August 2012 BMP Q&A

By: Todd Thomas, BMP Forester, Texas Forest Service

Q: While discussing streamside management zones (SMZs) with a co-worker, the topic of thinning came up.  I told him that the guidelines recommend leaving at least 50 ft² of basal area per acre.   He insisted that it was 50% crown cover.  Who is right?   Is there really a difference?

 A: Great question.  SMZs provide many critical functions in protecting water quality, so it is important that we don’t limit their effectiveness through over harvesting.  Not only do these areas slow and filter runoff, they also provide shade to streams, maintain bank stability, and lessen the impacts from raindrops that can lead to erosion. 

Now back to your question.  In essence, there is some truth to both answers, though in the event you have a friendly wager riding on this, I would have to side with you.  According to the Texas Forestry Best Management Practices Handbook, within the SMZ of perennial and intermittent streams, a minimum of 50 square feet of basal area per acre should be left.  The residual basal area should be evenly distributed throughout the SMZ in order to provide adequate protection to the stream. 

So what exactly is basal area?  This forestry term is primarily used as a measure of density and is defined as the cross sectional area of a tree stem in square feet at breast height (4.5 feet above the ground).  Since this can sometimes be a difficult concept for people to understand, especially those unfamiliar with forest measurements, the BMP handbook also includes a general rule of thumb that can be used.  Retaining 50% overstory crown cover within the SMZ can usually serve the same purpose, though in order to achieve this, you probably will have to leave a little more than half the trees in a forest that has reached canopy closure. 

Basal area, once you understand it, is very easy to measure, especially when you have the right tool.  A BAF 10 factor prism can quickly help you determine the residual density of the SMZ.  If you don’t have a prism, the BMP handbook includes a section on page 107 that provides information on how to calculate basal area. 

Remember, any time you are working in SMZs, special care is necessary in order to maintain their critical function.  While it is important to manage these areas, operators should continue to follow all related BMPs.  Roads and skid trails should be located outside of these areas when feasible and logging decks should be at least 50 feet from the edge of the SMZ.  Directional felling should also be used to minimize the amount of debris that enters the stream.  Logging slash that inadvertently enters the stream should be removed.  Lastly, minimize the number of bank trees that are harvested, as these help protect the integrity of the stream, provide shade, and stabilize the bank. 

Thanks for taking the time to increase your knowledge of BMPs and keep up the good work out there in the woods.  Also, please send in any BMP questions you may have, because chances are if you are unsure of something, there is someone else out there who has the same question.  Feel free to contact me at tthomas@tfs.tamu.edu or (936)-639-8180. 

*This article was published in the August 2012 issue of the Texas Logger