Module 12: Treatments to Increase Growth, Yields, and Forest Product Quality
- competition occurs both above- and below-ground
- thinning and weed control treatments can make soil resources and light more available to future crop trees (FCTs)
- vine removal and branch pruning can improve stem form
- how tree stem growth is measured
The silvicultural treatments discussed in this chapter are applied to increase the growth rates of future crop trees. This benefit must be weighed against the financial costs of the treatment, combined with potential deleterious impacts on biodiversity, watershed function, and other ecosystem services.
We must recall that net primary productivity is influenced by climate, soil, vegetation structure, and a multitude of other factors that vary a great deal among sites in the tropics. Commercial productivity (e.g., m3 of timber/ha/yr) also varies tremendously but to some extent can be increased by the forest manager. The various stand “improvement” treatments available to silviculturalists are designed to make light and soil resources more available to future crop plants by reducing competition.
Reducing competition for light
Tree growth rates are often limited by lack of light. Species differ in their light requirements, but most canopy trees grow most quickly when their crowns are exposed to at least 30-60% full sun. Growth rates can often be predicted from the degree to which a tree’s leaves are exposed to full sun, commonly categorized into to 5 crown exposure classes: dominant; co-dominant; sub-dominant; side-lighted; and suppressed. Crown width is also a good predictor of stem diameter increment rates in many species. Finally, heavily vine-laden trees generally grow slowly at least partially due to shade cast by vine leaves. Silvicultural treatments that result in increased crown exposure to light, more room for crown expansion, and decreased vine cover can all increase tree growth by reducing competition for light.
Reducing below-ground competition
Although root interactions are not obvious, many studies have demonstrated substantial growth responses from reduced root competition for water and soil nutrients. Graminoids may be particularly effective below ground competitors relative to their above ground biomass because they generally produce large numbers of very fine roots with high root length densities (cm root/cm3 soil) in the upper portion of the soil profile. Vines may also be serious below ground competitors with trees because they too produce mostly narrow diameter roots that rapidly colonize nutrient-rich patches in the soil. Deeper in the soil root length densities are low, root competition is less intense, and water is gradually more abundant, but concentrations of available nutrients are generally lower.
In natural forest management, silvicultural treatments are seldom prescribed specifically to reduce root competition. Nevertheless, increased availability of soil resources generally results from reducing the effects of shade on potential crop trees. For example, removal of trees and vines crowding a potential crop tree also increases local availability of water and nutrients due to reduced absorption by neighbors.
Seedling establishment can be greatly enhanced by reducing local root competition. Due to their limited root surface area relative to their surface area of transpiring leaves, seedlings transplanted into forest clearings during enrichment planting operations may be particularly susceptible to competition with neighboring plants. Mechanical site preparation techniques (e.g., disking), herbiciding of competitors, and even digging especially large planting holes can all reduce transplant shock in young seedlings by increasing the easily exploitable soil volume and decreasing local root competition.
Topic 12.4: Measuring and Reporting Growth Rates
Topic 12.5: Environmental Impacts of Stand Improvement Treatments