Wood density and hydraulic properties of ponderosa pine from the Willamette valley vs. the Cascade Mountains

The Willamette Valley (WV) race of ponderosa pine (Pinus ponderosa) is being widely planted for timber in the Willamette Valley, western Oregon, because it grows in habitats that are either too wet or too dry for Douglas-fir (Pseudotsuga menziesii). Compared to the eastern Cascade Mountains (CM). the WV has 3 to 5 times the annual precipitation and warmer temperatures year around. This study characterized the wood quality of the WV race (4 sites) and the CM (4 sites), and also compared the behavior of their wood for water transport for the living trees (1 site in the WV and 1 site in the CM). The average tree ages at the sites ranged from 30 to 83 years at breast height. Between rings 27 and 31, compared to the CM, the WV had denser wood (0.48 vs. 0.40 g/cm3), denser earlywood (0.41 vs. 0.36 g/cm3), and denser latewood (0.62 vs. 0.50 g/cm3), with no significant differences in mean latewood proportion (about 0.35) or mean growth ring width (about 2.5 mm). The pith-to-bark trend in density differed between regions. In the WV, total wood density, earlywood density, and latewood density increased with growth ring from the pith. In the CM, total wood density and latewood density decreased slightly with growth ring width, and earlywood density remained unchanged. An additional sample of younger trees (23 years at breast height) from a genetic trial in the WV in which the seed source was the CM, had low density wood in the first few rings (like the CM trees) but had a steady increase in wood density with growth ring number (like the WV trees). Specific conductivity (ks) of trunk wood was lower in the WV, consistent with its higher wood density and suggestive that the WV race is more drought-adapted than the CM populations. There was no decline in ks from outer to inner applied air pressure of 3.0 MPa, the WV and CM trees had lost 19% and 32% of their ks, respectively, again suggesting that the WV trees are slightly more drought-adapted than are the CM trees. At the other applied air pressures tested (0.5, 2.0, 3.0, and 5.0 MPa), there were no significant differences in loss of conductivity between the two sites. Trunk wood from breast height had a 50% loss of ks at 3.3-3.6 MPa. The loss of relative water content (100% - RWC) was about the same in both sites, except at 4.0 MPa, in which the CM trees had a larger loss of RWC than the WV trees. More work is needed on physiology to better understand the wood density/water transport relations. Ponderosa pine may be more interesting to study than other species because the earlywood, which transports most of the water, shows substantial density differences between geographic regions.

Duke Scholars

Author Jean Christophe Domec Environmental Sciences and Policy