Biomass and water storage dynamics of epiphytes in old-growth and secondary montane cloud forest stands in Costa Rica

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 wind-exposed slopes in the Cordillera de Tilarain (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.