Age- and position-related changes in hydraulic versus mechanical dysfunction of xylem: inferring the design criteria for Douglas-fir wood structure

Abstract

We do not know why trees exhibit changes in wood characteristics as a function of cambial age. In part, the answer may lie in the existence of a tradeoff between hydraulic properties and mechanical support. In conifers, longitudinal tracheids represent 92% of the cells comprising the wood and are involved in both water transport and mechanical support. We used three hydraulic parameters to estimate hydraulic safety factors at several vertical and radial locations in the trunk and branches: vulnerability to cavitation; variation in xylem water potential (psi); and xylem relative water content. The hydraulic safety factors for 12 and 88 percent loss of conductivity (S(H12) and S(H88), representing the hydraulic safety factors for the air entry point and full embolism point, respectively) were determined. We also estimated the mechanical safety factor for maximum tree height and for buckling. We estimated the dimensionless hydraulic and mechanical safety factors for six seedlings (4 years old), six saplings (10 years old) and six mature trees (> 110 years old) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Over the natural range of psi, S(H12) decreased linearly from treetop to a minimum of 0.95 at the tree base. Young and mature trees had S(H12) values 1.4 and 1.3 times higher, respectively, at their tips (juvenile wood) than at their bases (mature wood). Modeling analyses indicated that if trees were made entirely of mature wood, S(H12) at the stem base would be only 0.7. The mechanical safety factor was 1.2 times higher for the base of the tree than for the rest of the tree. The minimum mechanical safety factor-1.6 for the critical buckling height and 2.2 for the critical buckling load-occurred at the base of the live crown. Modeling analysis indicated that if trees were made only of mature wood, these values would increase to 1.7 and 2.3, respectively. Hydraulic safety factors had values that were less than half those for mechanical safety factors, suggesting that wood structure in Douglas-fir has evolved primarily as a result of selection for hydraulic safety rather than mechanical safety. The results suggest that forest managers must consider the role of juvenile wood in tree physiology to avoid producing plantations vulnerable to drought.

Maximum height in a conifer is associated with conflicting requirements for xylem design

“…Vulnerability data reported here are means of curves obtained from four to five samples per sampling height. We also used published values of trunk and branch vulnerability to embolism and aperture conductance (9,46). Regression Analysis.…”

Young conifer stands form a deer browsing refuge for an oak admixture: silvicultural implications for forest regeneration under herbivore pressure

“…Moreover, the height of saplings is determined by age; therefore, the effect of height may be difficult to distinguish from the effect of age in naturally regenerated trees of different age. For instance, wood density influences browsing incidence (Boulanger et al 2009), whereas wood density tends to change with seedling/sapling age (Domec and Gartner 2002). This paper presents data from an 8-year study of browsing incidence in even-aged pine-oak mixtures.…”

Effects of Harvesting Systems and Bole Moisture Loss on Weight Scaling of Douglas-Fir Sawlogs (Pseudotsuga Menziesii var. glauca Franco)

“…The large quantity of water lost suggests that even under alternative seasonal conditions, the effect of harvest system and time between harvest and delivery could prove significant when working in a weight scaling system. Tree age and size can affect wood characteristics and in turn water storage capacity [45].…”