Wetwood

Wetwood

Wetwood is traditionally considered a bacterial disease of living trees.  However, if you look carefully at the evidence, you find that wetwood protects trees from decay diseases and so it is beneficial in general.  On the other hand, it can lead to damage if pressure builds up and acidic liquid is forced out.  But we’re getting ahead of ourselves.

A young Abies concolor (white fir) with wetwood occupying the heartwood.

Wetwood can be defined as wood in living trees that:

  • is non-conducting but has a high moisture content and appears watersoaked
  • is somewhat darker in color than surrounding wood
  • has a fetid, fermentative odor
  • is colonized by bacteria
  • occupies the heartwood of some tree species on a normal basis:
    • conifers: firs and hemlocks primarily
    • hardwoods: elms, poplars, birches, oaks
  • also may form in response to wounding in sapwood

Wetwood is sometimes called “bacterial wetwood”, as if to distinguish it from nonbacterial wetwood.  All wetwood contains bacteria, so there is no distinction to be made.

The Bad

Wetwood has a bad reputation in the logging and wood products industries. Let’s face it: it stinks, is loaded with bacteria, and pressure may build up, squirting the foul liquid on a hapless logger (leading to the appellation piss-fir in some parts).  In landscape trees, the liquid may ooze from pruning wounds, cracks, etc., become colonized by a dog’s breakfast of microbes, become slimy, and may kill sapwood and bark that it contacts.  This condition is known as slime flux.

It is also associated with a variety of problems during wood products production [10]:

  • Wetwood is more difficult to dry than normal wood and requires more energy.
  • Wood dries unevenly and may warp and twist.
  • During kiln drying, acid vapors cause kiln corrosion.
  • It is associated with ring shake and honeycomb, two lumber defects.  Ring shake in elm leads to the term “onion elm” in the lumber trade.

Primarily because bacteria are found within it, in a living tree, wetwood was traditionally considered a tree disease caused by bacteria [2].  This was reinforced by the fact that, in some cases. it is associated with damage caused to the tree.  This occurs when the gasses produced anaerobically by the bacteria cause pressure to build up, expelling the liquid, which may damage living tissues on its way out.

Wetwood in Populus angustifolia (narrowleaf cottonwood).

The Good

On the other hand:

  • Wetwood is often the normal condition of heartwood of mature trees in species in which it occurs [4, 8, 10, 11].
  • Wetwood also forms in response to wounding or infection [1, 4, 6, 11].
  • Wetwood can be formed under conditions that preclude bacterial growth (in other words, it is NOT caused by bacteria) [4, 12].
  • Wetwood appears to be wet in part because of accumulation of calcium and magnesium salts, lowering the osmotic potential.  A drier transition zone with living parenchyma separates sapwood from wetwood [4, 11].  Water likely passes through the transition zone as a vapor to satisfy the osmotic potential in the wetwood.

The key points:

  • Wetwood is colonized by facultatively and obligately anaerobic bacteria [2, 8, 14, 15] that consume almost all available oxygen and bring the oxygen content far too low for fungal growth [5, 9, 13].
  • The bacteria also produce volatile, low-molecular-weight organic acids: acetic, propionic and butyric acids [7, 13].  These acids are responsible for the odor and kiln corrosion.
  • The combination of anaerobic conditions and inhibitory organic acids largely prevent fungal growth and wood decay in intact wetwood [3, 9, 13].

Thus, wetwood is formed by the tree itself during heartwood formation and as a response to wounding. It is a favorable environment for the growth of bacteria that create conditions inimical to the growth of root- and butt-rot fungi.

Rather than being a disease, wetwood appears to represent a mutualistic symbiosis.  Trees create conditions favorable for bacterial growth, while bacteria create conditions that  defend the tree from decay fungi.  Wetwood occasionally develops into slime flux and becomes damaging to trees.

Is it a coincidence that wetwood often occurs  in tree species that have heartwood with no effective decay-inhibitory extractives, like Abies (true firs), Tsuga (hemlocks), Betula (birches) and Populus (poplar) spp.?

References

1.
Campana RJ, Murdoch CW, Andersen JL. 1980. Increased development of bacterial wetwood associated with injection holes made for control of Dutch elm disease [Abstract]. Phytopathology 70(5):460.
2.
Carter CJ. 1945. Wetwood in Elms. Illinois Natural History Survey Bulletin 23(4):407–448. [Source]
3.
Coleman JS, Murdoch CW, Campana RJ, Smith WH. 1985. Decay resistance of elm wetwood. Canadian Journal of Plant Pathology 7(2):151–154. [Source]
4.
Coutts M, Rishbeth J. 1977. The formation of wetwood in grand fir. Forest Pathology 7(1):13–22. [Source]
5.
Murdoch C, Biermann C, Campana R. 1983. Pressure and composition of intrastem gases produced in wetwood of American elm. Plant Disease 67(1):74–76. [Source]
6.
Murdoch C, Campana R. 1984. Stem and branch distribution of wetwood and relationship of wounding to bleeding in American elm trees. Plant Disease 68(10):890–892. [Source]
7.
Murdoch C, Campana R, Biermann C. 1987. Physical and chemical properties of wet wood in american elm (Ulmus americana). Canadian Journal of Plant Pathology 9(1):20–23. [Source]
8.
Murdoch CW, Campana RJ. 1983. Bacterial species associated with wetwood of elm. Phytopathology 73(9):1270–1273. [Source]
9.
van der Kamp B, Gokhale A, Smith R. 1979. Decay resistance owing to near-anaerobic conditions in black cottonwood wetwood. Canadian Journal of Forest Research 9(1):39–44. [Source]
10.
Ward JC, Pong WY. 1980. Wetwood in Trees: A Timber Resource Problem. General Technical Report PNW-GTR-112. Portland, Oregon: USDA Forest Service, Pacific Northwest Forest and Range Experiment Station. 59 pp.
11.
Worrall JJ, Parmeter JR. 1982. Formation and properties of wetwood in white fir. Phytopathology 72(9):1209–1212. [Source]
12.
Worrall JJ, Parmeter JR. 1982. Wetwood formation as a host response in white fir. European Journal of Forest Pathology 12(6):432–441. [Source]
13.
Worrall JJ, Parmeter JR Jr. 1983. Inhibition of wood decay fungi by wetwood of white fir. Phytopathology 73(8):1140–1145. [Source]
14.
Zeikus J, Henning D. 1975. Methanobacterium arbophilicum sp. nov. An obligate anaerobe isolated from wetwood of living trees. Antonie Van Leeuwenhoek 41(4):543–552. [Source]
15.
Zeikus J, Ward J. 1974. Methane formation in living trees: a microbial origin. Science 184(4142):1181–1183. [Source]