Coordinated adjustments in Douglas-fir and ponderosa pine xylem efficiency, safety, and capacitance along a gradient of increasing aridity in Oregon

Background/Question/Methods

In the Pacific Northwest, the east side of the Cascade Mountain range has less precipitation and a more continental climate than does the west side.  We studied water storage, transport, drought resistance, and anatomical properties of trunkwood of mature trees of two coniferous species in populations from both the east and the west side of the Cascades.  Water transport efficiency was measured as specific conductivity (k_s, kg m^-1 s^-1 MPa^-1) in both axial and radial directions at three different sapwood depths; intrinsic water storage capacity was expressed as sapwood capacitance derived from sapwood moisture release curves and normalized on a sapwood volume basis (kg H₂O m^-3 MPa^-1).  Resistance to drought-induced xylem dysfunction was determined through acoustic emissions testing. Fluorescent microscopy was used to determine ray tracheid and ray parenchyma lumen areas, and a tree ring microscope was used to determine earlywood, latewood and growth ring boundary properties from the samples used to collect radial conductivity measurements. Sapwood width, capacitance, vulnerability to embolism, and axial and radial hydraulic conductivity were measured on 12-mm diameter cores from two stands per species in both the east and the west sides.

Results/Conclusions

Ponderosa pine (Pinus ponderosa) trees had 3-4 times wider sapwood than did Douglas-fir (Pseudotsuga menziesii) trees of similar diameter.  The west-side populations of both species had higher radial conductivity than did the east-side populations. There were very few correlations of ray characteristics (ray frequency, composition of parenchyma vs. ray tracheids, sizes of ray tracheids) with radial conductivity, suggesting that rays do not facilitate radial water movement in the living trees.  As expected, axial conductivity was higher in the east-side populations, and it was also higher in Douglas-fir than in ponderosa pine. Unexpectedly, sapwood was more vulnerable to embolism in the eastern populations of both species, and sapwood capacitance was about twice as great in east-side (about 580 kg m^-3 MPa^-1) as in west-side populations (about 274 kg m^-3 MPa^-1) for both species. Capacitance was positively correlated with mean embolism pressure and axial conductivity across species and populations.  This result suggests that coordinated adjustments in xylem efficiency, safety and water storage capacity may serve to avoid embolism along a gradient of increasing aridity.