Effects of root competition and canopy openness on survival and growth of tree seedlings in a tropical seasonal dry forest

The effects of seasonality, canopy openness and root competition were studied on seedling development of four canopy tree species, The study was undertaken in secondary dry forest in the Guanacaste Conservation Area, Costa Rica, with an annual rainfall of 1500 mm concentrated into a 6 month period. Four-month-old nursery-grown seedlings of Cedrela odorata, Hymenaea courbaril, Manilkara chide and Swietenia macrophylla were planted and monitored at one deciduous and one semi-evergreen secondary forest site. Four treatments were established in a 2x2 factorial design, combining thinning to increase light levels at the forest floor, and trenching to reduce root competition around the planted seedlings. Seedling growth occurred only in the rainy period and mortality was low during this period. Seedling mortality was high during the dry season, particularly in the first year. Seedling survival was generally lower in the deciduous than in the semi-evergreen forest site. Root trenching increased survival in the deciduous site but had less effect in the semi-evergreen site. Seedling survival was positively related to initial seedling size. Growth was positively affected by root trenching at both sites. High light levels improved seedling growth during the wet season, but had a rather negative impact on survival during the dry season causing exacerbated desiccation for C, odorata and M. chide, which both had higher survival in the unthinned than in the thinned treatments. Survival after 3 years was less than 3% for the former and about 10% for the latter species. Survival of H. courbaril and S. macrophylla was favoured by both thinning and trenching. H. courbaril had the highest survival of all studied species after 3 years (55%), followed by S. macrophylla (35%).