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A Group of Young Longleaf
Pine Trees Growing in a Forest Opening
(bolded words in text indicate key words
and concepts)
 Student
Information: If you were to look at the longleaf forest
from a bird’s perspective, it would similar to a big piece
of Swiss cheese. Over time, lightning, tornados, and hurricanes
create “holes” in the forest by killing trees. It is only
through the death of these older trees, that the younger forest
can grow.
Teacher Information: A quick
glance across the longleaf pine forest may lead one to believe that
all the trees are the same size. However, upon closer inspection
tightly clustered patches of longleaf pine seedlings will be observed.
Young longleaf pine seedlings actually take advantage of the forest
openings created by the death of adult (canopy)
trees to regenerate. These disturbances
occur somewhat regularly. Lightning kills one or two adult
trees frequently in the longleaf ecosystem while tornadoes and hurricanes
may kill hundreds of trees infrequently. “Forest gaps”
is the term that scientists give to these forest
openings. This picture shows young longleaf pine trees
accumulating in these gaps.
The reason for this phenomenon is twofold. Adult longleaf pine trees
drop seeds every few years. However, these living trees are also
dropping pine needles every day. Pine needles burn very well. Small
longleaf pine seedlings growing amongst all these pine needles have
a difficult time becoming established because frequent ground fires
sweeping through the forest burn them up. Because there would be
less accumulation of pine needles in forest gaps, fires are unable
to travel far into the opening—thus giving small seedlings
a chance to grow. You will also note how the growth of these seedlings
in these gaps appears to follow a “bell-shaped curve”.
The death of mature trees means that there is decreased competition
and increased resources available for longleaf pine seedlings. Compared
to “closed canopy” redwood, Douglas fir, or tropical rain
forests, when walking through a longleaf pine forest you will immediately
notice how open and sunny it appears. One would think that light
would not be a limiting resource
for seedlings. However, within these gaps light DOES increase as
does soil moisture and soil nitrogen (all elements needed for longleaf
pine seedlings to grow). The farther one travels into the gap, the
more of these resources are available. Seedlings in a gap center
are getting more light, soil moisture and nitrogen then ones growing
at the edge—and thus are larger.
Some of the longleaf pine seedlings clumped together in this picture
are observed in various stages of growth. Some longleaf pine seedlings
are in the grass stage. The
grass stage is a period when seedlings are not growing much aboveground
and instead are putting on a tremendous root system below ground.
In this stage, longleaf pine seedlings resemble a clump of grass—and
hence the name. Longleaf seedlings are very resistant to fire at
this stage. Trees may stay in the grass stage for several years
until enough resources become available so they can grow in height.
Other longleaf pine seedlings pictured are in the rocket
stage. This is a stage when rapid height growth occurs
in longleaf pine. For the rocket stage to start, excess resources
must become available around the seedling. By growing fast, the
tree is able to capture more resources then its neighbor, thus giving
it an advantage. Also, rapid growth allows the tree to get its top
above the frequent fires that move through the woods. Longleaf pine
remains somewhat susceptible to fire when they leave the grass stage
until they reach about 4 feet in height. Those trees that are stuck
somewhere in between the two growth stages are usually thinned out
by fire or stress over limited resources.
Key Words and Concepts (click
on for glossary definition): canopy,
competition,
disturbance,
forest
opening, gap,
grass
stage, regeneration,
resource,
rocket
stage.
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