Tracking forest statistics is an essential piece of understanding forest trends. Wood and environmental services from forests are critical to the success and welfare of human society.
How much forest is there? Has there been more, or less, than over recent decades? What types of forests exist and how has the mix changed? How diverse are Lake States forests compared to other forests in the United States?
These are just a few of the many questions commonly asked about forest resources. Some answers are easier to come by than others. Definitions, interpretations and understanding the statistical measures all color the results of an analysis.
For example, what exactly constitutes a forest? What are the parameters for acreage, volume, number of trees, growth rate, tree size, etc.? Which units will be used to describe the quantitative measures? What about the qualitative measures?
Most forest inventory statisticians, or forest biometricians, will have difficulty in providing an answer to a general question without a series of caveats. Even a simple question such as; “How many trees grow in Michigan?” needs clarification. How small of a tree should be counted? One of the billions of brand-new seedlings that have a low chance of surviving to the next growing season? When does a shrub become a tree?
The best source, and sometimes the only source, of statewide forest inventory data can be found in the USDA Forest Service Forest Inventory and Analysis Program. Since the 1930s, the Forest Service has been, more or less, systematically inventorying the forests of the United States. There is a lot of sampling and statistical design, sometimes controversial. Some state governments add money to the pot for additional sample plots in order to obtain information that is more precise.
Local inventories on a specific forest ownership might have different inventory systems that describe the forest in different ways. A forester preparing a forest management plan for a family forest will likely categorize forests in ways more useful to the specific owners. Private parcels are far too small to use the Forest Service data.
However, Michigan State University Extension recommends checking the Forest Service database for statewide and region-wide forest conditions. Each year, a short report is prepared for each state. Michigan’s most recent is for 2013, but the 2014 report should soon be available.
The thousands of sample plots were once measured every decade or so. However, since 1999, a fifth of the plots are re-measured annually on a five-year cycle. The annualized system provides critical data more often than the older system. The database is available online, but forestry and statistical expertise is needed to effectively navigate the tools.
By reviewing the annual Forest Service reports, you can answer many questions, complete with caveats and sampling errors. For example; “Volumes are for 5-inch and larger diameter trees. Number of trees and biomass are for 1-inch and larger diameter trees. Sampling errors and error bars shown in tables and figures represent 68 percent confidence intervals.” From there, various categories of volume estimates are given.
One is the half-million acre increase in forest land from 2008 to 2013, to a total exceeding 20 million acres, about 53 percent of Michigan’s land cover. Volume removals (mostly harvest) are reduced. Natural mortality has increased significantly, mostly due to losses in the ash and beech resources. Growth exceeded removals by over 100 percent each year, as it has for decades.
The most common forest type is maple/beech/birch, mostly what we call our “northern hardwood” type. The most common tree species are sugar maple, red maple, white-cedar, red pine and red oak. Over 60 percent of the forest is privately-owned, including over two million acres of corporate forest. Individuals own about 45 percent of the forest. The area of large-diameter trees continues to expand.
The descriptive profile varies geographically across the state. Forests of the western Upper Peninsula are quite different from forests in the southern Lower Peninsula. This is, of course, not a surprise. However, the inventory statistics help define just what those differences are.
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