Broad-scale and long-term forest growth predictions and management for native, mixed species plantations and teak in Costa Rica and Panama

Native tree species and species mixtures are key elements for biodiversity conservation by forest plantations. Yet, introduced species planted in monoculture still dominate plantation forests in many regions around the world and especially in the tropics. In Costa Rica and Panamá, Tectona grandis (teak) is the most planted species, occupying 49% and 64% of the forest plantation area. Here, we analyzed growth performance of four neotropical native species (Dalbergia retusa, Dipteryx oleifera, Hieronyma alchorneoides, Vochysia guatemalensis) in mixture with teak or as an alternative to teak plantations. For the first time, we parametrized a forest growth model for these native, neotropical tree species that was based on a large database which covers different climate, soil and management conditions as well as prolonged monitoring. We parametrized the mixture version of the process-based forest growth model Physiological Principles Predicting Growth (3-PGmix). We then developed management scenarios for pure and mixed plantations, where we simulated mixtures of T. grandis with D. oleifera and D. retusa as well as a mixture of D. oleifera, H. alchorneoides and V. guatemalensis as an alternative to teak plantations. With average 9% of model error, 3-PGmix qualified as a tool for making growth predictions for these native tree species and teak. Except for the very fast growing V. guatemalensis, growth of the native species at harvest age was lower than previously suggested by other studies in pure plantations. With two exceptions, all species showed good growth in mixture and a potential for implementation. Mixture simulations were highly sensitive to the 3-PG fertility rating input parameter. In all our simulated mixtures, we observed a trade-off between volume production and dbh growth, where dbh growth was inversely related to the tree density of the most productive species in the mixture. For mixtures of natives with of a close to baseline thinning T. grandis plantation, diameter growth of the natives was strongly reduced by the presence of teak. When comparing the tree-by-tree mixtures to their respective monocultures, the time of comparison was important, since, in monoculture, fast-growing species can be replanted during the long rotation periods of the slower growing species. This can reverse findings of increased volume production during early ages of plantation development.