By Bill Cook
Riparian zones are those transition areas between waterways and uplands. The boundaries are commonly defined by bank and shoreline characteristics, which can be variable and not always distinct.
Riparian management zones have a variety of vegetation types, but the ones of concern are the forested RMZs in a forested landscape. This definition excludes the many RMZs that lie in agricultural or urban and 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 runoff 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 demands of the 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 because forests and fish are closely linked. With forests as integral elements of healthy watersheds and the production of clean water, forestry has more influence on fisheries than fisheries management.
RMZ woodlands often have saturated soils that require special precaution during management operations. Riverine RMZs commonly accumulate silt and sediments from annual high flows and flooding that the physical structure of trees and shrubs help mitigate. 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 including otters, bats, beaver, amphibians, turtles, mink, fishers, a host of birds and many other species.
Manage with caution
Timber management can enhance all of these values, but special precautions must be considered because 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 that change along different stretches of the RMZ and therefore require each RMZ to be evaluated independently. 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 and highly-stocked RMZs provide better services when thinned, allowing more light into the system. The creation of tree snags, big “wolfy” trees and large downed logs can also be helpful.
Similar to other forested systems, disturbance is the key to regeneration and maintaining productivity in the RMZ. However, operators need to take special care to avoid damaging sensitive soils while harvesting trees to 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, which can be 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. Treetops (slash) should not be randomly left in water, although in some cases, larger-diameter wood can enhance and rehabilitate stream habitat, but in all situations 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.
Cook writes for Michigan State University Extension.