Mixed Species Plantations

Species diversity can contribute to protection against pests and diseases in plantations such as those of cedar and mahogany (see Chap. 2). Species diversity can also improve resource use efficiency through complementarity. Most reforestation in the tropics is with monocultures of species such as pine, eucalyptus, and teak (Evans 1992, 1999). In monoculture plantations, the main interaction between individual trees is competition for nutrients, water, and light. In mixed-species plantations, differences in utilization of resources in space and time may benefit the ecosystem through greater primary production. When mixtures combine tree species that differ in growth requirements and production, they can reduce interspecific competition and can outyield monospecific stands (Kelty 1992). Stratified mixtures that combine rapidly growing overstory species with slow-starting but higher-producing species are likely to exhibit greater total productivity than pure stands of shade-intolerant species (Smith 1986). Mixed stands can improve the survival and growth of a species in nutrient-poor soils (Matthews 1989; Binkley et al. 1992).

Mixed species plantations have been established at several locations with varying results (Wormald 1992). However, results from a number of field experiments suggest that mixed designs can be more productive than monospe-cific systems (Burkhart and Tham 1992; Wormald 1992; Montagnini et al. 1995 a; Montagnini and Porras 1998). In addition, mixed plantations yield more diverse forest products than monospecific stands, thereby helping to diminish farmers' risks in unstable markets. Farmers may prefer mixed plantations in order to diversify their investment and as a potential safeguard against pests and diseases, in spite of the technical difficulties of establishing and managing

Fig. 6.5. A 5-year-old mixed plantation at La Selva Biological Station, Costa Rica. The species are: Jacaranda copaia (the tallest trees), Vochysia guatemalensis (the second largest trees with round canopies), Calophyllum brasi-liense (smaller tree in the lower layers), and Stryphnodendron mi-crostachyum (not shown in the picture). (Photo: F. Montagnini)

Table 6.3. Biomass and carbon sequestration by 12 tree species in pure plots and in mixtures of four species each and their respective estimated rotation length at La Selva Biological Station, Costa Rica. Plantations were 6-6.5 years old at time of thinning for biomass determinations. Biomass values are means of four replicate plots per treatment (standard errors between parentheses). (Adapted from Shepherd and Montagnini 2001)

Table 6.3. Biomass and carbon sequestration by 12 tree species in pure plots and in mixtures of four species each and their respective estimated rotation length at La Selva Biological Station, Costa Rica. Plantations were 6-6.5 years old at time of thinning for biomass determinations. Biomass values are means of four replicate plots per treatment (standard errors between parentheses). (Adapted from Shepherd and Montagnini 2001)

Aboveground biomass

Mean annual Mean annual Estimated

stem carbon rotation

Stems Total

increment sequestra- length (Mg/ha/yr) tion (years) (Mg/ha/yr)

0 0

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