Standing dead trees
Michigan has large volumes of standing dead trees. The distribution varies among species, sizes, and geographically around the state. These stories speak volumes about how these resources are valued.
The volume of standing dead trees in Michigan forestlands amounts to 2.2 billion cubic feet, or the equivalent of about 28.7 million cords. By comparison, Michigan’s annual harvest is roughly five million cords. If all the dead standing trees were laid-out in cords, side by side, that pile would run nearly 22,000 miles, almost around the Earth at the equator. Another way to imagine that cord volume, four feet deep, is that it would cover a township of 36 square miles. This volume represents about 6.5 percent of the standing live volume. So, it’s no small amount of wood and it’s quickly accumulating. Yet another way to look at this standing dead volume is how many wood-based mills it might support. That 28.7 million cords could provide enough feedstock for about 24 large paper mills, or 100 large sawmills. Of course, the forest doesn’t accumulate 28.7 million cords every year. It’s built-up over time.
Now, standing dead trees are valuable for habitat. Many species of wildlife will use the trees for cavity nesting, roosting, an insect food source and other purposes. The larger the dead tree, the better. However, almost two-thirds of the dead tree volume is in the smaller size classes, under a foot in diameter. It’s important to note that wildlife populations do not necessarily increase with higher numbers of dead trees. Many species are territorial. Eventually, those dead trees fall over and create important on-the-ground habitat for a different suite of wildlife. Over the decades, the wood decays and becomes part of the soil, although most of the nutrients in a tree are not in these woody trunks and roots.
The major species groups of standing dead trees are the ashes, aspens, and fir-spruce, together accounting for nearly half the volume. We can thank the emerald ash borer for much of the dead ash. The spruce budworm may be able to take part of the credit for fir and spruce. Five individual tree species account for 40 percent of the standing dead volume; green ash, quaking aspen, white ash, balsam fir, and paper birch. Maples comprise nearly a third of all Michigan live volume, but account for only nine percent of the standing dead trees. This suggests the maple resource is currently healthy and vigorous. Maple-dominated forest types are among the most commonly managed, which would help explain this healthy condition.
There is more to tree mortality than standing dead trees. Every year, trees die. Of course, eventually all trees die, even our favorites. This annual mortality rate runs around the equivalent of six million cords, which is also more than what is harvested. Only over the last few years has Michigan’s mortality volume exceeded the harvest volume. Not surprisingly, the patterns of annual mortality are similar to those of the standing dead tree volumes. Most of the annual loss occurs in the smaller diameter trees and is among the ashes, aspens, and fir-spruce.
Five tree species account for 46 percent of the annual mortality volume; green ash, quaking aspen, white ash, balsam fir and red maple. This is almost the same list as for the standing dead volume. However, when weighed against the live volume, which species have taken, proportionally, the greatest hits? The top ten volume losers, by percentage of live volume, include the three ashes, American elm, and beech. These five have suffered from exotic species. Balsam fir and white spruce have been hit by the native spruce budworm. The other three, to round-out the top ten, are paper birch, jack pine, and quaking aspen. This mortality has been due to old age and successional changes.
If even a million cords of the annual mortality, of the six million, could be harvested, that would be enough wood to supply a large pulp mill, many sawmills, or provide heat and hot water for 200,000 homes. There are more factors in better utilizing forests and enhancing the economy than just the forest inventory. However, understanding the magnitude of the potential supply is a good place to start. With an amenable set of socio-economic conditions, Michigan’s renewable forests could sustainably contribute much more to human welfare, forest health, wildlife habitat, and clean water. So, in the end, it’s more about the people, than the forest.