By: Hughes Simpson, BMP Forester, Texas Forest Service
Q: I have been reading the Texas Forestry Best Management Practices (Bluebook) and am a little confused about some of the Streamside Management Zones (SMZ) guidelines, especially regarding thinning within these areas. Would you mind trying to clarify these guidelines?
A: Certainly! SMZs are very important areas that should be established when conducting forestry operations. These areas provide many benefits to us, mainly through the protection of water quality by filtering runoff water before it enters waterways. To offset potential monetary losses for establishing SMZs, tax incentives were passed by the 76th Texas Legislature to encourage these practices.
Partial harvesting (thinning) is acceptable within the SMZ. A minimum residual density of 50 square feet of basal area per acre should be retained. Basal area is generally measured using a 10 factor angle gauge (prism) and is based on the theory of the probability of selection being proportional to tree size. There is no minimum diameter limit for residual trees.
If you are not familiar with the concept of ' 'basal area" or do not have a prum, a. general rule of thumb that you can use is to retain a minimum of 50% of the original Crown cover. This is easily accomplished by looking up at the canopy and visualizing how it will look after the harvest while you are marking the trees to be cut. Keep in mind that in order to stay Within the guidelines, you usually must remove less than half of the original timber in the SMZ.
When thinning the SMZ, it is important to remember that the residual trees should be evenly distributed. Large gaps or open patches where the basal area falls below 50 square feet per acre should be avoided when possible. This can reduce the filtering and shading capacity of the SMZ. It is also important to plan for the damage and accidental felling of "leave" trees while thinning within this zone. SMZs that do not have an original basal area of 50 square feet per acre should not be thinned.
Placing a higher priority on removing the more valuable timber (mature pine) over lesser valuable timber (scrub hardwoods) can help reduce the financial burden of leaving an SMZ. However, harvesting this timber under certain site conditions may cause additional water quality impacts. Operating heavy equipment while the ground is saturated may cause rutting and sedimentation. If you have a question regarding BMPs, please contact me at (936) 639-8180.
* This article was published in the March 2003 issue of the Texas Logger