Dokument

Summary:
Elevational gradients in tropical mountains are suitable systems for studying spatial variations in plant diversity. Due to their great abundance, diversity, and sensitivity to environmental changes, bryophytes are appropriate organisms to explore relationships between diversity patterns and environmental fluctuations. The present study undertakes an analysis of bryophyte diversity and its functions. Moreover, it evaluates the importance of considering bryophytes on different substrates to assess the effects of the microenvironment on the distribution of diversity. The study addresses the following specific questions: 1. How does bryophyte species diversity change with elevation, and how elevational patterns differ between substrate types? 2. How do the community composition and beta diversity of bryophytes on different substrates vary along an elevational gradient? How does elevation influence species association for a particular substrate type along a mountain slope? 3. How do bryophyte biomass and water-holding capacity change with the increase in elevation while accounting for the effect of bryophyte substrates? The variations in the aspects of diversity and ecosystem functions were assessed along an elevational gradient on the Baru Volcano, Panama. Eight study sites were established from 1900 m to 3300 m, with elevational intervals of 200 m between sites. At each elevation, forest structure and climate data, as well as cover per bryophyte species from six substrate types in 600 cm2 plots were recorded. From these plots, bryophyte samples were collected, deposited in plastic bags, and transported to the laboratory where biomass and water-holding capacity were determined and early stages of species identification were carried out. The obtained results revealed that: i) bryophyte species richness consistently decreased towards the highest elevation; ii) elevation explains bryophyte community composition along the whole elevational gradient, while substrate types explain variations in short elevation ranges; and iii) bryophyte biomass and water-holding capacity consistently increased towards the highest elevation. The present work demonstrates that bryophytes respond to the environmental variations drawn by a tropical elevational gradient, varying in species richness and community composition. Total richness of species at different elevations and substrate types decreased with increasing elevation. Species richness patterns were dependent on the scale of analysis, and substrates differed from each other only when considering total number of species aggregated per plots. The pattern of decrease in species richness was related to a gradual change in the composition of the communities. Changes in community composition were mainly explained by elevational variations and to a lesser extent by differences related to substrate types. Different substrates were more crucial in explaining community composition only in short elevational ranges (the four lowest and four highest elevations). Environmental aspects related to a transition zone of forest vegetation at 2500 m were associated with high rates of species turnover and differentiation between communities from the higher and lower area of the mountain. The continuous change of species along the gradient induced a change of typical species per type of substrates and within each elevation. Community turnover results in variations in ecosystem functions that bryophytes perform along the elevational gradient. Bryophyte biomass and its water-holding capacity increased towards higher elevations. Being the terrestrial communities those that registered higher water-holding capacity. Considering different substrates is relevant in the analysis of the bryophyte diversity since each of these micro-environments provides with different extent of information on the richness of species, composition of communities, and functions within the ecosystem. Species turnover induced a high ecological differentiation between lowest and highest elevation communities, causing modifications even in the association of species for a specific substrate. Bryophyte ecosystem functions varied with elevation due to changes in biomass, with different intensity in each substrate. Consequently, epiphytic and terrestrial bryophyte communities performed functions to different degrees within the mountain. Besides, these functions are performed by different communities at both ends of the gradient and also with varying effectiveness. Modifications in the climate, such as those expected under climate change scenarios, would imply changes in different aspects of bryophyte diversity and their functions within the mountain ecosystem. If substrates differ in their elevational patterns of species richness, changes in substrate availability present an additional pathway for the climate to shape the diversity of bryophytes on tropical mountains. Therefore, a better understanding of the spatial variation in bryophyte diversity in these mountains is essential to elucidate the effects of environmental change on this crucial group of plants and its implications for ecosystem functioning. Our data suggest that considering the elevational gradient in the tropical mountain is key to the conservation of diversity and maintenance of ecosystem productivity.

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