Abstract:
The state of Oregon has two distinct climate types bisected by the crest of the Cascade Mountain range. The western side of the Cascades experiences high levels of precipitation and mild temperatures due to the maritime influence of the Pacific Ocean, while the eastern side lies in the rain shadow of the Cascades and is subject to more continental temperatures and arid conditions. Several tree species have subspecies or varieties adapted to the contrasting climates, including Douglas-fir (Pseudostsuga menziesii), ponderosa pine (Pinus ponderosa) and the shore pine and lodgepole pine subspecies of Pinus contorta. These species provide a unique opportunity to study adaptations of tree hydraulic architecture to contrasting climates. The current study has two major foci: the first being an investigation of the water storage and transport properties and resistance to drought-induced xylem dysfunction of ponderosa pine and Douglas-fir from east and west-side populations, the second being an investigation into the sapwood radial water transport properties in east and west-side populations of all three species listed above coupled with an analysis of ray structure and frequency in an attempt to elucidate the radial flow pathway.
Water transport efficiency was measured as specific conductivity (k[subscript 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 and is reported as percent of cumulative acoustic emissions (%UAE) as a function of xylem water potential (Ψ). 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.
Axial k[subscript s] values were not significantly higher in ponderosa pine than Douglas-fir (5.46 and 5.30 kg m⁻¹ s⁻¹ MPa⁻¹ respectively) and axial k[subscript s] was lower from western populations of both species. Resistance to drought-induced embolism expressed as the xylem water potential causing 50% loss of k[subscript s] (Ψ₅₀) was the same in eastern populations (Ψ₅₀=-2.0 MPa for both species) but was significantly higher in western populations of ponderosa pine (Ψ₅₀=-3.26) and Douglas-fir (Ψ₅₀=-2.35 MPa). Axial k[subscript s] showed a strong positive relationship with Ψ₅₀ values across species and populations (r²=0.40, p<0.0001). Capacitance values did not vary significantly between species of west-side populations or east-side populations, but east-side populations were significantly higher than west-side populations. Capacitance showed a strong positive relationship with Ψ₅₀ (r²=0.65, p<0.0001), suggesting that capacitance acts to buffer water column tensions thereby reducing the necessity for reliance on structural adaptations to reduce Ψ₅₀.
Radial k[subscript s] values were highest in west-side populations and generally decreased with distance inward from the cambium, except for west-side ponderosa pine and Douglas-fir in which radial k[subscript s] increased with increasing depth into the sapwood. East-side populations of all species tended to have smaller ray tracheid lumen areas but ray tracheids comprised a larger proportion of total area and were more numerous per area. Ray tracheids were smaller and less numerous in Douglas-fir than in the pines, which had similar lumen areas, although ray tracheids comprised a larger proportion of total area in shore/lodgepole pine. Ray parenchyma lumen areas tended to be larger in west-side populations but ray parenchyma frequency was lower and thus comprised a smaller proportion of total area in east and west-side Douglas-fir. Total ray density was higher in west-side varieties, and correlated with radial conductivity. The strongest correlation was between radial conductivity and number of growth ring boundaries in the sample (p=0.002, r² =0.11). This result suggests that rays may not be as important in radial conductivity as thought, since growth ring boundaries should not affect water movement through ray cells.
