Publications
Natural Regeneration Using Seed Trees
One of the most serious forest management problems in Mississippi is a lack of proper regeneration. This is especially true on nonindustrial, private forestland holdings. Unless a stand of desirable crop trees is regenerated on these lands, all the intermediate management available is relatively useless. It all starts with regeneration—but many landowners do not realize they may have several options and that regeneration work should start long before the harvest of the current crop of trees.
The reasons for not regenerating these areas are varied, and for the most part, not valid. One of the most common reasons given is the belief that trees will regenerate themselves with no managerial input. This is true, but without active regeneration efforts, we have no control over what species will occupy the site—or how long it may take for trees to cover the area.
Another major concern in regeneration is the cost. While money certainly is a valid concern, it should not be prohibitive to all regeneration options. When landowners learn the cost of site preparation,seedlings, and planting could be $150 to $200 per acre,many of them immediately stop considering any regeneration work. Does regeneration always cost so much? The answer is “no,” but the alternatives to high-cost systems require planning and effort from the forest manager or landowner before harvest—not just after the timber is cut.
The landowner may have alternatives for regeneration. These alternatives will be determined by the following:
- what is occupying the site currently
- what is desired on the site
- when regeneration efforts will begin
The key to regeneration is to manage your stands so that you have alternatives, then use the most cost-effective system for the desired crop.
Seed Tree Regeneration
The seed-tree system is a harvest/regeneration activity in which an appropriate number of individual trees are left across an area to provide seed for the production of the next crop of trees. This method is perhaps the best choice for the natural regeneration of species that develop well in full sunlight. (This is a great choice for southern pines.)
In terms of earlier criteria, it is important to evaluate any given stand to determine whether or not seed-tree cutting is a good alternative. If you have the desired species of adequate quality, then with proper management, seed-tree cutting can work successfully on most land holdings. Research has shown that seed-tree cutting does not improve desirable hardwood regeneration in most situations.
Preharvest Activities
For seed-tree cutting to work well, you must control
undesirable vegetation to allow the desirable seedlings
to establish. This work usually starts 4–5 years before
harvest, with annual or biannual prescribed burns (in
the pine stands) that will give some control of undesirable
species and also prepare a seedbed. It is important
to remember that most light-seeded species
require the mineral soil to be exposed to have proper
germination and seedling establishment.
Injection or other timber stand improvement (TSI) methods should be used on larger stems that cannot be controlled with the burning unless they (larger stems) are to be sold in the harvest and plans are made for later control of undesirable sprouting. When the preharvest preparation activities are complete, the landowner is ready to mark the trees to be left for seed production.
Desirable Seed Trees
A landowner wants to leave the absolute best trees of
the desired species on the site. The following characteristics
are desirable for all seed trees: tall, straight,
well-pruned, well-developed crown, average seed production,
disease free, superior volume growth, and
wind-disseminated seeds. Volume production is
extremely important and, since most species used with
this method develop in even-aged stands or even-aged
groups, trees can be compared to others in the immediate
area to evaluate their total growth and development.
Historically, landowners were not actively involved in selecting seed trees, and poor-quality stems were left, resulting in poor regeneration from application of the method. Overall, you must leave the best trees available on the site. If you do not have these “superior” trees available on the area, use another form of regeneration to start the next stand. With proper management and careful thinning during the next rotation, seed-tree cutting should be a viable option when regeneration is next required.
How Many Trees to Leave?
The number of required trees is a function of the following:
- Desired amount of regeneration
- Seed production by individual tree
- Expected seed survival
- Height of trees
- Projected tree mortality (loss)
Critical Facts
- Average number of cones per bushel (can count
conelets to estimate seed production for the following
year)
Loblolly = 35
Longleaf = 25
Shortleaf = 40
Slash = 30 - Average number of seeds per bushel of cones
Loblolly = 18,000
Longleaf = 4,000
Shortleaf = 34,000
Slash = 9,500 - Seed survival
1–3 percent of sound seed should produce seedlings (varies due to site, seed, drought, insects, predation) - Seed distribution
Southern pines will distribute seeds a distance of twice the tree’s height.
Example:
How many loblolly pines should be left to regenerate
an area if seed production is expected to be one bushel
per tree and average tree height is 75 feet? Number of
seedlings desired = 2,000–3,000/A
Number of seed produced/tree = 18,000
Expected seed survival = 2 percent
Seedlings produced per tree = 18,000 x 0.02 = 360
Number of trees required = 2,000 v 360 = 6 trees/A *
Height requirement = no restriction
* You may want to leave an extra two to four trees per acre for potential loss (windthrow, bark beetles) or seed-crop reduction.
