful in constraining interpretations of shade tolerance of the Eocene trees.

In modern temperate-region conifers, earlywood tracheid diameters reflect turgor pressure achieved and auxin availability in cambial initials. Typically, temperate region conifers show a rapid increase in earlywood tracheid diameter in spring, followed by a reduction in diameter by mid or late summer, coinciding with a reduction in soil moisture (Larson 1963, Jagels et al. 1994). Preliminary investigation of tracheid diameter along radial files indicates larger tracheid diameters in the middle of the growing season suggesting that the fossil Metasequoia may have had maximum moisture availability in mid summer. Alternatively, this may be a signal of light-controlled auxin transport. While the best way to distinguish between these possibilities is through controlled experiments, comparison of tracheid dimensions with the d ¹³C isotopic signatures of early, middle and late wood would provide useful guidance in interpreting seasonal moisture availability and moisture stress (see objective 2, above).

There appears to be many stems that are nearly intact which will be exposed by excavation, but precise heights may be difficult to determine from the excavated specimens since the uppermost portion of the trees often appear decayed, or broken and scattered. This will cause uncertainty in stem dimension reconstruction efforts. Tracheid length and maximum diameter are positively correlated with tree height across genera, and to some extent, within genera (Panshin and de Zeeuw 1980). We propose to measure these in the three outer rings of NLR's of varying heights to see if there are within-genus relationships that would be useful for reconstructing tree heights. Tracheid dimensions in Eocene trees of similar diameter growth rates (very slow growing trees have shorter tracheids) can then be used to estimate tree height for comparison with reconstructions of recovered specimens.

Ray volume is a measure of stored carbohydrate needed when trees begin growth in the spring. In northern temperate regions deciduous conifers have higher ray volume than co-occurring evergreen conifers (e.g., Larix sp. 10-11%; var 2.1% vs. Picea rubens 4.9% var. 2-4%; Pinus strobus 5.4% var. 0.4%; Thuja occidentalis 3.4% var. 0.6%; Panshin and de Zeeuw 1980). In contrast, in warm temperate regions, deciduous and evergreen conifers do not differ in ray volume (e.g., Taxodium distichum 6.6% var. 2.6%; Sequoia sempervirens 7.9% var. 2.5%; Pinus taeda 7.6% var. 1.6%; Panshin and de Zeeuw 1980). It is worth exploring whether ray volumes in modern Metasequoia vary in response to temperature regime, and if this parameter can be used as another estimator of the Eocene climate of Axel Heiberg Island. Based on the relationship observed in modern conifers, we expect that ray volumes of the Eocene deciduous and evergreen conifers should be similar if they grew in a warm temperate climate.