Canopy development in tropical tree plantations: a comparison of species mixtures and monocultures

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In this study, the early development of monocultures and mixtures of Cedrela odorata, Cordia alliodora, and Hyeronima alchorneoides was measured to determine what interspecific differences in structural characteristics lead to good ecological combining ability in fast-growing tropical tree species. Plantations were established at high density (2887 trees/ha) on fertile alluvial soil in the humid Atlantic lowlands of Costa Rica. The three species are native to the region and were selected to represent a range of crown structure, leaf morphology, and phenology. Height growth was rapid for all three species, with dominant heights (mean height of tallest 20% of trees) of monocultures reaching 7–8 m at age 2.0 yr, and 14–15 m at age 4.0 yr. At age 2.0 yr, monocultures had nearly reached their maximum leaf area index (LAI), with Hyeronima forming a denser canopy (LAI of 4.5) than the other two species (LAI's of 2.5). In mixtures, a partially stratified canopy developed with the tallest Cordia forming an upper canopy stratum above Cedrela and Hyeronima. Mixtures reached an LAI of 3.9 by age 3.0 yr, approaching the level of the Hyeronima monoculture. Compared to their growth in monoculture, Cordia was taller and Hyeronima was shorter in mixture, but both species developed larger mean crown size and breast-height diameters in mixture. In contrast, Cedrela was smaller in all dimensions in mixture than in monoculture. The poor growth of Cedrela in mixture likely resulted from the early onset of interspecific competition coupled with damage from attacks of the shootborer Hypsipyla grandella. Cedrela seedlings recovered their rapid growth in monoculture after early insect attacks, but were suppressed in mixtures by the competition from surrounding trees of Cordia and Hyeronima which are not susceptible to the shootborer. The compatibility of Cordia and Hyeronima was related to the formation of a stratified canopy in mixture. Cordia has more rapid seedling height growth than Hyeronima, but forms a more open canopy with low LAI and semi-deciduous foliage. This results in the interception of sufficient solar radiation in the upper canopy to allow high productivity of Cordia, and yet adequate transmission of radiation to the dense, evergreen crowns of Hyeronima for rapid growth of that species in the lower canopy. This combination of characteristics is likely to lead to compatibility in other sets of tree species.









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