Leaf defoliation of tropical dry forest tree seedlings - implications for survival and growth

The effect of herbivory on survival and growth of seedlings of four species, Cedrela odorata, Hymenaea courbaril, Manilkara chicle and Swietenia macrophylla, was studied in secondary dry forests in Guanacaste, Costa Pica. Potted seedlings were planted at two sites 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. C. odorata and S. macrophylla were repeatedly severely defoliated by insects, while H. courbaril became less affected. M. chide did not show any significant signs of defoliation and was not further analysed. Defoliation levels were generally higher in both thinned and trenched treatments, and also positively correlated with larger initial seedlings sizes. Decreased growth rates caused by defoliation were seen in S. macrophylla and C. odorata in the growing season. Defoliation of more than 50% in combination with abiotic factors, particularly drought, were sufficient to contribute to retarded seedling development and increased seedling mortality of C. odorata and S. macrophylla.