4. WHY ARE NUTRIENT LIMITATIONS SO COMMON?

Simply, nutrient limitations develop when a stand's potential nutrient use cannot be met by soil nutrient supply. Consideration of a stand's potential nutrient use and nutrient supply indicates a large disparity between nutrient needs and supply in southern pine stands (Allen et al., 1990). Typically, nutrient availability is rather high following harvesting and site preparation as these disturbances provide suitable conditions for rapid decomposition and release of nutrients from the accumulated forest floor and slash material. Use of nutrients by crop trees is minimal owing to their small size, low leaf area, and lack of site occupancy. However, as leaf area development and stand growth accelerates, use of nutrients increases rapidly. At the same time, the supply of readily available nutrients is being rapidly sequestered within the accumulating forest floor and tree biomass. Furthermore, as the canopy closes, the environmental conditions conducive to high nutrient availability are no longer present (Piatek and Allen 1999, 2001). Consequently, a stand's nutrient requirement for maximum growth generally outstrips soil supply (particularly for N) around time of canopy closure. As nutrient supply diminishes, leaf area production and, in turn, growth become regulated (and limited) by the available nutrient pools. It is not surprising then, that the majority of field trials in intermediate-aged southern pine stands (8 to 20 years old) have shown strong responses to additions of N and P (Martin et al. 1999, Amateis et al. 2000). In young stands, the development of nutrient limitations is still possible when levels of available nutrients (particularly P) in the soil are low and the soil volume exploited by roots is small. In addition, as other silvicultural treatments (e.g. vegetation control and/or tillage) are used to improve water availability, crop tree growth and use of nutrients will be increased at young ages. Fertilization will then be needed to sustain rapid growth on all but the most fertile sites.

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