AMELIORATION OF DEGRADED RAIN-FOREST SOILS BY PLANTATIONS OF NATIVE TREES

Empirical evidence from temperate Europe and North America indicates t hat old-field succession, resulting in reforestation and a protracted period of forest fallow, can improve soil condition on lands once badl y degraded. However, the ability of trees to ameliorate degraded sites in the tropics is widely debated. In 1988 eight native tree species: Hyeronima alchorneoides Allemao, Inga edulis Mart., Pentaclethra macro loba (Willd.) Ktze, Pithecellobium macradenium Pittier, Stryphnodendro n microstachyum Poepp. et Endl., Virola koschnyi Warb., Vochysia guate malensis J.D. Smith, and Vochysia ferruginea Mart. and three exotic tr ee species: Acacia mangium Willd., Gmelina arborea L., and Pinus tecun umanii (Schw.) Equiluz et Perry were planted on abandoned pasture land in northeastern Costa Rica cleared of rain forest 25 yr earlier. Four replicates of 0.25-ha plots of the 11 species and a control were esta blished in a randomized complete block design. All species survived fa irly well, but some grew much better than others. The soil was thoroug hly sampled before trees were planted and again 4 yr after trees were established. Although bulk density decreased significantly beneath eig ht of the 11 species, organic C increased significantly under only thr ee species. Significant increases in base cations also occurred beneat h the majority of species. Nitrogen did not increase as much as expect ed beneath the N2-fixing legumes; however, the availability of P was s ignificantly enhanced beneath both species of Vochysia. The changes in soil properties were dramatic, and significant amelioration of these degraded soils occurred in a short time beneath the majority of specie s.