Early growth of native and exotic trees planted on degraded tropical pasture

Our goal was to assess the potential of two exotic and five native tree species in reestablishing trees to degraded land in the humid tropics of southern Costa Rica. Our study site was deforested in the 1950s. Steep topography and high rainfall led subsequently to severe erosion, which exposed nutrient-poor, acid, unproductive subsoil. Our site is typical of many tropical overgrazed pastures that have been established in high rainfall areas on ultisols worldwide. Restoration of useful trees to such lands is difficult when erosion is severe. We established 30 experimental blocks in 1994 across our study site that varied in topography and degree of erosion. We analyzed survival over 7 years and height after 3 and 7 years. Pinus tecunumanii, non-native to Costa Rica but native to Central America, was the outstanding performer with best growth and survival among all blocks. Several of the Costa Rican natives, all considered valuable for timber, survived and grew moderately well (Vochysia guatemalensis, Terminalia amazonia, Calophyllum brasiliense). We assessed the effect of erosion on survival and growth. Survival and/or growth negatively correlated with degree of erosion in five of the seven species. P. tecunumanii, an ectomycorrhizal species, survived and grew uniformly well and independently of erosion. T. amazonia survived better but grew worse on more eroded sites. These results suggest the following recommendations for projects that attempt to restore trees to tropical degraded cattle pastures similar to our site: areas that have suffered less extreme erosion could be planted successfully with three of the native species studied here: V. guatemalensis, T. amazonia, or C. brasiliense, perhaps in mixtures; P. tecunumanii is suitable for planting on the most deeply eroded, bare areas and therefore might ameliorate hostile micro sites more quickly than other species; our results suggest that the pine should be tested as a site-preparing species by intermixing seedlings of more shade loving species two or more years after planting the pine; a system of crop rotation might be sustainable, planting pine as a rapidly growing frequently harvested species and interplanting more slowly growing but more valuable species.