TROPICAL FOREST STRUCTURE AND COMPOSITION ON A LARGE-SCALE ALTITUDINAL GRADIENT IN COSTA-RICA

1 Forest inventory data were collected in 1988-89 from permanent plots in undisturbed tropical forest along an altitudinal transect on the n orthern slope of Volcan Barva, Costa Rica. Plot altitude ranged from 3 0 m at the base to 2600 m near the summit. 2 A total of 14 plots with a total area of 23.4 hectares was censused; all stems greater than or equal to 10 cm d.b.h. were tagged, identified, mapped, and measured in diameter and height. 3 Altogether 11 478 live stems greater than or e qual to 10 cm d.b.h. were encountered in the 1988-89 census, represent ing 561 species in 91 families, 4 Canopy height was greatest at 300 m, decreasing both above and below that altitude, and reaching a minimum at the summit. Mean stem diameter remained constant from the base of the gradient to 1500 m a.s.l., increasing slightly at higher altitudes . Large-diameter trees were least abundant at middle altitudes. Basal area was greatest near the summit, exceeding 40 m(2) ha(-1) in the two highest plots. 5 Diversity was highest at 300 m, with 149 species and 55 families per hectare. There was a progressive decrease both above and below this altitude in species richness, species diversity, number of families, and the number of species per family. Diversity was lowe st at the summit. 6 Five life-forms were recorded: dicot trees compris ed 78.0% of stems; palms 14.9%; tree ferns 5.2%; lianas 1.5%; and hemi -epiphytes 0.4%. Life-form distribution varied markedly over the gradi ent. 7 Species composition varied continuously with altitude, as shown by a detrended correspondence analysis ordination of data from 375 20 -m x 20-m subplots. There were no discontinuities, nor evidence of dis crete floristic zones. 8 No Species was distributed over the entire 26 00-m range of altitudes. The species with the greatest amplitude, Ardi sia palmana, occurred over 75% of the gradient (a range of around 2000 m). A total of 203 species were recorded from only a single 1-ha plot . 9 The altitudinal range of species did not vary with altitude: the r ange of lowland species was similar to that of montane species, Specie s of high-diversity assemblages (encountered at low altitude) were sim ilar in altitudinal niche breadth to species of low-diversity assembla ges (found at high altitude).