Abstract |
- In deciduous species, water exits stems mainly through leaf traces
attached to the outer growth ring and yet we know that water ascends
throughout the entire cross-section of the sapwood. There is an increasing
amount of information on sap flow and sapwood hydraulic properties from
separate studies, but little information exists on how flow and hydraulics vary
radially along the sapwood within a tree. In this study, we determined axial sap
flux (3) and axial specific conductivity (ks) at five different radial locations within
the sapwood, spedfically at 1.2, 2.0, 3.5, 5.0 and 7.0 cm inward from the
cambium, and used these values to estimate the tension gradient in the water
column at those radial positions. Four hardwood species growing in the Pacific
Northwest were used for this study: Acer macrophyllum Pursh (bigleaf maple,
n=8), Alnus rubra Bong. (red alder, n=8), Populus trichocarpa Torr. & Gray x
Populus deltoides Bartr. ex Marsh. (cottonwood, n=4) and Arbutus menziesii
Pursh (Pacific madrone, n=7). Field and laboratory study were performed during
2003 and 2004, and values were pooled for these two years. Sap flux decreased
from the outer to the inner part of the sapwood in all species. Arbutus, followed
by Acer, showed the highest differences within all analyzed depths. Alnus had a
slight declining sap flux profile along the first three depths, with a marked
decline from 10.3 g m2 s1 to 5.7 g m2 s1 in the two inner positions. Populus
showed the highest sap flux values, averaging 32.1 g m2 s1 in the two outer
depths and these values fluctuated as 23.7 g m2 s', 26.3 g m2 s1 and 23.9 g m
2 1, along the radius. Specific conductivity showed a similar radial pattern in
Alnus, Populus and Arbutus, declining from the outer sapwood inward. Of
these species, Alnus had the most uniform profile, decreasing, on average, by
l2% at each depth. In Arbutus and Populus, k had quite high variance, and
exhibited the lowest values in the inner depths. In contrast, Acer k5 fluctuated
from the outer to the inner sapwood. In general, there was a relationship
between J and k5 measured at the same locations for Alnus, Arbutus, and
Populus. Arbutus had the most significant linear relationship between J and k
at all analyzed depths, with r2= 0.93, followed by Alnus and Populus (r2= 0.79).
Acer showed a fluctuating pattern, with a random J/k relationship by depth (r2
< 0.001). Axial tension gradient appeared to be constant across the sapwood
radius in Alnus and Arbutus, but it was non-uniform in the inner positions in
Acer and Populus. Gradients remained constant in Acer at the first two depths
then declined toward the 3.5 cm position, where the highest tension values
occurred at the 5.0 cm position. There was a marked increase in tension gradient
toward the inner depths in Populus. The observed Jand k5 patterns along the
sapwood also demonstrated the resistance to water flux in the radial direction.
This demonstrates, in the studied species, the existence of different resistances
to water flux in the radial direction. This tension gradient along the sapwood
demonstrates the presence of measurable resistance to water flow in the radial direction. This resistance may also affect wood utilization and processing,
especially when dealing with drying and treating wood. This study also showed
the importance of accurately estimating J along the entire sapwood in order to
estimate whole-tree water use.
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