Abstract
The cohesion-tension theory of water transport states that hydrogen
bonds hold water molecules together and that they are pulled through
the xylem under tension. This tension could cause transport failure in
at least two ways: collapse of the conduit walls (implosion), or rupture
of the water column through air-seeding. The objective of this research
was to elucidate the functional significance of variations in tracheid
anatomical features, earlywood to latewood ratios and wood densities
with position in young and old Douglas-fir and ponderosa pine trees in
terms of their consequences for the safety factors for tracheid implosion
and air-seeding. For both species, wood density increased linearly
with percent latewood for root, trunk and branch samples. However, the
relationships between anatomy and hydraulic function in trunks differed
from those in roots and branches. In roots and branches increased hydraulic
efficiency was achieved at the cost of increased vulnerability to
air-seeding. Mature wood of trunks had earlywood with wide tracheids
that optimized water transport and had a high percentage of latewood
that optimized structural support. Juvenile wood had higher resistance
to air-seeding and cell wall implosion. The two safety factors followed
similar axial trends from roots to terminal branches and were similar for
both species studied and between juvenile and mature wood.
