By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington and Yamhill Counties
If you’ve ever been out on a field tour with a bunch of foresters, you probably heard one of them use the term “site productivity” in describing a particular forest, or comparing two different forests. But to the person without a lot of formal forestry background, site productivity may be a vague concept at best. However, it is an underlying attribute that turns out to explain a lot of what we observe in our forests: what types of trees thrive, which seem to have problems, what amount of competition our seedlings face, and more. So let’s take a closer look at site productivity.
Essentially, site productivity refers to the amount of vegetation that a particular site can grow. In forestry, it’s usually expressed in terms of wood production, but technically all vegetation counts. In Oregon, we have a wide variety of forests that range from low to high site productivity (see map). You can intuit site productivity somewhat with your eyes: forests in eastern Oregon tend to have sparse, lower-growing vegetation between the trees. In the Coast Range, there is biomass everywhere – tall trees, ferns, berry thickets, and thick carpets of moss.
Site productivity is largely influenced by the climate, the soil, and the terrain. For example: copious rainfall and mild winter temperatures favor plant growth; cold temperatures at high elevations do not. South-facing slopes tend to be hotter and drier. Silty loam soils are generally highly productive for conifer growth.
Site Index and Site Class
Foresters have quantitative measures to describe site productivity- site index and site class, which can help to predict expected wood volume grown over a period of time. Site index is based on tree heights. That is because for conifers, there tends to be relatively little variation in height growth of a given tree species on a given site, regardless of how close together the trees are spaced. (In contrast, diameter growth is highly dependent on stand density.)
Researchers in early decades of the 20th century measured thousands of trees of different ages on different sites. From their data they constructed curves that would predict how tall trees would grow by a certain age, on sites with different gradients of productivity. Then, to compare one site to another, an index age is used (typically 50). So on a site with a 50-year Douglas-fir site index of 110, Douglas-fir trees would be expected to reach 110 feet tall at age 50.
Site classes are simply groupings of site index, with site class I being the most productive and V being the least.
Recently we explored the concept of site productivity with our Master Woodland Manager trainees. At two different sites we calculated site index by measuring the heights of trees that were roughly 40 years old, and then found where they fell on a site index chart. At the first site, a higher elevation Coast Range location with 85 inches of annual rainfall, we estimated site index to be about 130, or a high site class II. At the second site, which was lower elevation and receives only about 50 inches of rainfall, it was about 95, or a low site class III.
So, equipped with the right tools and knowledge, you too can estimate your forest’s site index. But, it might not match up with published values. This brings us to the last factor that influences site productivity (at least in terms of wood production), and that is management. It is really the only factor that we can control.
Remember, the forests where those scientists did their work to construct site index tables were natural in origin. But contemporary forest management practices typically result in trees that grow much faster than their naturally-originating counterparts. For example, tree breeding programs have selected genetic stock that is fast-growing, and that is the majority of what is produced by forest nurseries today. Secondly, we emphasize vegetation management in reforestation so that trees reach the free-to-grow stage as quickly as possible. This reduces management costs and ensure better compliance with the Forest Practices Act.
As a result, much of our managed forest landscape is probably out-performing the site index tables to some degree. It’s like Lake Wobegon, where all of the children are above average. So take your forest’s expected site productivity figures with a grain of salt.