Wayne Clatterbuck, Professor, Silviculture and Forest Management
That forests have always been subject to disturbance tends to be the norm rather than the exception. Without human influence, forests have experienced fire, insect and disease outbreaks, drought, flood untimely frost, ice storms, and varied effects of the wind. With human influence, forests have also experienced varied cutting practices and conversion to other land uses. The longevity of forests increases the risk of forest damage through these disturbances. However, despite that disturbances are frequent and rampant on the landscape, forests persist. Forests are resilient and adaptable, but they do change based on disturbances.
Factors which affect response to disturbance include the history of the forest, the timing of the timing of the disturbance, the extent and intensity of the disturbance, and the climatic, soil and plant regenerative conditions following the disturbance.
The history of the forest can have a major impact on its response to disturbance. A mature or overmature forest responds more slowly than a younger forest. A vigorously growing forest under management generally responds better than a poorly managed degrading forest. A recently thinned forest responds differently than does an overstocked forest. Normally, the better managed, healthier forest will be more resilient in its response to disturbance than an unhealthy, poorly managed, or stressed forest, regardless of the type of disturbance.
Most natural disturbances (flood, drought, frost, ice, insects and disease outbreaks) are unpredictable. The normal energy budget for trees, and therefore the forest, involves energy initially for root growth in the early spring, followed by emphasis on shoot elongation and height growth in late spring to early summer, with stem and diameter growth in mid- to late summer, and roots having a second opportunity to grow when the photosynthetic system shuts down in late fall. Roots can continue to grow during the winter if available moisture and moderate temperatures prevail. The same disturbances occurring at different time of the year have different influences on forests because different aspects of tree growth are affected.
For example, floods that occur during the dormant season can impact root growth and tree health affecting shoot growth the following year. Floods occurring during the growing season can stress and kill trees because of oxygen being limited in the soil that is needed by roots and the respiratory system in addressing the demands of tree foliage. Late season defoliation by insects has less effect on same year's growth, but could influence subsequent year's growth. Although acorns of both the red and white oak groups fall from trees in the autumn, acorns for the white oak group germinate immediately while those in the red oak group require a period of cold stratification over winter before germinating in the spring. Disturbances in late fall could impact white oak acorn germination more than that of red oak, while the reverse is true for disturbances in the spring.
Combinations of disturbances (e.g. drought and insect attack) add more stress to the health of the forest and make the forest less able to resist further future impacts. The late season freeze during Easter weekend in 2007 when buds and leaves were expanding depleted energy reserves affecting tree health, especially when trees tried to releaf afterward. Seed production declined or was non-existent because the freeze also affected flowering. Subsequent crowns were sparse. Many trees died the following three or more years because they could not overcome the stress from the initial freeze disturbance and their inability to replace these energy reserves in subsequent years.
The growing conditions following the disturbance will affect both regeneration and growth of remaining trees. After a complete disturbance where most of the trees are felled and removed, the regeneration conditions for seed, sprouts and advanced growth (existing small seedlings) can influence the species that become the next forest. Usually those trees that are shade intolerant (ex. pines, yellow-poplar, cherry) will proliferate compared to those that are intermediate (oaks) or more shade tolerant (beech and maple). During an incomplete disturbance where some standing trees remain, the size of the gap will influence the amount of sunlight for the regenerating trees. Crowns of adjacent trees will expand to utilize the additional growing space. Wider gaps support trees that are more shade intolerant, narrower gaps support those that are more shade tolerant.
Although, we cannot control the timing, extent or intensity of natural disturbances, we should be able to assess the impacts of the disturbance and take subsequent management actions that are beneficial for future growth of the forest. Man-made disturbances should be scheduled at times to fulfill management objectives. For example, a forest should be harvested with a regeneration plan in place. Forests harvested during poor seed years may not produce a forest that is desired. If advance regeneration is present and the harvest is conducted after a good seed year, regeneration success is usually ensured. However, if the harvest is conducted in a year of poor seed production, the disturbance may compound the stresses already present and the likelihood of successful regeneration of desired species is diminished.
Forests are amazingly resilient and adaptable to a range of disturbances. Forests endure. Forests may change based on the growing and regeneration conditions before and after disturbances. Most all forests in Tennessee will incur one or more natural disturbances in our lifetime. Our ability to attend to and accept forest changes and promote healthy forests following disturbances will challenge us in taking full advantage of the benefits offered by forests.
A timber harvesting checklist
Wayne Clatterbuck, Professor, Silviculture and Forest Management
If you are contemplating a timber sale, make sure that the following checklist items are assessed prior to the timber sale.
1. Contact a knowledgeable and reliable source: A professional forester.
2. Establish property lines and determine the timber sale area.
3. Develop a forest management plan that includes assessing future regeneration.
4. Appraise the timber to be harvested to determine market value
4. Know tax codes and how your timber sale may impact your taxes.
5. Protect water quality through implementation of best management practices.
6. Formulate a written contract before the timber sale.
7. Plan for your new forest before harvesting your timber.
Consideration of these guidelines will assist in having a successful timber sale that meets your needs. For more information, refer to UT Extension publication PB1790 .
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