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Criterion 1 Indicator 2
Extent of Area Covered by Different Forest Types and Age Classes
Forests are made up of many different species of trees, and different structural classes. The diversity of forest types and successional stages provides habitats for many different species of plants and animals across the landscape. Thus, the diversity of forest types can be an important measure of total ecosystem health.

Can This Indicator Be Quantified?
Forest types were derived from the tree species information in a digital "map" of land cover provided by the Oregon Department of Fish and Wildlife (ODFW). Tree diameter information, which is highly correlated with age and successional stage, was combined with the forest type data for this indicator. A geographical information system (GIS) was used to combine and analyze the two digital data sets, and then to calculate the area covered by various forest types in six tree diameter classes.
Figure 2-1. Area of forest types, by size class, in western Oregon
Data in the land cover map was aggregated into 10 tree species groups (see Map 1-1). The diameter class data was derived from satellite imagery, as classified by the Oregon Forest Industry Council (OFIC). We aggregated the data into 6 size classes representative of different successional stages. These statistics were developed only for western Oregon.
Most of northwestern Oregon’s forests are dominated by stands of Douglas-fir. In a representative stand (Site Index 160 — low Site Class II), Douglas-fir naturally reaches a diameter of 5 inches about age 15, 10 inches at age 40, 15 inches at age 70, 20 inches at age 100, 25 inches at age 150, and 30 inches about age 200 (McArdle, 1930). Site index is a measure of stand productivity. Douglas-fir stands will reach these average diameters earlier if the site index is higher than 160, or later if the site index is lower than 160. If the stand is actively managed, by thinning, for example, then the trees can reach these diameters sooner.
The table below shows the total number of acres of Douglas-fir stands in northwest Oregon, and the size classes of these stands. Forty-two percent of the Douglas-fir stands are in early successional stages (i.e., average tree diameter is less than 10 inches), slightly more than 39 percent are in mid-successional stages, and about 16.7 percent are in the older successional stages (i.e., average tree diameter is greater than 20 inches).
Table 2-1. Acres of Douglas-fir stands in northwest Oregon, by size class
SpeciesSize (dbh inches)Area (acres)Area (as percent of class area)
True Fir; Mt. Hemlock0-5141,36611.88
Total 1,168,414 
Most southern Oregon forests are a Douglas-fir and mixed conifer forest type. These forests are on poorer sites than northwest Oregon forests, and therefore do not grow as fast. The western slopes of the Coast Range have about 1.8 million acres of deciduous or mixed deciduous tree-conifer stands.
Red alder is the most common hardwood species in western Oregon. Pure stands of red alder start changing to other forest types naturally by the time the red alder reach about 15 inches in diameter, and the alder rarely exceed 20 inches in diameter (Worthinton, 1960). As alder trees die they are normally replaced with hemlock or Douglas-fir. About 90 percent of the deciduous and western mixed (mixed deciduous-conifer) stands in Figure 2-1 are in size classes equal to or less than 15 inches in diameter.
A spruce-hemlock forest grows naturally on the coastal strip in northwest Oregon. As shown in the table below, slightly less than half the stands (47 percent) are in the earlier successional stages (i.e., average tree diameter less than 10 inches), about 43 percent of the stands are in mid-successional stages, and about 10 percent are in the older successional stages (i.e., average tree diameter is greater than 20 inches).
Table 2-2. Acres of spruce-hemlock forest along northwest Oregon’s coastal strip, by size class
SpeciesSize (dbh inches)Area (acres)Area (as percent of class area)
Spruce; Hemlock0-564,78116.09
Total 383,165 
Most true fir and mountain hemlock forest stands are located in the high elevation zone of the Cascade Range. On these poorer sites, trees grow slowly. Some stands where the trees are only an average 10 to 15 inches in diameter may be over 100 years old, and stands where the trees have reached 15 inches or more in diameter are almost always over 100 years old. The following table shows the total number of acres in this forest type, by size classes.
Table 2-3. Total acres of true fir-mountain hemlock forest, by size class
SpeciesSize (dbh inches)Area (acres)Area (as percent of class area)
Douglas Fir0-51,202,39717.55
Total 6,733,870 
For eastern Oregon, Pacific Meridian Resources classified tree size classes using data from satellite imagery, in a study done for the Oregon Department of Forestry. GIS was used to combine and analyze information on forest types, and acres of these types in four diameter classes and a low density class (low density of trees, diameter not specified). The results are shown in the next figure.
Figure 2-2. Area of forest types, by size class, in eastern Oregon
Because eastern Oregon generally has lower productivity sites, trees grow more slowly than in western Oregon. In a ponderosa pine stand with a site index of 100, it takes about 65 years for trees to reach an average 10 inches in diameter, and 150 years for trees to reach an average 20 inches in diameter (Wycoff and Atterbury, 1974). Most forests in eastern Oregon are either ponderosa pine or mixed conifer, and many stands are managed with uneven-aged silvicultural methods that leave a variety of size classes in the same stand. The majority of the timberland in eastern Oregon is occupied either by low-density stands or by trees that are 10 to 20 inches in diameter.

