The issue of sustaining productivity over successive cycles of plantation establishment, growth, and harvesting has been discussed by several authors. This may be especially critical in tropical regions with fast-growing tropical tree plantations that incorporate considerable amounts of nutrients in their biomass over a relatively short period of time. Plantation forestry can cause large reductions in soil fertility through the excessive removal of living biomass, particularly if nutrients in tree crowns are lost through harvest or site preparation (Fol-ster and Khanna 1997; Wadsworth 1997). The harvest of forest products represents a nutrient cost to the site (Wang et al. 1991). This problem can be particularly serious when plantations are established on poor soils.
Management strategies to conserve site nutrients and enhance plantation sustainability may include preferential planting of tree species that do not place high nutrient demands on the site (Wang et al. 1991; Montagnini and Sancho 1994). Large differences may exist in nutrient use efficiency among tropical tree species. For example, in studies in Puerto Rico, Wang et al. (1991) found that Casuarina spp. was twice as efficient as Leucaena spp. for nitrogen (N), three to four times as efficient as Albizia and Leucaena for potassium (K), and about twice as efficient as all of the studied species for magnesium (Mg). A major practical issue in the next few decades is going to be the question of how much wood can be produced per unit of nutrient by a given plantation species under a regime of continual exploitation. To design viable tropical plantations, focusing on efficient use of nutrients at a stand level may be as important as considering production rates (Wang et al. 1991; Montagnini 2000). Nutrient removals in harvest are especially critical for the productivity of successive rotations of plantations at the same site. Knowledge of plantation species nutrient content in plant parts can give guidance to management considerations applicable at harvest (Box 6.1).
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