Early growth of native and exotic trees planted 4 on degraded tropical pasture

11 12 Abstract 13 14 Our goal was to assess the potential of two exotic and five native tree species in reestablishing trees to degraded land in the 15 humid tropics of southern Costa Rica. Our study site was deforested in the 1950s. Steep topography and high rainfall led 16 subsequently to severe erosion, which exposed nutrient-poor, acid, unproductive subsoil. Our site is typical of many tropical 17 overgrazed pastures that have been established in high rainfall areas on ultisols worldwide. Restoration of useful trees to such 18 lands is difficult when erosion is severe. 19 We established 30 experimental blocks in 1994 across our study site that varied in topography and degree of erosion. We 20 analyzed survival over 7 years and height after 3 and 7 years. Pinus tecunumanii, non-native to Costa Rica but native to Central 21 America, was the outstanding performer with best growth and survival among all blocks. Several of the Costa Rican natives, all 22 considered valuable for timber, survived and grew moderately well (Vochysia guatemalensis, Terminalia amazonia, Calo23 phyllum brasiliense). 24 We assessed the effect of erosion on survival and growth. Survival and/or growth negatively correlated with degree of erosion 25 in five of the seven species. P. tecunumanii, an ectomycorrhizal species, survived and grew uniformly well and independently of 26 erosion. T. amazoniasurvived better but grew worse on more eroded sites. 27 These results suggest the following recommendations for projects that attempt to restore trees to tropical degraded cattle 28 pastures similar to our site: 29 areas that have suffered less extreme erosion could be planted successfully with three of the native species studied here: V. 31 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 33 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 35 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 37 interplanting more slowly growing but more valuable species.