For as long as people have been using rivers and streams for transportation, logs and trees that had fallen into the water were considered a nuisance or a hazard. In many parts of the country, including Vermont, waterways were cleared of woody debris to allow for the easy movement of timber to the saw mills.
Up until about 75 years ago, it was common practice to harvest timber right up to the edge of a stream. While this practice kept land open for farming and provided wood for the mills, it left no trees along the stream banks. The result of our past land use is a landscape crossed by streams with little or no woody debris flowing through relatively young forests. It turns out this is not a good thing for the ecology of our streams.
Since the 1980’s scientists have been studying what happens when whole trees fall into streams and rivers. Trees in streams, called Large Woody Debris or LWD, greatly influences how a stream works and the habitat it provides for fish, amphibians and insects.
Using the stream’s natural flow regime, LWD in headwater streams: stores, sorts, and distributes sediment (sand, gravel, cobble and boulders); creates a diversity of habitat features such as pools and riffles; and provides hiding cover for stream dwelling fish and wildlife. LWD also traps and retains organic matter that falls into the stream. The organic matter is consumed by aquatic organisms while adding nutrients to the aquatic ecosystem. A large number of fish, amphibian and insect species can occupy the pools and riffles created by woody debris.
So, how much LWD should there be in a stream? Current scientific research indicates that under natural conditions, upland streams on the Green Mountain National Forest should have as many as 175-230 pieces of LWD per mile of stream.
The Green Mountain NF fisheries staff have looked at nearly 300 miles of stream habitat in more than 50 streams and found that almost every stream has less than 53 pieces of LWD per mile – only a third to a quarter of what could occur naturally. Stream habitat assessments also showed that along with the lack of LWD came a deficiency in cover, pool quantity and quality, and sometimes, high levels of sand in spawning and rearing habitats.
To address the habitat deficiencies created by the un-naturally low quantities of LWD in GMNF streams, the Forest Service has implemented habitat restoration by the placing LWD back into our streams. Placements of LWD were done in the upper sections of Griffith and Jones Brooks between 1996 and 2002. Whole trees are placed by hand or heavy equipment and anchored with cable and rebar in medium to large streams. In small stream, trees can be felled right into the channel and require no anchoring.
Habitat monitoring conducted by the Forest Service at restoration sites completed in the past few years indicates that LWD placements that approach the expected natural quantities greatly increase habitat complexity and cover, in addition to pool area and quality. Because LWD is a significant pool-forming agent in forested mountain streams, low LWD loadings result in low quantities of pool habitat that is critical to our native Brook trout.
While we have clearly be successful at increasing amounts of LWD to several streams, the real measure of success is how the fish respond to the habitat changes created by more LWD. Brook trout population monitoring in restoration sites shows that the addition of LWD increased spawning and rearing habitat, resulting in increased trout abundance. Brook and Rainbow trout abundance increased by an average of 50% in LWD project sites compared to untreated control sites across the Forest.