Snags, or standing dead trees, like the one near the center of the above photograph, provide food and shelter for a richness of life. Birds nest in snags, and small mammals make homes there as well. Moss and lichen will live on the bark, while fungus and insects soften the core of the tree. A large snag could stand for 150 years or more before it finally topples to the forest floor.
At right is a blow up of the above tree showing a large diameter hole near the top. Woodpeckers, like the Pileated woodpecker, excavate holes in snags for feeding and nesting. The woodpecker digs a new nest hole each year so in subsequent years other birds or mammals may move into old cavities.
Snags are so important to forest ecology that British Columbia has a program to tag and protect such trees as "Wildlife Trees". These valuable resources are marked with a yellow symbol which identifies and protects them.
Many forest creatures can only live in large, old trees, and snags can provide the height and structure they require. The Spotted owl is an example of a bird that requires old trees for nesting, often setting up in snags, living in the hollow of the broken top.
Bald eagles use large trees for perching and nesting, often having a second nest nearby in a companion big tree. Eagle nests can weigh up to 1000 pounds - you need a big tree to be able to hold such a massive structure of branches.
The threatened Marbled Murrelet also needs old growth trees.
"Throughout the murrelets’ North Pacific range, most occupied nests have been found in tall trees. They were simple cups in the moss on thick branches, 20 to 40 m above the ground. Only fairly old Douglas fir and Sitka spruce have branches thick enough for murrelet nests, although other trees may offer suitable platforms or cavities.
Trees with thick branches near the top usually occur in stands of old growth and mature forests that are rapidly being cut for timber. If it turns out that British Columbia’s murrelets are not concentrated in colonies and have no alternative nesting habitat, the need to protect sites for murrelet nests will be another reason to preserve more of British Columbia’s coastal old-growth forests. However, a great many areas still remain to be explored for nests." http://www.hww.ca/hww2.asp?id=55&cid=7
When we save large old trees, whether they are dead or alive, we are also saving a vast richness of life that depend on such trees. Each ancient tree is an ecosystem to itself, and we are just now realizing their true importance. The snag pictured at top enriches Francis/King Regional Park, Saanich B.C.