Aspen and White
Birch
Both aspen and white birch are sun-loving pioneer species that grow in
even-aged stands and commonly grow together in a mixed stand. White
birch is often referred to as paper birch. When we talk about aspen, commonly
called popple, we are typically referring to quaking (trembling) aspen
and bigtooth aspen. Quaking and bigtooth aspen are very similar in how
and where they grow. The only exception is that bigtooth aspen has the
potential to produce sawlogs where quaking aspen is typically only pulpwood.
Balsam poplar, also called Balm-of-Gilead, is another related popple species
that looks similar to aspen but grows in wet soils, so management scenarios
for balsam poplar would be significantly different from those discussed
here.
Aspen and white birch can grow in a wide variety of conditions ranging
from dry nutrient poor soils to wet nutrient poor soils, but they typically
do best on nutrient rich sites with well-drained soils. From a financial
perspective, these rich mesic sites, which grow the best aspen and birch,
are typically well suited to grow species that can be considerably more
economically lucrative, such as mixed hardwoods. Nonetheless, there are
a wide variety of reasons why you might want to manage for aspen and birch
on your woodland. For example, aspen is valuable for some game species
such as grouse and whitetail deer. Additionally, birch is an increasingly
uncommon species with high biodiversity values as well as value for non-timber
forest products such as birch bark. Both aspen and birch can be desirable
for aesthetic reasons with their white bark adding color to the woodland
landscape, particularly during winter.
(Mature aspen stand – Aspen2.jpg)
Aspen
is a somewhat unique tree with an ability to aggressively sprout young
new aspen trees from existing root systems when the existing overstory
aspen trees are either harvested or knocked down by a wind or ice storm.
Entire groves of aspen trees commonly develop from a single root system.
This means that large groups of aspen trees can be essentially one organism
growing together as a clone. Therefore, any remaining aspen trees will
negatively impact the growth and vigor, as well as the abundance of new
young aspen sprouts. Even if not from the same clone, overstory shade
has a negative impact on the development of aspen seedlings.
White birch can also re-sprout into young new trees when the overstory trees are killed, but white birch will sprout from the stump rather than the roots, and birch does not sprout as aggressively as aspen. White birch is typically much more successful at regenerating from seed. White birch seeds germinate best on moist bare soil that has been either lightly scarified or lightly burned and in a very light partial shade from a few remaining overstory trees. Once germinated, however, shade above white birch seedlings will significantly impact their growth and vigor.
The
inability of both aspen and white birch to tolerate shade means that they
will not regenerate in the shade of their own overstory. Therefore, without
some form of significant disturbance, aspen and white birch will die-off
and be replaced by more shade tolerant species. More details on the characteristics
of aspen and white birch are available from the USDA
Forest Service Manual of Silvics.
The different strategies of aspen and white birch for regenerating new seedlings can make managing them in a mixture very difficult. Aspen is typically managed with the clearcut system while white birch is typically managed with a seed-tree or shelterwood system. The successful application of one of these management prescriptions will likely favor one species over the other, particularly over successive rotations. Aspen will sprout vigorously from a clearcut and will likely out-compete most white birch sprouts. On the other hand, successful regeneration of white birch seedlings through either a seed-tree or shelterwood will require vigorous control of other competing vegetation, particularly aspen sprouts. Therefore, most landowners are forced to choose a management strategy that will focus on one species over the other.
Aspen is fairly easy and inexpensive species to manage. It is a short-lived species that is best maintained by clearcutting while the overstory remains sufficiently healthy and vigorous to produce hearty sprouts. Failure to clearcut aspen at an early enough age will significantly reduce the growth, vigor, and abundance of new sprouts. Additionally, old-age aspen is highly susceptible to interior decay fungi, so harvesting old aspen may not produce good financial returns. The optimum age to clearcut aspen is primarily driven by site quality and ranges from about 45 to 70 years of age. Aspen will thrive to an older age on higher quality sites and bigtooth aspen will thrive to a somewhat older age than quaking aspen. Aspen is also highly susceptible to root damage and compaction, so the best regeneration results will be achieved when clearcutting is performed during frozen soil conditions. Additionally, aspen roots contain the most energy and will regenerate new seedlings best when their leaves are off, which is another reason to harvest aspen during the winter. Intermittent thinning harvests are not common in aspen.
The Zdanovecs in Marathon County manage a considerable amount of aspen on their woodlands, and their management objectives focus on wildlife, particularly roughed grouse. The Zdanovecs promote grouse habitat on their woodlands by maintaining a wide variety of ages of aspen across their property through regularly scheduled aspen clearcuts. Follow the link here for more information on managing aspen for grouse habitat.
White birch is also relatively short-lived reaching old age at between
about 60 to 80 years old depending on site quality, and higher quality
sites are able to support stands to the older end of the range. White
birch loses vigor as it approaches old age, so it will be less able to
produce large amounts of viable seed. Additionally, old-age white birch
is highly susceptible to interior decay fungi. Therefore, regenerating
a white birch stand before it reaches an excessively old age can have
important biological and economic ramifications to the landowner. If well
managed, white birch can produce valuable sawlogs. Thinning (the removal
of some of the trees to promote the growth of the remaining trees) can
begin in birch stands as the dominant trees reach 5 inches diameter (dbh).
Thinning should focus on the removal of poorly formed, unhealthy, and
smaller trees while promoting the growth of dominant, healthy, well-formed
trees. Make certain to protect residual trees from logging damage in order
to maintain their health, vigor, and highest crop tree values when they
are eventually harvested. Once an old-aged condition is reached, a seed-tree
or shelterwood harvest should be conducted to regenerate a new stand.
Shelter or seed trees should be selected based on the health of their
crown and not on the quality of the stem. In order to establish new seedlings,
scarification of the soil and control of understory competition will be
essential. Control of understory competition may require the judicious
use of herbicides.
More exact guidelines for managing aspen and white birch are available through the DNR’s Silviculture and Aesthetics Handbook.