Managing Riparian Forests
Article #261, November 2017
By Bill Cook
A range of human and ecological benefits come from managed forests. Michigan’s riparian forests are no exception, although these woodlands are too often left as “no management” buffer zones.
Riparian zones are those transition areas between waterways and uplands. The boundaries are commonly defined by bank and shoreline characteristics, which can be quite variable and not always distinct.
Riparian management zones (RMZs) have a variety of vegetation types, but the ones of concern, here, are the forested RMZs in a forested landscape. This definition excludes the many RMZs which lie in agricultural or urban/residential landscapes.
In addition to the usual mix of benefits, RMZ woodlands are largely responsible for supplying nutrients to the adjacent aquatic systems, protecting them from excessive run-off, and occasionally dropping trees into the water to help provide structure for fish habitat. The canopy that shades trout streams helps keep water temperatures low enough to support the high oxygen-demanding fish. RMZs are also the source of most of the nutrients that support the aquatic life.
Many water quality measures are influenced by forested RMZs. Forests and fish are closely-linked. Forestry has more influence on fisheries than fisheries management. Forests are integral elements of healthy watersheds and the production of clean water.
RMZ woodlands often have saturated soils that require special precaution during management operations. Riverine RMZs commonly accumulate silt and sediments from annual high flows. The physical structure of trees and shrubs helps mitigate flooding and high water flows. Vernal pools (temporary spring ponds) are common and particularly valuable in RMZs.
These woodlands have high levels of biological diversity and serve as important wildlife travel corridors. The ecozone between the upland and aquatic habitats will have wildlife species from both habitats. Otters, bats, beaver, amphibians, turtles, mink, fishers, a host of birds, and many other species find critical or preferred habitat conditions within RMZs.
Timber management can enhance all of these values, but special precautions must be considered. Treating RMZs as “set aside” areas may not provide the same level of ecological quality as those that are managed, similar to other kinds of forests.
Forested RMZs in forested landscapes have highly variable characteristics. These characteristics change along different stretches of the RMZ. Therefore, each RMZ needs to be evaluated. Boilerplate guidelines typically do not take into account this diversity and are poor substitutes for professional assessment.
Timber management is appropriate in forested RMZs. Long-lived conifer species should be encouraged. Progress towards later successional forest types can be accelerated. Highly-stocked RMZs provide better services when thinned, allowing more light into the system. Tree snags, big “wolfy” trees, and large downed logs can be created.
Similar to other forested systems, disturbance is the key to regeneration and maintaining productivity. However, operators need to take special care to avoid damaging sensitive soils. Tree harvesting in RMZs also may provide sufficient light to encourage certain exotic species. This is an increasing risk in most forest management systems, but can be especially harmful within RMZs.
Properly-applied forest management practices outside the RMZ, including clearcutting, have little impact on water quality measures when healthy RMZ woodlands are in place. Timber harvests can leave more canopy closer to water than farther away (called variable retention), assuming the forest type responds to this type of practice.
Good RMZ management cannot replace best management practices outside the RMZ. These water and soil quality best management practices are explained in a manual available from the Michigan DNR.
Some less-than-optimum practices include no-cut buffers, arbitrary RMZ widths, and soil rutting and compaction. Tree tops (slash) should not be randomly left in water, although in some cases larger diameter wood can enhance and rehabilitate stream habitat. Vernal pools should not be disturbed.
Lastly, human habitation often frequents RMZs in the form of homes, camps, lawns, and resorts. Drastic changes occur to woodland structure and composition. Maintaining undeveloped RMZs is becoming increasingly important, as well as better managing those developed RMZs.
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This website is maintained by Bill Cook, Michigan State Extension Forester/Biologist.
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