Mixed
Hardwoods in Northern Wisconsin
The northern mixed hardwood forest is a magnificent blend of a wide variety
of broadleaf species dominated by white ash, yellow birch, red (soft)
maple, basswood, and sugar (hard) maple. Beech may also be abundant in
eastern Wisconsin close to Lake Michigan. These hardwood species represent
a range of abilities to withstand shade with white ash and yellow birch
being the least tolerant of shade and sugar maple and beech being the
most tolerant. With some tolerance to shade in the northern hardwood tree
species, stands in this timber type may display an uneven-aged structure.
A higher degree of uneven-aged structure is typically reflected in an
increasing dominance of the most shade tolerant species, sugar maple and
beech. Younger, more even-aged northern hardwood stands often contain
a wide diversity of species with greater abundance of ash, yellow birch,
red maple, and basswood. Oak, cherry, butternut, and bitternut hickory
may even be present in northern hardwood stands before an uneven-aged
structure has developed. Hemlock is also a semi-common species associated
with northern hardwood stands that have a history of at least fairly regular
small-scale disturbance.
Northern hardwood stands are best suited to deep, well-drained, and nutrient rich sites. With decreasing site quality, size, growth, quality, and vigor of the hardwood trees decreases as well as the abundance and dominance of sugar maple and beech. More details of the growth characteristics of species in the mixed northern hardwood forest can be found in the USDA Forest Service Manual of Silvics.
Northern hardwood forests represent a wide variety of values. High quality northern hardwood lumber is economically among the most valuable forest products in North America. Northern hardwood forests also produce economically important non-timber products such as maple syrup. Northern hardwood forests are excellent habitat for a variety of wildlife including the Red Shoulder Hawk, and many people value the aesthetics of a northern hardwood forest for their fall colors.
The
variety of values represented by the northern hardwood forest is matched
by the variety of management objectives and options available to the landowner.
Nonetheless, the most common management prescription in the northern hardwoods
is single-tree selection, which seems to promote all of the values of
the northern hardwood forest mentioned above. Single-tree selection harvesting
promotes the growth, health, vigor, and quality development of sawtimber.
Selection harvesting may produce a short-term decline in maple syrup production,
but it will help maintain the long-term viability of a sugar bush. Some
preliminary research from the DNR also suggests that selection harvesting
is fully compatible with the habitat requirements of the Red Shoulder
Hawk and may actually promote their populations. Additionally, a healthy
vigorous stand of northern hardwoods maintained under selection harvesting
will produce more vibrant fall colors than a stagnant, stressed stand
not under any management control.
To the casual observer, single-tree selection systems may appear to be very simple, but reality is quite the opposite. Many people mistakenly believe that single-tree selection systems require only that a stand is periodically logged to reduce stocking by removing a selection of the largest trees, but this simplistic approach to managing northern hardwoods will slowly degrade the quality of the stand over a long-term basis. In reality, great care should be taken to maintain a stand where trees of all sizes dominate equal areas of space across a stand. For example, it takes many more seedlings to dominate an equal amount of space as one large old tree with a giant spreading crown, and seedlings underneath a large old tree don’t count, because they aren’t dominating that space. The biological justification for this goal is that while northern hardwood trees may have the ability to survive in deep shade, they will be healthiest, most vigorous, and achieve their greatest quality potential if they grow without overtopping competition. Therefore, when performing a selection harvest in northern hardwoods, the installation of canopy gaps is recommended.
Canopy
gaps require that when a large overstory tree is removed, all of the trees
that have been growing at least partially underneath the canopy of that
tree, regardless of size, are also removed. In other words, canopy gaps
are essentially mini clearcuts that are the size of the canopy of one-to-three
adjacent large trees (25-to-70 feet across). Gaps larger than this in
northern hardwood stands may actually delay regeneration of these gaps,
but larger gaps up to ½ acre should be installed for the regeneration
of hemlock. Between three to eight gaps per acre should be installed in
northern hardwood forests, and stands should be entered once every 10-to-15
years. Trees should be selected for harvest not only to remove the lowest
quality trees but also to release growing space for the highest quality
crop trees.
While uneven-aged single-tree selection systems are most common in northern hardwoods, there is a growing acceptance of the application of even-aged management strategies in northern hardwoods under some circumstances. For more details on managing northern hardwood stands, go to the DNR Silviculture and Forest Aesthetics Handbook.