2. Materials and methods2.1. Study siteSpecies productivity statistics was generated from data basesof 13 trees experimental blocks of introduced and native species, established in four ecological regions in 1994 in the Southern Region of CostaRica. Priority insites election was given to farms located within the mosaic of abandoned pasturelands within the Rio Terraba Watershed along four eco-regions.In this study an Eco-region combined climate(Hold ridge Life Zones System - [6] and soil (USDA soil taxonomic classification, [33,43] and as the two main biophysical factors that hypothetically define four environments for growth. Eco-region 1 (Tropical Moist Forest) and2 (Tropical Wet Forest) represent the most challenging environmental conditions with their ustic soil moisture regimes and lengthy dry season each year;soilsofvery low fertility, very acid, clayey texture, compacted, and after the last few decades of land use, a loss of aggregation [28]. Ecoregion 3 (Premontane Rain Forest) is dominated by volcanic soils, Andisols with loamy texture and medium tohigh fertility. Because these sitesareathig here levation, the dry season exerts much less water stress on plants. Eco-region 4 (Tropical Wet Forest) has only one experimental plot, which is located on ayoungalluvialInceptisol,withloamytexture,highfertilityand perudic moisture regime [13].A simultaneous analysis on the distribution of aboveground biomass, nutrients uptake and C content of the selected tree species was conducted in year 2000 in four experimental blocks located in Eco-region 2, around the Canton of Buenos Aires, in the Puntarenas Province of Costa Rica at approximately 500 m a.s.l. These blocks are located within the Holdridge Tropical Wet Forest Life Zone, with a mean annual temperature of 24.3 C, annual precipitation of about 3300 mm with a dry season of <3 months from January to April. The experimental blocks were established on deep residual soils, which are clayed, very acid and leached that are mapped as Haplustults and locally described to be Oxisols [28]. Soil most relevant chemical properties for the upper 15 cm layer have the following averages and standard deviation: pH in H2O 5.10 0.07, effective cation exchange capacity 3.52 2.2 in cmol kg1 and aluminum saturation 68 16 in percentage. A complete description of specific site locations and physical characteristics is found in [13].2.2. Plant materialTree species selection was based on elimination trials results of 41 native species, preference by farmers, economic value, seedling availability and previous forestry trials results obtained in other comparable regions of Costa Rica[3,9e11,14,15,23]. The selected native species for this study were: Terminalia amazonia (J.F. Gmelin) Exell, Vochysia ferruginea Mart., Vochysia guatemalensis Donn. Sm. and Hieronyma alchorneoides Allemao. Two introduced species were also selected for their economic value, proven adaptability and often excellent growth: Gmelina arborea Roxb. and Pinus caribaea Morelet var hondurensis (Barret y Golfari).
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