干旱区生态环境知识资源中心

The relationship between diversity and composition of neotropical bromeliad communities and abiotic and autecological factors is still poorly understood. In this study, I related point diversity (mean species number per 400 m(2) plot), alpha diversity (total species number per site), representation of life-forms (epiphytes, terrestrials, saxicoles), and six ecophysiological types of bromeliads at 74 forest sites in the Bolivian Andes and adjacent lowlands to 12 environmental factors reflecting mostly climatic conditions. A total of 192 species, including 108 epiphytes, 106 saxicoles, and 49 terrestrials, were recorded. Extrapolation revealed that the actual total species number in the region is at least 24% higher than recorded, especially among terrestrial (71%) and saxicolous (40%) species. Epiphytes were more fully sampled because of their tendency towards larger range size and because they are distributed more evenly where they occur. Overall, the environmental factors explained up to 61% of the observed variance, reflecting all expected relationships such as the increase of tank bromeliads in wet habitats and of atmospheric bromeliads in arid regions. Point diversity almost always showed higher regression coefficients than alpha diversity, possibly as a result of more complete sampling in small plots and because the abundance of individual species (which influences point diversity) may be more closely related to abiotic factors than species richness. Despite somewhat lower alpha diversity in arid areas, point diversity peaked in dry habitats, presumably due to the scarcity of competing epiphytic orchids, ferns, and aroids that lack the extreme adaptation of bromeliads to drought stress and low nutrient availability. The decline of epiphytic bromeliad diversity at high elevations appears to be linked to low temperatures, particularly increasing frost frequency. It is hypothesized that the low diversity of bromeliads in humid tropical lowland forests is caused by low photosynthetic rates due to high water stress and low light availability combined with high respiration losses. In combination, these factors would impede the maintenance of a positive carbon balance.