ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

The primary reason for larch selection and apparent avoidance of Douglas-fir is
related to decay characteristics of the two species. After tree death, Douglas-fir
sapwood decays rapidly along with the heartwood (Wright and Harvey 1967). A protec-
tive "shell" of sapwood is missing in snags of that species. In western larch, the
sapwood decays very slowly, perpetuating a shell around decaying heartwood for many
years. Cavities in Douglas-fir deteriorate more rapidly and are more susceptible to
predation (a predator is able to gain access to the nest chamber by enlarging the
entrance, which is in decaying wood). No live Douglas-fir were used as nest sites,
but 1 ive larch were used more of ten than were larch snags. Nest cavities in 1 ive
larch were usually excavated through living wood tissue into decaying heartwood
rather than in a dead top or a dead branch. With one exception (a Black-backed Three
-toed Woodpecker nest), Douglas-fir snags were used only by the "weak" excavators
(flickers, chickadees, and nuthatches). They seem to require very soft wood for
successful completion of cavities. In instances where the weak excavators nested in
trees species such as western larch, they typically used cavities abandoned by
stronger excavators. Mountain Chickadees and Red-breasted Nuthatches usually nested
in old nest holes of Yellow-bellied Sapsuckers (woodpeckers nearly always excavate a
new nest cavity each spring).
In western larch, selection related to tree condition was directed toward
broken tops (fig. 5). Although there were nearly equal numbers of intact-top and
broken-top larch snags (and mean dbh of both groups exceeded 15 in (38 crn)) , there
were about 5 times more nests in broken-top snaqs. This selection process probably
involves decay presence. A broklen top provides an avenue of entry for decay orqanisms
(Hepting 1971 ) . Top breakage in western larch commonly resul ts from 1 iqhtninq
(fig. 6), snow or ice accumulation, or windstorm. Broken tops also may result from
breakage at a point weakened by pre-existinp decay or the presence of a nest cavity
(Conner et a1 . 1975).
1 1 Relative density
BE Nest trees In = 183)
. . .G
I top top 1 I fop dead top live top I
SNAGS L l VE TREES
TREE CONDITION
Figure 5. -- Dee corzdikion preferences, western larch nest trees, all
hole-nesting bird species cornb