To a forester, the methods used to harvest and regenerate a forest stand are called silvicultural systems. To a timberland owner, these harvesting and regeneration methods are management activities. A silvicultural system is a planned program to establish a new crop of trees and to enhance their growth during the production cycle.
The practices for harvesting and replacing timber stands are classified according to the type of harvest cut used to regenerate a new crop artificially or naturally.
Seed-tree Harvest
The large, mature trees are harvested in one operation to leave five to fifteen dominant, seed producing trees per acre. Seed trees should have well-developed crowns and be good seed producers. If you select seed trees several years prior to the final harvest, you can prepare them for maximum seed production by giving them more growing space for crown development.
The seed-tree system is used for light-seeded species like pine and ash that are wind firm and prolific seed producers. The site must be prepared to expose bare soil and remove all competing vegetation. You can use prescribed fire in pine stands and tree injection applications before the final crop is harvested to reduce regeneration cost substantially. If you do not remove competing vegetation before the final harvest, you may be forced to use expensive mechanical or chemical site preparation practices.
The value of the seed trees may exceed the cost of artificial regeneration and you may lose several seed trees to lightning, wind, and insects before the new crop is established. In such cases, you may be better off selling the seed trees and using the money to prepare the site for planting. Good seed crops come every two to five years so competing vegetation must be controlled before the final harvest, or retreated after the seed trees are released to assure adequate seed germination and survival.
The seed-tree system is an acceptable natural regeneration alternative for you if you apply proper management practices several years prior to the final harvest.
Shelterwood Harvest
The shelterwood harvest is a natural regeneration system in which a new crop is established under the protection of 18 to 40 overstory trees. The overstory trees are removed in a series of two or more harvest cuts. The last harvest removes the shelterwood after the new crop is well established. During the preparatory thinning, trees of sawtimber size are selectively marked and cut to favor the better, dominant, seed-producing trees that will shelter the new crop.
The shelterwood system is the most versatile natural regeneration system because you have many opportunities to market the old stand and regenerate a new crop. Trees can be removed in several thinnings over time depending upon regeneration and market conditions. The seed trees have enough volume to make logging economical. If market conditions are bad, the growth rate and volume of the seed trees are sufficient to justify carrying them for many years. There is usually enough seed distributed across a stand to assure rapid regeneration. Since the new crop is established under the old crop, there is no delay between the final harvest and the start of the new crop. In fact, the new crop may be several years old before the high value seed trees are harvested.
Advance planning for site preparation is necessary, but since the forest floor remains partially shaded during the preparatory thinning, site preparation can be accomplished before the seed fall is needed.
Single-Tree and Group Selection Harvests - The selection harvests are natural regeneration systems in which individual trees or small groups of trees are harvested to create small openings with sufficient sunlight to allow seed germination or root sprout development. Group selection is actually clearcutting on small areas that do not exceed an acre or two in size.
The selection system can be used for pines or hardwoods. Managed properly, it can be a desirable alternative for the timberland owner because frequent thinnings can provide periodic income while the cost of natural regeneration is relatively low. The major disadvantage is that harvest volumes per acre may be low which will increase logging costs. Improper use of the selection system can lead to "high-grading." Quality seed trees must be left in the openings or around the perimeter to provide seed and sprouts, while the poorest quality trees should be removed to provide space for the new crop.
Clearcut Harvest
An even-aged silvicultural system in which the old crop is completely harvested (cleared) at one time from a large area (generally more than a few acres). Regeneration of pine stands is often done "artificially" by planting or seeding. Natural seeding is possible from seed produced by trees around the clearing or seed stored in the duff-litter layer on the forest floor. Hardwood stands are regenerated naturally from seed and sprouts. A few hardwood species can be planted successfully, but they require intensive cultivation for many years.
Clearcutting is a valid management practice for species that require full sunlight for seed germination, sprout development, or seedling survival, such as the southern pines, yellow poplar, oaks, and most commercially important hardwoods. It is the only alternative in stands that do not have enough desirable species for natural regeneration. Clearcutting often requires expensive site preparation practices, but it has several advantages over other harvest methods when properly used. Large harvest volumes reduce logging costs, and this increases the timberland owner's income from a sale. Clearcutting followed by mechanical site preparation practices increases seedling survival and early growth for prompt, successful regeneration of the new crop. Planting gives the proper spacing for complete utilization of the growing space and improves the growth rate of individual trees.
Some timberland owners have clearcut their timber because they did not know about other alternatives. After a clearcut you have two choices: (1) spend a lot of money to prepare the site and replant or (2) do nothing. Clearcutting is an alternative for a timberland owner only if you can afford the cost of site preparation and planting or seeding and if you are willing to wait 12 to 15 years for the next round of income.
Site preparation costs can be reduced or eliminated if timber stand improvement practices (TSI) such as prescribed burning and cull tree removal are used prior to clearcutting. If site preparation costs were kept low, the cost of tree planting or direct seeding would not be too unreasonable. However, the proper use of selective thinning and other TSI practices can eliminate the need for clearcutting in most forest stands.
Diameter Limit/High Grading
Diameter limit harvests remove all trees larger than a specified diameter. Diameter limit cuts are sometimes used instead of marking trees and can lead to a host of problems. They in effect "high grade" the stand, removing the larger quality trees while leaving small, slow growing trees with no regard to spacing. Tracts that have been high graded a few times will have to be clearcut for any chance at producing an adequate timber crop.
