In dry forests east of the Cascades, large snags and large down logs play an incredibly important role in maintaining ecosystem health. They are like the forest's immune system, providing habitat for the birds and ants that eat the insects that defoliate trees. Large snags and down logs are much more useful than smaller snags and down logs. Large snags also stay standing much longer than smaller snags.

After large snags have been standing for a few years, woodpeckers will make enough holes in them that they will be like bird condominiums for other insect eating birds. In eastern Oregon forests most 15 to 20 inch diameter snags will only stay standing for about ten years or so. Some larger snags can stay standing for much longer. Very large fire-hardened snags can remain standing for 50 years or more.

When a snag falls after ten years, a live green tree must die and replace it. Since it can take seventy or eighty years to grow a twenty inch tree on the eastside, we must provide six green trees as replacements over time for each snag that we want to maintain.

The best forest scientists in eastern Oregon recommend that four snags over 15 inches and two snags over 20 inches should be maintained on every acre of eastside forest. This means that we must maintain 6 times 6 or 36 green trees per acre as replacements for the snags that will fall over time.

But there is a problem with only leaving 36 green replacement trees. The green replacement trees do not die fast enough to replace the snags in ten year intervals. Only two of the 36 green replacement trees are likely to die every ten years. In order to have enough green replacement trees to provide adequate snags to protect the forest from insect attacks, you must leave about three times as many green trees so that an adequate number of them will die naturally and replace the fallen snags. This means that we must leave about one hundred and eight 15 to 20 inch diameter trees in order to insure that enough of them will die naturally to provide six snags per acre over time.

An insect infested forest is not a sustainable forest. If we wish to sustain the insect killing birds and ants that are essential to a healthy forest, we must provide adequate snag and down log habitat.

The numbers of leave trees recommended in the discussion above are probably on the low side. While I discussed green tree replacement for snags, we must still provide for down logs. Since the best forest scientists in eastern Oregon recommend that we maintain more than 33 large down logs per acre, it is difficult to see how we will get them out of 6 snags per acre.

Down logs provide more than habitat for the ants that eat defoliating insects. When a down log finally rots into the ground, the underground wood stores a tremendous amount of moisture. In dry forest types, underground wood supplies most of the late season moisture that is available in the soil. Underground wood also supplies most of the nitrogen in the soil for dry forest types.

The above scenario also assumes an 80 year rotation. This would mean that there will be no old growth on managed land and that the species that depend on low elevation old growth will be left without habitat.