Aim Humid tropical alpine environments are crucial ecosystems that sustain biodiversity, biological processes, carbon storage and surface water provision. They are identified as one of the terrestrial ecosystems most vulnerable to global environmental change. Despite their vulnerability, and the importance for regional biodiversity conservation and socio-economic development, they are among the least studied and described ecosystems in the world. This paper reviews the state of knowledge about tropical alpine environments, and provides an integrated assessment of the potential threats of global climate change on the major ecosystem processes.Location Humid tropical alpine regions occur between the upper forest line and the perennial snow border in the upper regions of the Andes, the Afroalpine belt and Indonesia and Papua New Guinea.Results and main conclusions Climate change will displace ecosystem boundaries and strongly reduce the total area of tropical alpine regions. Displacement and increased isolation of the remaining patches will induce species extinction and biodiversity loss. Drier and warmer soil conditions will cause a faster organic carbon turnover, decreasing the below-ground organic carbon storage. Since most of the organic carbon is currently stored in the soils, it is unlikely that an increase in above-ground biomass will be able to offset soil carbon loss at an ecosystem level. Therefore a net release of carbon to the atmosphere is expected. Changes in precipitation patterns, increased evapotranspiration and alterations of the soil properties will have a major impact on water supply. Many regions are in danger of a significantly reduced or less reliable stream flow. The magnitude and even the trend of most of these effects depend strongly on local climatic, hydrological and ecological conditions. The extreme spatial gradients in these conditions put the sustainability of ecosystem management at risk.

Epiphytic biomass, canopy humus and associated canopy water storage capacity are known to vary greatly between old-growth tropical montane cloud forests but for regenerating forests such data are virtually absent. The present study was conducted in an old-growth cloud forest and in a 30-year-old secondary forest (SF) on windexposed slopes in the Cordillera de Tilará n (Monteverde area) in northern Costa Rica. Epiphytic vegetation in both forests was dominated by bryophytes. Epiphyte mat weight (epiphyte biomass and canopy humus) at the stand level was 1,035 kg ha -1 in the SF and 16,215 kg ha -1 in the old-growth forest (OGF). The water contents of epiphytic bryophytes in the OGF were determined gravimetrically in situ and showed maximum values of 418% ± 74 (SD)% of dry weight and minimum values of 36% ± 10 (SD)%. Maximum stand water storage of non-vascular epiphytes and canopy humus at Monteverde was estimated at 0.36 mm for the SF and 4.95 mm for the OGF. Epiphytic bryophytes exhibited more dynamic wetting and drying cycles compared to canopy humus. Maximum water loss through evaporation was 251% of dry weight (bryophytes) and 117% of dry weight (canopy humus) within 3 days of sunny weather without precipitation. Despite the high potential water storage capacity of epiphytic bryophytes and canopy humus the actually available storage is likely to be much smaller depending on antecedent rainfall and evaporative conditions.

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