- I studied the water relations traits, twig conductivity, C'3 isotopic composition,
and wood density of three conifer and five angiosperm species in western Oregon. This
study took place from spring 2002 to fall 2003 on four sites. Species were selected to
represent the diversity of drought tolerance of woody plants in western Oregon. The
objectives of this work were to describe the seasonal variation and species' response to
the dry season and to determine suites of functional traits that vary in a similar manner
across species. Pre-dawn (ψpd) and mid-day (ψmd) water potentials decreased
throughout both summers for most species; however, the diurnal change ΔΨ) in water
potential decreased for conifers and increased for angiosperms during the same time.
Quercus garryana and Arbutus menziesii reached the lowest ψmd and the forest species
had the highest values. Stomatal conductance (gw) was highest, and remained high
during the summer, for Q. garryana, consistently lowest for Acer macrophyllum and
Corylus corn uta in the forest, and intermediate for A. menziesii. Osmotic potentials at
full (ψsf) and zero turgor (ψsz) decreased from early to late summer, and increased from
late summer to spring for the four evergreen species sampled. ψsf and ψsz were lowest
for Pseudotsuga menziesii, Q. garryana, and A. menziesii and highest for the forest
species. The index of elasticity (IE) also increased (i.e., tissue elasticity decreased) summer, and continued to increase for most species from late summer to spring.
Throughout the summer, A. macrophyllum and C. cornuta had the most elastic tissue
(low IE), while the more drought resistant species had less elastic tissue. The twig cross-sectional
arealleaf area ratio (Huber Value) was, in general, lowest for the conifers, C.
corn uta, and A. macrophyllum and highest for Q. garryana and A. rnenziesii, and did not
change much with season. The specific conductivity (ks) of twigs was lowest for
conifers, intermediate for C. cornuta and A. menziesii and highest for Q. garryana, A.
menziesii, A. macrophyllum, and Fraxinus latfolia in the mid-summer and changed little
during the summer. The leaf-specific conductivity (k₁) was lowest for conifers,
intermediate for angiosperms in the forest and F. latfolia, and highest for Q. garryana
and A. menziesii. During the summer, there were few significant changes in k₁ for most
of the species. In spring, k₁ was very high for two deciduous species due to their small,
immature leaves. δ¹³C values were lower for forested species than clear-cut species and
indicated that, within each habitat, intrinsic water use efficiency (WUE) was higher for
conifers than for the co-occurring angiosperms. Wood density was lowest for A.
macrophyllum (high percentage of pith resulted in an underestimation of wood density in
this species), highest for Q. garryana, and intermediate for the remaining species.
The eight species in this study shared common relationships between leaf water
potentials, wood density, osmotic potential, cell wall elasticity, and expressions of
conductivity, despite their differences in drought resistance. Among the five
angiosperms, Ψsz and ΔΨ decreased as maximum g increased and the maximum gw
increased as the Huber value, k₁ and tissue elasticity decreased, across species. For
species in the forest and clear-cut, δ¹³C increased with decreasing early summer ψmd and increasing ΔΨ.
Across all species and seasons, Ψmd decreased with decreasing Ψpd;
however, the slope of decrease was greater for ring-porous species than for diffuse porous
or conifer species. Tissue elasticity decreased with decreasing minimum ψsz for all
species except Q. gartyana, which had higher tissue elasticity than would be predicted
based on the other species. Across all species except A. macrophyllum, mean ks and
mean k₁ increased with increasing wood density. Decreasing ψmd, ψsf, and ψsz were
associated with increasing wood density. The maximum ΔΨ increased with increasing k
and k₁, while the minimum ψsz decreased with increasing Huber Value.