CANOPY DEVELOPMENT IN TROPICAL TREE PLANTATIONS - A COMPARISON OF SPECIES MIXTURES AND MONOCULTURES

In this study, the early development of monocultures and mixtures of C edrela odorata, Cordia alliodora, and Hyeronima alchorneoides was meas ured to determine what interspecific differences in structural charact eristics lead to good ecological combining ability in fast-growing tro pical tree species. Plantations were established at high density (2887 trees/ha) on fertile alluvial soil in the humid Atlantic lowlands of Costa Pica. The three species are native to the region and were select ed to represent a range of crown structure, leaf morphology, and pheno logy. Height growth was rapid for all three species, with dominant hei ghts (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, monocultu res had nearly reached their maximum leaf area index (LAI), with Hyero nima 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 wi th the tallest Cordia forming an upper canopy stratum above Cedrela an d 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 m onoculture, Cordia was taller and Hyeronima was shorter in mixture, bu t both species developed larger mean crown size and breast-height diam eters in mixture. In contrast, Cedrela was smaller in all dimensions i n mixture than in monoculture. The poor growth of Cedrela in mixture l ikely resulted from the early onset of interspecific competition coupl ed with damage from attacks of the shootborer Hypsipyla grandella. Ced rela seedlings recovered their rapid growth in monoculture after early insect attacks, but were suppressed in mixtures by the competition fr om surrounding trees of Cordia and Hyeronima which are not susceptible to the shootborer. The compatibility of Cordia and Hyeronima was rela ted to the formation of a stratified canopy in mixture. Cordia has mor e rapid seedling height growth than Hyeronima, but forms a more open c anopy with low LAI and semi-deciduous foliage, This results in the int erception of sufficient solar radiation in the upper canopy to allow h igh 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 li kely to lead to compatibility in other sets of tree species. (C) 1998 Elsevier Science B.V.