7/24/2011

Super-Sized Douglas-fir Snags Important Bird Habitat

One giant, old growth large diameter snag
Up the Sooke River in a not-so-secret location, lies one of the largest Douglas-fir snags I have seen, and is a tree I like to visit from time to time. A snag is a dead, standing tree, and the one above is a prime example of a large diameter, old growth snag.

This centuries old tree probably snapped in a windstorm - what is left could stay standing for another century or more. In that time it will provide habitat for a whole ecosystem of interacting organisms and food webs. Although the tree itself may be dead, the structure itself is rich with life.

Birds in the coastal forest depend on these Douglas-fir snags. Up to 34 different hole-nesting species of birds use Douglas-fir snags for roosting or nesting. Woodpeckers are especially dependent on snags as they use them for roosting, nesting, and for feeding. They eat the insects living in the dead wood.

A study conducted in the 1970s in the coastal Douglas-fir forest found that snags provide crucial habitat for hole-nesting birds:
"On the average, hole-nesting birds used Douglas-fir snags over 60 cm in dbh (diameter at breast height - 1.3 meters/4 ft above the ground) and over 15 m tall for foraging and nesting; these snags usually had broken tops, few or no branches, decayed sapwood and heartwood, and less than 100% bark cover. Snags of this size and type occurred primarily in forests over 110 years of age; consequently, use of snags by hole-nesting birds was concentrated in older forests (>110 years old).
Density and species diversity of hole-nesting birds increased with forest age. Density of hole-nesting birds was positively correlated with mean dbh of snags. Intensive management of Douglas-fir forests does not allow for the production or retention of large snags. A reduction in the number of large snags could reduce populations of hole-nesting birds."

No snags, no hole-nesting birds.

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