6 Wood Discoloration

8.2 Tree Wounds and Tree Care 175
wounds and penetrating microorganisms by phenolic compounds, terpenoids,
and resin (Tippett and Shigo 1981).
In hardwoods, the defense reactions depend on physiologically active parenchyma
cells. The water-conducting system is protected against damage by
tyloses, plugs or membranes, and phenolic substances or suberin are deposited
on the cell wall or in the lumen (Schmitt and Liese 1993).
For the graphic understanding of the spatial cut off within a tree, Shigo
developed the CODIT model (Fig. 8.7; Shigo and Marx 1977; Shigo 1984),
which stands for “compartmentalization of decay in trees”. The model means
that the tree protects itself from penetrating microorganisms by four inhibiting
walls and that the spatial expansion of discoloration and decay is determined
by the anatomical structure of the wood. The axial “walls 1” with the weakest
partitioning effect are formed by the closure of the vessels and pits above and
underneath a wound by gums and tyloses. The tangential “wall 2” stem-inward
occurs by the annual ring borders and by the sapwood/hardwood boundary.
The radial “walls 3” are caused by the lateral wood rays. The most effective
compartmentalization is by “wall 4”, also termed barrier zone, formed by the
cambiumaftertheinjurywithincreasedparenchymacontent.
The CODIT model interprets the tree-own reactions as compartment formation
against microorganisms. It seems, however, more biological that the
tree protects itself first from penetrating air, particularly since wood fungi
can only settle the tissue if air is present. With changed definition, the term
CODIT has been expanded by Liese and Dujesiefken (1989, 1996): the D does
not only stand for decay, but also for damage and covering desiccation as well
as dysfunction.
The histological changes that occur in wood and bark as wound reactions
in hardwoods are schematically shown in Fig. 8.8.
The parenchyma cells die at the surface of the damaged wood area. The
tissue beneath the wound plane also dies, without mobilizing reserve materials,
Fig.8.7. CODIT model with walls 1 to 4
(after Shigo 1979)
www.taq.ir