Recovery of biodiversity in pure and mixed plantations with native species in Costa Rica

Mixed plantations may offer a more favorable environment for natural regeneration than pure plantations, due to their multi-strata architecture. Mixed plantations may have a higher variety of environments for seed dispersers and potentially create a greater variety of ecological niches allowing for the establishment of diverse regeneration. At La Selva Biological Station, mixed plantations with native tree species had relatively high abundance and high numbers of regenerating species in their understory, in comparison with pure plantations (Fig. 6.18). Higher plant species richness accumulated under Vochysia guatemalensis, Virola koschnyi, Termina-lia amazonia, Hyeronima alchorneoides, and Vochysia ferruginea, all species commonly planted by farmers in the region. Natural regeneration was higher in understories with low or intermediate light availability. Most of the seeds entering the open pastures were wind-dispersed, while most seeds entering the plantations were bird- or bat-dispersed. This suggests that the plantations facilitate tree regeneration by attracting seed-dispersing birds and bats into the area.

The different species of the plantations created different conditions of shade and litter accumulation, which in turn affected forest regeneration (Carnevale and Montagnini 2002). Competition from grasses is a major factor influencing woody invasion under these plantations. High accumulation of litter on the plantation floor may help diminish grass growth and thus encourage woody invasion under the species' canopies.

High establishment and maintenance costs are potential disadvantages of the use of plantations for accelerating natural regeneration, given the intensive management that is needed, especially during the first 2-3 years (Montagnini et al. 1995 a). On the other hand, silvicultural manipulations (thinning, enrichment planting) may increase the economic as well as the ecological value of the regenerating forest. In addition, the value obtained when harvesting the products at the time of thinning or at the final harvest gives an economic incentive for the establishment of these systems.

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