Management of site nutrients in plantation forestry

Losses of nutrients during harvest may far exceed the rate of their replenishment by weathering of minerals in soils or by input via precipitation, especially when rotations are short (Folster and Khanna 1997). However, the amounts of nutrients in various tree tissues (foliage, branches, stems) differ substantially. Results of studies of plantation biomass and nutrients in a humid tropical Costa Rican site showed that the relative tree tissue nutrient concentrations for all species were foliage >branches >stems (Stanley and Montagnini 1999; Montagnini 2000). In tropical plantations, the nutrients in the tree crowns generally account for a higher proportion of the total aboveground nutrients in the stand than do those in other compartments. Though branches and foliage summed together represented only 25-35% of total tree biomass, they generally represented about 50% of total tree nutrients (Fig. 6.2). To reduce the nutrient cost of harvests, site tree tissue biomass conservation should be prioritized as follows: (1) foliage, (2) branches, and (3) stems. Leaving branches and leaves on the site at the time of harvest, rather than harvesting the whole tree, would typically reduce the nutrient cost of log harvest by one-half. Additionally, the slash left on the ground would act as mulch, helping to improve soil conditions, and potentially increasing the number of rotations before fertilization or fallow would be necessary. The amount of nutrients represented by branches or foliage that may be left behind varies between nutrients, species, and sites. Adjusting harvest regimes in consideration of the differing nutrient contents of these tissues can be an effective means of managing site nutrients (Wang et al. 1991; Montagnini and Sancho 1994; Folster and Khanna 1997; Nykvist 1997; Stanley and Montagnini 1999; Montagnini 2000). Additional considerations for nutrient conservation would include, in the case of N, avoiding burning since N can be lost by volatilization before being incorporated into the soil.

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