| American beech
(Fagus grandifolia)
in the Hubbard Brook forest: |
American beech is one of the dominant canopy species of the northern hardwood forest found at all but the highest elevations in the Hubbard Brook Experimental Forest. It is one of the most shade tolerant hardwood species in the forest, and therefore, has increased its presence in the canopy as the forest has matured since logging in the late 19th and early 20th centuries. Beech regeneration is very persistent in the Hubbard Brook forest and beech saplings now fill the understory in most areas, excluding the regeneration of other tree species.
1.
Beech abundance in the forest community
2.
Beech regeneration
3.
Beech bark disease
4.
Beech leaves and branching pattern
5.
Beech tree rings
6.
Beech tree cross-section
1. Beech abundance in the forest community
Beech is a canopy replacing species, meaning it can regenerate
in the shade of a forest canopy, including a canopy of beech. It
commonly regenerates both by seed and by root sprouting. However,
we believe that the vigorous beech regeneration at Hubbard Brook is mostly
of sprout origin. While seedlings must rely on the development of
their own root systems, the young beech that have sprouted from the roots
of a mature tree are nourished from the much larger root system of the
parent tree. Therefore, these young trees have the resources to sustain
relatively rapid growth in their early years and quickly become healthy,
vigorous saplings.
We have conducted several collections of beech saplings since 1987 for the purposes of dimension analysis to determine their rates of growth. Distances between bud scale scars were measured to determine rates of extension growth (this will eventually link to an explanation of how extension growth is measured), and a cross section was taken at breast height (1.37 m) to determine rates of diameter growth (will eventually link to explanation) and age (will eventually link to explanation) at breast height. Our studies of beech saplings at Hubbard Brook reveal it takes an average of only 12 years for the saplings to reach breast height (1.37 m). Once the trees are 1 to 3 cm in dbh, they put on between 11 and 22 cm in extension growth each year, and an average of 0.44 mm in diameter each year.
A study of sapling population has been conducted on W6 since 1965. There was an explosion of small beech (<50 cm tall) in 1965, before the arrival of BBD, and perhaps rampant root sprouting subsequent to the arrival of BBD (we have no data between 1965 and 1977, by which time the disease was in full swing). The graph below shows beech and sugar maple stem densities for saplings between 1.5 and 9.5 cm dbh. We can follow this beech population explosion through the diameter classes. Most of the trees had reached the 2 cm diameter class by 1992, after which the density of this size class began to decline. The front of the "hump" has moved into successively larger diameter classes in successively later years, reaching the 6 cm diameter class in 1997, as evidenced by the first significant increase for that size class.
Over the same
time period, sugar maple sapling densities have been steadily declining.
Beech is very effective at intercepting light in
the understory, and we believe it is the shading effect of beech that is
the cause of this decline in sugar maple regeneration. Canopy damage
from the ice
storm of 1998 may provide a chance for the sugar maple regeneration
to recover--sugar maple can grow more quickly than beech at higher understory
light levels. However, understory beech at Hubbard Brook is extremely
vigorous and seems to be poised to take full advantage of any extra light
reaching the understory. It may not be until this cohort of beech
replaces the canopy that sugar maple regeneration has another chance to
thrive.
Note: The stems/ha scale for sugar maple is only 20% of the beech scale. Also, note the steady decline of sugar maple in most diameter classes between 1965 and 1997. |
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These photos show varying degrees of beech bark disease (BBD). The first photo shows normal beech bark which is very smooth, even on very large trees. The second photo shows isolated "cankers". The disease is a pathogen complex involving a scale insect and a nectria fungus. The insect pierces the bark to feed, creating a place for the fungus to enter at a later date. The fungus begins to grow within the bark, resulting in the round scars. The last photo is of bark severely affected by the disease, where individual "cankers" have grown together indicating that the fungus has spread completely around the bole. Fungal activity interrupts the tree's normal physiological processes, and a severely infected tree will most likely die. Trees that do not die will remain weak and become more susceptible to wind damage.
Beech bark disease first arrived in North America around 1890 in Nova Scotia. By 1932 it was present in Maine, and by the the 1970s it had swept through the Hubbard Brook forest. Typically BBD has spread rapidly through an area with an "advancing front" of the scale insect, followed by a "killing front" of heavy infestations of the scale insect and nectria fungus. Within the killing front many of the larger trees will die or be left in a weakened state. The extremely smooth bark of small young trees is less susceptible to infestation by the scale insect, so these trees often remain healthy. In the wake of these fronts is left an "aftermath zone", where populations of both the scale insect and the nectria fungus remain in reduced numbers. Although less active, BBD remains in the forest, infecting individual trees as they grow large enough to become inviting to the scale insect.
This is the white "woolly" wax that is secreted by the
scale insect in the early stages of beech bark disease.
Beech bark disease links:
4. Beech leaves and branching patterns
Beech generally spends the early part of its life in
the forest understory where light is limited. Therefore, the architecture
of saplings is adapted to these low light conditions. Branches tend
to grow outward as well as upward, and for this reason growth in the length
of the main stem is termed "extension" growth rather than height growth.
Leaves are "two-ranked", meaning they are arranged on either side of the
twig, each twig's leaves lying in a single plane. Spreading branches
with leaves in a single plane allow beech saplings to create many layers
of photosynthetic tissue oriented perpendicular to the light filtering
down from the canopy. This growth form insures that much of the light
not captured by an individual's higher layer of leaves will be captured
by a successively lower layer, making beech very efficient at intercepting
any available light that reaches the understory.
This core was
taken from the bole of a beech tree. Each ring represents a year. By measuring
the widths of the rings and multiplying by two, we can estimate the amount
of diameter growth in any given year. These rings are fairly easy
to distinguish, but the smaller rings of slower growing trees can be very
difficult to read.
This cross section was taken at breast height (1.37 m)
from the bole of a beech on the edge of a rain gauge clearing (shown actual
size). The tree was cut in October 1990 and the outermost ring was
from that year. Notice that the oldest six rings visible (at the
bottom of the picture) are relatively wide indicating a period of fast
growth. Then growth slows for the next 18 years. The fairly sharp
increase in growth in 1969-70 is probably the result of the removal of
near-by trees related to the maintenance of the rain gauge clearing.
(Bluish marks are traces of writing from a permanent marker.)
This cross section was taken at the base of a beech sapling.
(Magnification = 3.5)