© 2007 Heron Publishing—Victoria, Canada
Bole water content shows little seasonal variation in century-old Douglas-fir trees
Peter A. Beedlow (1,2), David T. Tingey (1), Ronald S. Waschmann (1), Donald L. Phillips (1) and Mark G. Johnson (1)
1. U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA / 2. Corresponding author (beedlow.peter@epa.gov) / Received March 22, 2006; accepted June 4, 2006; published online February 1, 2007
Summary
Purportedly, large Douglas-fir trees in the American Pacific Northwest use water stored in bole tissues to ameliorate the
effects of seasonal summer drought, the water content of bole tissues being drawn down over the summer months and replenished
during the winter. Continuous monitoring of bole relative water content (RWC) in two 110–120-year-old Douglas-fir trees with
ThetaProbe impedance devices provided an integrated measure of phloem–sapwood water content over 4 years. Seasonal changes
in RWC closely tracked cambial activity and wood formation, but lagged changes in soil water content by 2–3 months. The RWC
in the combined phloem and sapwood markedly increased during earlywood production in the late spring and early summer to maximum
values of 64–77% as plant available soil water (ASW) decreased by ~60%. With transition and latewood formation, RWC decreased
to minimum values of 59–72%, even as ASW increased in the fall. The difference between minimum RWC in the fall and maximum
RWC in midsummer was only ~5%. Seasonal changes in bole RWC corresponded to cambial phenology, although decreasing AWS appeared
to trigger the shift from earlywood to latewood formation.
Keywords:
basal area increment, bole water storage, cambial phenology, relative water content, soil water, summer drought, ThetaProbe,
wood formation.
