Why So Many Brown Trees?
Article #133, June 2008
By Bill Cook
Reports of, and queries about, unusual numbers of dead and dying trees have been making their way to conservation offices for several weeks this spring. The short explanation involves the past three dry growing seasons, but there are many contributing factors.
A stressed tree becomes increasingly vulnerable to attacks by insects and diseases, similar to any other living organism. Most trees will be able to sustain the stress of the past three dry summers. However, trees with other sources of stress have begun to succumb in greater numbers. People are beginning to notice. Keep in mind that all living things die.
Besides droughty conditions, stressful factors include tree age, tree species, soil type, residential and roadside environments, past construction activity, normal insect activity, individual genetics, etc. In nature, a tree with low vigor attracts a range of pests that the tree could otherwise resist. Root rots are particularly common and will take advantage of an unhealthy tree that has less resistance. The root rots kill portions of the tree, sometimes killing the whole tree.
Low vigor means weaker tissues, inviting bark beetles, wood borers, and other insects. If those insect populations mushroom, then weaker trees will die. We saw that a few years ago as bark beetles were the last straw among many tamaracks, following a dry season and larch casebearer outbreaks (needle eaters).
Roadside trees are more commonly noticed and are more vulnerable because of ice-melting chemicals, vehicle exhaust concentrations, and altered water levels from the road. Residential environments are particularly hostile to most tree species. A general rule-of-thumb cuts the normal lifespan of a tree in half if it grows in a town or along a street.
Normal insect population outbreaks, which won't commonly kill trees, will have a much greater impact on trees already pre-disposed by weak health. This spring, cankerworm outbreaks have chewed the leaves from maples and other trees. Sometimes, this will be enough to kill the tree if the tree is already in bad shape.
In red pine plantations, owners have often neglected to properly thin the stands. The overcrowded condition leaves the pines with small crowns. Health is further diminished by the recent dry summers. This is a perfect recipe to attract bark beetles, which can now successfully attack the tree and pockets of dead pine begin to appear.
So, the answer to why a tree is dying this spring can have many reasons, most of them accentuated by the past three dry summers, and usually complicated by individual site conditions. Unfortunately, there are not many things that can be done. A good growing season may be the best remedy.
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Bill Cook is an MSU Extension forester providing educational programming for the entire Upper Peninsula. His office is located at the MSU Upper Peninsula Tree Improvement Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at firstname.lastname@example.org or 906-786-1575.
by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005
J Road, Escanaba, MI 49829
906-786-1575 (voice), 906-786-9370 (fax), e-mail: email@example.com
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