STEM AND BRANCH DISEASES

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occurs in all species. There also are cases where decay fungi may enter stems
through root grafts, and in this case Wall 4 may not be formed because no wounding
is involved
Ecological Role of Decays
Heart and butt rot fungi cause substantial economic losses. However, they also
play important ecological roles in altering the speed and direction of forest succession,
changing vegetation composition, altering nutrient cycling patterns and
long-term productivity, and providing wildlife habitat for cavity-nesting birds,
small mammals, and bats (Hennon 1995, Bull et al. 1997). In many natural forests,
heart, root, and butt rots cause bole breakage and uprooting or induce trees to die
standing, thus creating small gaps. Decay fungi can be considered to be agents of
disturbance like fires and insects, particularly between long periods of episodic
large-scale disturbances. They are particularly important in temperate and boreal
forests, where fire return intervals are long, and in tropical forests.
In coastal Alaska old forests eventually convert to low-productivity bogs if
they exist for long periods without disturbances, like uprooting, that result in soil
mixing (Hennon 1995). Accumulation of organic matter restricts moisture
drainage in this high-rainfall area, resulting in gradual tree death. If trees are
uprooted, soil mixing can occur and productivity can be maintained. Heart rot
fungi on the other hand may contribute negatively to long-term productivity in
these stands because heart-rotted trees are not generally uprooted.
Wood decay fungi initiate a complex succession of organisms in logs and
standing boles involving other fungi, N-fixing bacteria, invading plants and mycorrhizal
fungi, and animal herbivores, fungivores, omnivores, and predators
including salamanders, bats, and bears. Heart rot fungi are extremely important
for cavity-nesting birds.
Brown and white rot fungi also play different ecological roles. Decomposition
is more complete with white rot fungi, and tree boles can become completely
hollow in the presence of white rot fungi. Brown rot residues, which are primarily
composed of lignin, last considerably longer and are important sources of soil
organic matter. Nitrogen fixation in brown rotted decay is an important source of
N to forest ecosystems. Brown rotted logs also can provide wildlife habitat for
long periods of time; hundreds of years in large logs. By increasing the variability
of structure in forests, heart rot fungi indirectly enhance habitat for many
wildlife species. For example, gaps created in forests in southeastern Alaska allow
more light to reach the forest floor, increasing understory vegetation browse for
black-tailed deer (Hennon 1995).
Interactions among fire, decay fungi, and mountain pine beetles strongly control
the vegetation dynamics in the lodgepole pine stands that form an edaphic climax
over large areas of the infertile “pumice plateau” in south-central Oregon
(Gara et al. 1985). Periodic fires create fungal infection courts in damaged roots;
with time, advanced decay caused by Basidiomycete decay fungi develop in the
butts and stems of these trees. The mountain pine beetle (Dendroctonus ponderosae)
preferentially selects and kills these trees during the flight season. As