Studies on experimental mixed plantations at La Selva Biological Station Costa Rica

A number of studies have been conducted in experimental plantations of native species in mixed and pure-species designs at La Selva Biological Station in the Caribbean lowlands of Costa Rica (Fig. 6.5). At 2-4 years of age, mixed plantations had greater growth and lower pest damage than pure stands for three of the twelve species tested, and there was neither damage nor differences between pure and mixed conditions for the other species. The costs of plantation establishment were lower for the slower-growing species in mixed than in pure stands (Montagnini et al. 1995 a). In comparison with pure stands of the fastest-growing species, mixtures had relatively high yields, with values similar or even higher than pure stands. When plantations were 9-10 years of age, most species had better growth in mixed than in pure-species plantations. However, the slower-growing species grew better in pure than in mixed stands, both on an individual tree and on a stand basis. Mixed plantations (combinations of three to four species) ranked among the most productive in terms of volume (Piot-to et al. 2003 b). Studies on soil fertility carried out in the same experimental setting showed that mixed plantations had a more balanced nutrient stock in the soil: 4 years after planting, decreases in soil nutrients were apparent in pure plots of some of the fastest-growing species, while beneficial effects such as increases in soil organic matter and cations were noted under other species. The mixed plots showed intermediate values for the nutrients examined, and sometimes improved soil conditions such as higher organic matter (Montagnini and Porras 1998). The role of pure and mixed plantations on biomass accumulation and carbon sequestration was also examined, measuring biomass of trees that were cut when performing thinnings at 3 and 6 years of age. The mixtures of four species ranked high in comparison with the pure-species plots of faster-growing species (Table 6.3). The mixtures of four species gave higher biomass per hectare than that obtained by the sum of 0.25 ha of each species in pure plots (Montagnini and Porras 1998; Shepherd and Montagnini 2001).

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