The lands (and the trees that grow on them) managed by the USFS and BLM are governed by numerous laws that require these lands to be managed sustainably.  The O&C Act, which governs the management of over 2 million acres of federally owned forestland in western Oregon, is very specific on this point.  It states that the O&C lands “shall be managed . . . for permanent forest production, and the timber thereon shall be sold, cut, and removed in conformity with the principal of sustained yield for the purpose of providing a permanent source of timber supply”.  So what is the “principle of sustained yield”?

In its simplest form, this means that the federal agencies cannot cut more trees than it can grow.  The next question is how one knows that they are doing this?  That’s complicated.  It is done by collecting reams of field data which are compiled and feed into computer models developed by teams of PhD’s from a wide range of specialties.  Understanding the interplay between all of these biological, social and technical factors form the basis of Forestry Programs developed by our major universities.  Short of that, I will give you my synopsis of a Sustained Yield 101 course.  I will start by assuming that I own ten acres of land and want to grow Christmas trees to supplement my income.  These same principle apply to growing trees to produce building materials.

To determine my “sustained yield” of Christmas trees, the first thing I need to know is the productivity of the soil.  My ten acres consists of one acre of really good soil, eight acres of OK soil and one acre of rather poor soil.  The next thing I need to know is how long it will take to grow the size of Christmas tree I want to sell.  To determine this I need to know the characteristics of the trees I am going to plant (genetics), what supplements I might add to the soil (fertilizers), what management practices I will employ as the trees grow (thinning, weed control, etc.) and how big of a tree I want to grow. I then combine the soil productivity and the tree data to find out how long it will take to grow the size of tree I want and how many trees per acre I can expect to have.

Doing this I find that I can grow 1,500, six foot trees in five years on my one acre of really good soil, 1,250 trees on my OK soil and 750 trees on my poor soil.

I decide to start small and just plant my one acre of good soil.  What is my sustained yield?  Well it is 1,500 trees every five years.  Most of the time, sustained yield is displayed on an annual basis and is referred to as the Allowable Sale Quantity (ASQ).  This is because our federal forestland is also managed under another forestry principle which is called “non-decline, even flow management”.  What this does is ensure that the communities and businesses that rely on a predictable, constant supply of timber will not be hurt by huge swings in the amount of timber being sold every year.  On our case, it means that instead of selling 1,500 trees every five years we would sell 300 trees every year. (1,500 divided by five years)  Our ASQ would then be 300 trees.

What would happen if I wanted to plant all ten acres?  I could grow 1,500/acre on one acre, 1,250 /acre on eight acres and 750/acre on one acre.  Doing the math, this pencils out to 12,250 trees every five years or an ASQ of 2,450 trees.  In order for this to truly be sustainable, 300 trees would come off the one good soil acre, 2,000 from my eight OK soil acres and 150 from my one acre of poor soil.  I would also have to immediately replant the acres that I cut.  Planting baby trees is the heart of sustained yield management.

As you can see, calculating the sustained yield combines many factors and is based on the assumption that you start with bare land.  What happens to the sustained yield when you start with land that already has trees on them? The answer is…nothing.  Sustained yield is calculated on the potential growth based on the myriad of factors mentioned above.  What existing trees do determine is when you can start selling at the sustained yield level.  In our Christmas tree example, we can’t start until five years from now.  When our federal agencies ask the same question, they look at the size of the inventory that they start with and determine if it can be metered out at the sustained yield level.  If not, they must lower their ASQ until such time as the inventory is sufficient to maintain the sustained yield level.  If the inventory is very high, they could actually sell more than their sustained yield level for a period of time but the principle of non-decline, even flow prohibits this.  -Ross Mickey