In Oregon forests, the number of large trees has been decreasing over time, due to timber harvest and the historical trend to manage forests on harvest rotations of 40 to 80 years. However, on federal forest lands, new management policies have greatly reduced the amount of timber harvest (i.e., the Northwest Forest Plan). With these changes, the long-term trend on federal forest lands will be toward larger trees, with larger average diameters.
On private forest lands, planned harvest rotations will generally keep stands from developing larger than an average 15 inches in diameter. With the advent of satellite imagery, these trends will be easier to track in the future.

Data Source and Availability
The data used for this indicator is readily available from state and federal land management agencies. Digital data used to create the IUCN (World Conservation Union) land protection classes was obtained from the following sources.
  • Oregon Department of Fish and Wildlife. GAP II data from State of Oregon GIS Service Center. http://www.sscgis.state.or.us/data/themes.html
  • Interior Columbia Basin Ecosystem Management Project (http://www.icbemp.gov/spatial).
  • State of Oregon GIS Service Center (http://www.sscgis.state.or.us).
  • USDI Bureau of Land Management.

Reliability of Data
The ODFW data set is the most reliable data yet developed for classifying Oregon’s different forest types. It was derived from 23 Landsat Thematic Mapper satellite scenes collected from 1991 to 1993. The minimum mapping unit is 100 hectares.
The OFIC data for western Oregon was derived from satellite imagery scenes collected in 1994. The individual 25-meter pixels have been classified, and the overall accuracy of the data layer is about 84 percent.

The current ODFW digital land cover data for Oregon was derived from 25-meter resolution satellite imagery covering the entire state. This data was then classified and aggregated to a minimum mapping unit size of 100 hectares. The OFIC data is also derived from 25-meter resolution satellite imagery; it covers only the western half of the state.

Recommended Action for Data Collection
It is important to get data like the OFIC tree size data for the eastern part of the state. In order to track trends and detect changes in the future, it is critical to use consistent techniques, classification schemes, and definitions over time. Otherwise, many apparent "changes" over time could result from using different definitions or classification schemes. Satellite imagery is available starting from 1973 and is updated annually, but currently there is no plan to interpret and classify this data on a regular basis so that there will be a consistent historical record of Oregon’s forests.


Selected References
McArdle, Richard E., and Walter Meyer, Donald Bruce. 1930. The yield of Douglas-fir in the Pacific Northwest. USDA Forest Service PNW Forest and Experiment Station, Portland, OR. Bulletin No. 201, 74 pp.
Worthinton, N. P., and F. A. Johnson, G. R. Staebler, W. J. Lloyd. 1960. Normal yield tables for red alder. USDA Forest Service PNW Forest and Experiment Station, Portland, OR. Research Paper 36, 29 pp.
Wycoff, E., and T. Atterbury. 1974. Age-site regression equations for base 100 tables of normal yield for Douglas-fir, western hemlock, red alder, ponderosa pine, and lodgepole pine. Crown Zellerbach Corporation, FMSS, 90 pp.