© 2004 Heron Publishing—Victoria, Canada
Gas in stems: abundance and potential consequences for tree biomechanics
Barbara L. Gartner (1, 2), John R. Moore (3, 4) and Barry A. Gardiner (5)
1. Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331, USA / 2. Corresponding author (barbara.gartner@orst.edu) / 3. Department of Forest Resources, Oregon State University, Corvallis, OR 97331, USA / 4. Forest Research, P.O. Box 29237, Christchurch, New Zealand / 5. Forestry Commission, Northern Research Station, Roslin, Midlothian, Scotland EH25 9SY, U.K. / Received March 20, 2003; accepted May 23, 2004; published online September 1, 2004
Summary
Secondary xylem of woody plants has a large volumetric proportion of gas occupying spaces that would otherwise be filled with
water. We examined whether these gas-filled voids have a mechanical role by either decreasing the fresh mass the tree must
support (by replacing some of the water with gas) or by providing inexpensive filler to increase stem diameter (thereby increasing
the second moment of area at the expense of the modulus of elasticity and modulus of rupture). Calculations from published
data show that temperate softwood species (n = 26) average 18 and 50% gas by volume for sapwood and heartwood, respectively; temperate hardwood species (n = 31) average 26% gas by volume in both the sapwood and heartwood; and tropical species (n = 52) with mixed sapwood and heartwood have 18% gas by volume. In this paper, we develop equations to show how gas affects
the mechanical behavior of tree stems, and describe model results to show how gas affects mechanical stability, based on mass
and stem diameters for six 34-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. For the same applied load, modeled stems in which the gas space was filled with water differed in
their surface stresses by < 2% from modeled stems in the native state (partially gas-filled), indicating no practical benefit
from a reduction in stem mass due to gas. A second modeling scenario compared the native state to stems in which gas was removed
and stem diameters decreased (and material properties adjusted to concur with the increased wood density) to conserve mass.
Removal of the gas-filled voids resulted in up to 41% higher surface stress for the same applied load, caused by a decrease
in the second moment of area greater than the increase in modulus of elasticity. Trees with gas removed had higher modulus
of rupture, but could withstand up to 14% lower maximum wind forces than trees in their native state, suggesting a biomechanical
role for the gas if the model assumptions are valid. The gas content may, however, have evolved in response to pressures unrelated
to biomechanics. We discuss some of its potential effects on sapwood physiology.
Keywords:
embolism, heartwood, sapwood, tree stability, water content, wind.
