Effects of site management operations on the nutrient capital of a eucalypt plantation system in South Africa: scientific paper

Abstract

The Karkloof Project is a case study of the effects of intensive site management operations during the interrotational period, on (a) the nutrient capital of the system, and (b) the availability of growth resources (nutrients and water) in a commercial Eucalyptus grandis stand in South Africa. This paper specifically focuses on the nutrient contents in various pools of the system, namely the soil, the forest floor, and the above- and below-ground biomass. The effects of nutrient removal through harvesting operations, slash management or slash burning were examined in relation to estimates of readily plant-available nutrient pools in the system. The removal of individual nutrient elements through harvesting plus slash burning was calculated for a regime of one planted crop followed by two coppice crops. In this regime, slash burning (if used) and fertilization are normally only implemented immediately before replanting. The combined losses of harvesting and burning (averaged per crop cycle) amounted to 13, 25, 11, 5 and 3% of the readily available pools for N, P, K, Ca and Mg, respectively. The system is thus well buffered against the depletion of most macronutrients over the short term. Despite this fact, the cumulative effect of nutrient removal through successive rotations could add up to substantial amounts over long periods of time. Nutrients removed from the system need to be replenished to ensure sustained productivity in perpetuity. The comparatively large percentage loss of P is small in actual quantity (ca. 20 kg ha-1 per crop cycle) and is commonly mitigated to some degree by recommended fertilization practices. Losses of Mg are very small relative to available Mg pools. However, N, K and Ca losses are not replenished under current management regimes and management will need to deal with this to ensure long-term ecological sustainability. Strategies to ensure sustainable supplies of these nutrients are discussed.


Keywords: Intensive silviculture, Eucalyptus grandis, Harvesting, Slash burning, Fertilization


Southern African Forestry Journal No.199 2003: 15-26