3. RESOURCE LIMITATIONS REDUCE PRODUCTIVITY

What are the limitations to pine production in the southeastern U.S.? It is clear that leaf area levels are low enough in many stands that the capture of light is restricted resulting in poor productivity (Vose and Allen 1987, Albaugh et al. 1998, 2004, Jokela and Martin 2000). In addition, growth efficiency, the production of stemwood per unit of leaf area, is also less than optimum. From a resource availability perspective, water availability, whether too little or too much, has historically been considered the principal resource limiting pine productivity in the South. While, this is true for most sites for young seedlings (Dougherty and Gresham 1988) and for specific soil types (e.g. very wet or very dry soils) throughout the rotation, recent analyses suggest that chronically low levels of available soil nutrients, principally nitrogen and phosphorus, and additionally potassium and boron on loamy or sandy soils, are more limiting to growth in established stands than water limitations (Albaugh et al. 1998, 2004, Sampson and Allen 1999, Jokela and Martin 2000). Both water and nutrient limitations can reduce leaf area through reduced foliage production or early senescence and they can also affect growth efficiency through effects on photosynthesis and carbon allocation. Water availability is thought to have less effect on leaf area than nutrient availability because most leaf area production occurs in the spring when soil water availability is high and evapotranspiration demand is low. In contrast, water availability is thought to have a greater effect on growth efficiency because photosynthesis of existing leaf area can be reduced by drought during summer months when soil water availability may be low and evapotranspiration demand is high (Albaugh et al. 2004, Sampson and Allen 1999).

The key to optimizing leaf area and growth efficiency and thereby achieving optimum value is the development and implementation of site specific silvicultural prescriptions. Depending on the site and market conditions, silvicultural treatments may include planting high quality seedlings or plantlets from the best families of the right species to a site where competing vegetation has been suppressed and where the soil may have been tilled and/or fertilized to improve early tree growth. Application of appropriate silvicultural treatments does not stop with plantation establishment. Optimum value production on most sites will only be possible with competing vegetation suppression, repeated nutrient additions, thinning, and pruning – treatments that provide needed resources to the appropriate number of quality crop trees. Forest managers are now recognizing that intensive plantation silviculture is like agronomy; both the plant and the soil need to be actively managed to optimize production. Fortunately, financially attractive options exist to increase production through the management of both genetic and site resources. Improving stand nutrient supply through fertilization is a very attractive silvicultural option as nutrient limitations are very widespread.

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