The aspect of streams and regeneration harvests matter when looking at potential changes in stream temperature. Should south-facing streams get wider SMZ’s? (Photo by Steve Kallesser)
Of our neighboring states, Pennsylvania also has minimum SMZ width of 25’ (doubled in special watersheds, including municipal water supplies). That minimum SMZ width expands as slope increases. New York and Maryland both have SMZ widths between 50’ to 150’, based on slope. Delaware’s SMZ width is 50’ to 100’ based on both slope and erodibility of soils.
Excepting stream crossings (covered in a different section), machinery use within the SMZ is severely restricted. While machinery use is not absolutely prohibited, no skid trails are allowed. By combining forest access road, skid trail, buffer strip, and stream crossing BMP’s with those for SMZ’s, sediment delivery is minimized or avoided completely.
Research on SMZ effectiveness has focused on several areas, the first of which are termed “breakthroughs.” These are areas where flow of sediment-laden water overwhelms the ability of the SMZ to capture the sediment, and it enters the steam channel. These breakthroughs are often gullies, swales, and other dormant water channels that only flow during heavy water events. Other potential breakthrough areas were identified as low areas with minimal litter cover. Otherwise, they are caused by poorly planned drainage (particularly inadequate slowing of runoff) from roads.
Lakel et al. (2010) experimented with 25’, 50, and 100’ SMZ’s without thinning, and a 50’ thinned SMZ. All of the various widths performed equally well in trapping sediment. This suggested to the authors that SMZ effectiveness for controlling sediment is dependent on factors other than SMZ width. Such factors include slope, lack of ground disturbance (equipment use), and addressing potential breakthroughs prior to harvest.
Rivenbark and Jackson (2004) examined ephemeral water flow paths in the Georgia Piedmont region (clearcut and site-prepared harvests) to study where and why BMP’s fail to correct sediment pollution. They found that in some cases, overland flow penetrated SMZ’s of more than 100’ wide. Their conclusion was that BMP’s in Georgia needed to further address maximizing ground cover, improving road runoff dispersal, increasing resistance to overland flow paths, and selectively increasing SMZ widths in problem areas.
Therefore, an area of exploration for improvement of existing BMP’s would be to see if SMZ’s should be applied to gullies and swales, or if those areas would be better served by expanding the use of the filter strips in another part of the manual. Another would be to maximize ground cover and increase resistance in potential breakthrough areas. Based solely on sediment control, so long as potential breakthroughs are addressed, current SMZ widths appear to be justified by current literature.
Two studies are often cited to justify SMZ widths of greater than 100’, namely Jones et al (1999) and Peterman and Semlitsch (2009). However, it is important to note that Jones et al were looking at non-forest uses (primarily low-intensity cattle pasture, and row cropping), not forest harvesting. They were also looking at disturbance along riparian corridors that could be measured in kilometers, particularly 1-3 kilometers of non-forest land use along a riparian corridor. This does not correspond well with the intensity of forestry in New Jersey.
Peterman and Selitsch (2009) studied a National Forest in North Carolina, examining streams with no SMZ’s, SMZ’s of 30’ and 100’, and an unharvested control area. They found that, in terms of larval salamanders, that a 30’ SMZ was as ineffective as no SMZ, and that there were no real differences between the 100’ SMZ and the unharvested control. It is critical to note that all of the study areas had regeneration harvests on both sides of the stream. If regeneration harvests are rare in New Jersey, it should be doubly noted that regeneration harvests where both sides of the same stream are affected at the same time (or even within 20 years) are exceedingly rare. Also, in New Jersey, only two species of protected salamanders exist who exist as larvae in streams (eastern mud salamander and long-tailed salamander). It may be easier to make a species-specific recommendation on SMZ-width rather than adjust SMZ’s for the entire state.
In terms of requirements to retain trees within the SMZ, the 1995 manual has no specific requirement, except: “Protect trees that provide necessary stream bank stabilization and shade.” It would be difficult but not impossible to defend a regeneration harvest within an SMZ, but much more likely harvesting would be limited to thinning down to the “B” line on the appropriate stocking guide.
New York also has no specific targets for retaining trees within the SMZ. That state’s SMZ is divided between two zones. The first zone is 15’ to the top of the streambank, within which activities are extremely limited (although tree cutting is not excluded), and a second zone that is the balance of the width of the SMZ is which equipment use is limited. Different age classes and habitat structure is encouraged, as is tree species diversity.
Maryland and Delaware both have “minimum” basal areas of 60 sq.ft./ac., with the caveat that in Delaware it can be brought lower for salvage and sanitation cuts. Maryland requires a permit (and justification) if the basal area is brought below that figure. Pennsylvania has a two-tiered SMZ system where no tree cutting is allowed within 10’ of the top of banks and spring seeps, and 50% crown cover must remain in the balance of the SMZ.
Although the primary purpose of the SMZ is the capture of sediment, a secondary purpose is regulation of stream temperature by capturing sunlight in the forest canopy and other layers. Two questions immediately come to mind: What is the natural amount of sunlight reaching the stream/forest floor? And what is the aspect of the stream in relation to the harvest site?
Given the fire history of most of the major forest cover types in New Jersey, particularly upland oaks and hard pines, a very strong case could be made for the amount of light reaching the forest floor when a stand is at 60% relative density to be “natural.” That relative density coincides reasonably well with 60 sq.ft./ac. of basal area. (These BMP’s do not preempt existing BMP’s for Atlantic white cedar.)
Where the regeneration harvest area is to the south of a stream or other state open water (the stream is south-facing), in the northern hemisphere it is expected that more light will enter the newly-created temporary edge of the SMZ than a north-facing stream.
Clinton (2011) studied several east-facing harvest areas with no SMZ, a 33’ SMZ and a 100’ SMZ. Compared to a reference site, only the site lacking an SMZ had significantly different water temperatures. Wilkerson et al (2006) studied several sites with buffers ranging from none, 36’ and 75’ adjacent to clearcuts, as well as partial cuts where about half of the basal area was removed. Excepting those without SMZ’s, temperatures were not statistically different between pre-harvest and post-harvest conditions, even for south-facing sites. (One 36’ SMZ had an average aspect of SE.)
Moore et al (2005) conducted a review of literature in regards to riparian microclimate and stream temperature response to forest harvesting. That review suggested a buffer of one-tree-height should be reasonably effective in reducing impacts to microclimate and stream temperature. Given the information described above in subsequent studies, that one-tree-height buffer might be reasonable to protect south-facing streams from significant increases in water temperature.