Distribution of Trees
Select and mark trees with adequate distribution to
provide seeds fully to the entire area. The direction of
the prevailing wind at the time of seed fall dictates (to
a large extent) the distribution and location of the
seed trees.
Trees should be left along the boundary that is on the side of the source of the prevailing wind. Trees on the area to be harvested should be left along this boundary; or, if the adjacent stand of trees is deemed of acceptable quality, the landowner could expect seeds to blow into the regeneration area from that source. Conversely, no trees are needed on the edge opposite from the direction of the prevailing wind.
Perhaps the easiest way to approach the problem of seed-tree distribution is to consider them in terms of rows that run perpendicular to the prevailing wind. For loblolly pine, these rows can be spaced approximately two times the height of the trees themselves. For example, if the average height of the seed trees is 75 feet, the rows of these seed trees should be about 150 feet apart. In terms of positions within the rows, seed trees are typically spaced a distance apart that is equal to the height of the trees. So, in the example of 75-foot trees, the distance between the trees within these rows would be 75 feet.
Use with Hardwoods
A basic premise of seed tree operations is that the seed
will be spread across the harvested area by wind. For
this to work, the trees have to be a light-seeded
species.
Most light-seeded hardwoods are not desirable for timber management, but this method could be used for a few, including yellow poplar, green ash, cottonwood, and sycamore. However, most of our hardwood species that are desirable for timber production and/or wildlife habitat are heavy-seeded.
Species such as oaks and black walnuts have seeds that are far too heavy to be spread by the wind. For this reason, seed tree harvesting is not a good choice for regenerating most of the desirable Southern hardwood species.
Postharvest Activities
After harvest activities are completed, another prescribed
burn helps reduce the debris load on the site
and make final preparations of the seedbed. It is
important to note that this burning should be before
seed fall, which typically occurs in late October or
November in Mississippi.
Evaluate seed-tree areas during the winter following the first postharvest growing season. By doing this work in the winter, it is easier to see the tiny green seedlings, since they are contrasted against the dormant, brown vegetation on the area. It is also easier to access and move across the site.
As soon as adequate regeneration—in terms of density and stocking—is established, remove the seed trees. If seed trees are not removed, they will have a negative impact on the development of the established regeneration. They also will continue to disseminate seeds across the area, and that probably will cause an overstocking of seedlings.
On small parcels of land, it may be difficult to market these seed trees. It is essential to remove them or even cut them and leave them, if necessary, to promote the development of the next crop of seedlings.
If adequate regeneration is not present after 2 years, you need to burn the site to control undesirable vegetation and prepare a new seedbed. This could happen if your planned regeneration year turns out to be an extremely droughty period, or if an unexpected flood takes the seeds away from the area, but that is not usually the case.
Application of the Method
Most forested areas in Mississippi could be successfully
regenerated using the seed tree method, but there
are exceptions. Seed tree regeneration should not be
used in areas that flood frequently, on those areas with
steep topography, or on shallow soils.
On areas that flood frequently, the seeds will be washed away, and areas with steep topography may have heavy rainfalls that cause the seeds to wash down-slope. In the areas where seeds collect, seedling density is unacceptably high and low density of seedlings is observed across the remainder of the area. Areas with extremely shallow soils will suffer from excessive damage to seed trees due to windthrow.
Remember, these trees are highly vulnerable to the forces of nature once they are left on an area with no surrounding vegetation. This method works best in areas of fairly level topography with adequate soil moisture during the growing season for seedlings to survive and develop.
Summary
Overall, seed-tree regeneration presents an excellent
opportunity for most of the forestland owners in
Mississippi who want to grow pine or other lightseeded
species. It is important to remember two major
items before you attempt to use seed-tree regeneration.
First, regeneration is not free. It is much less expensive than many of the artificial regeneration alternatives, but the prescribed burning and timber stand improvement work cost money. In addition to the out-of-pocket costs, the timber sale revenue may be less, since the area was not clear-cut. The latter cost or loss of revenue may be small and/or negligible in many cases, but it should be critically analyzed.
Second, landowners should not try to use this method if they currently do not have a good quality stand of trees on the area. Trying to take a poor-quality stand and regenerate a high-quality stand using the seed-tree method typically results in disaster.
The seed tree method has limitations, but it also presents great opportunities. Working within those limitations and realizing the opportunities benefit the forest resource of Mississippi and the landowners who properly apply the method.
By Dr. Andrew W. Ezell, Professor and Head, Forest and Wildlife Research Center.
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Publication 1816
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture.
Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director
(POD-02-11)