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Deadwood Brings New Life

Friday, September 14, 2001

By David Suzuki

It may be hard to believe, but trees can actually create more habitats for various species after they die than when they are alive. Recent studies have found that dead trees are crucial to the overall health of a forest ecosystem. And amazingly, as reported recently in the journal Science, their influence extends beyond the edge of the forest to rivers, estuaries, and even the ocean.

For decades, scientists have recognized the importance of standing dead trees, or snags, as habitat for a variety of birds, small mammals, and insects. Fungi and bacteria first colonize these dead trees, followed by insects and other organisms like the pileated woodpecker. The woodpecker digs deep into the trees in search of carpenter ants to eat. In the process, it creates holes that serve as habitat for other creatures, such as chickadees, bluebirds, and bats, who cannot dig holes themselves.

Researchers in the United States have found up to 100 snags per hectare in old-growth forests. These trees can stand for more than 40 years, and once they fall, they decompose on the forest floor, creating new habitats for up to 300 years more. Studies of coastal temperate rainforests have found at least 80 species that depend directly on deadwood for their survival.

In an old-growth forest, rotting trees sprouting new saplings are a common sight. It is often assumed that these rotting logs, dubbed nurse logs, provide nutrients that help the seedlings grow. But studies at the University of British Columbia have found that nutrients released by rotting trees are largely unusable to seedlings. Most nutrients in the soil actually come from fallen needles and leaves. The real benefit of nurse logs is their ability to protect seedlings from pathogenic fungi in the soil, which can kill seedlings but cannot survive in the deadwood.

The more we discover about our forests, the more we find that our previously held assumptions were often wrong. Look at dead trees in rivers, for instance. It was commonly believed through much of the 20th century that these trees clogged streams and destroyed fish habitat. Now we are finding the opposite to be true. Large fallen trees can remain stuck in streams for more than a thousand years, collecting sediment, preventing erosion and gathering debris that provides nutrients for organisms. These trees also form dams and waterfalls that eventually create deep pools - perfect habitat for fish.

And if deadwood reaches the ocean, it helps spawn even more life. Marine invertebrates feast on the wood, attracting other creatures from little fish to birds to sharks. Eventually these floating piles of wood can become miniature ecosystems, some of which have been known to float for years, attracting so much sea life that fishing boat captains seek them out in their search for fish.

Unfortunately, the deadwood capable of lasting for the longest length of time — and thus creating habitat for the greatest diversity of species — are big old-growth trees, which are becoming more and more rare. Most of North America's virgin forests have been logged, much of it within the last 50 years. It will take centuries for very large trees to return. That means there will be less deadwood to provide habitat for birds, salmon, and other creatures and even less of it seeding the oceans with new life.

Deadwood is crucial to the health of forest ecosystems and beyond. Yet in focusing on trees for their value as timber, logs, or pulp, with rotation cycles of 70 to 80 years, we fail to see the real life cycle of a tree that extends far beyond the time of death. If we are to develop a truly sustainable model of forestry, we must recognize the interdependence of our ecosystems and change our practices accordingly.