Aquatic bryophytes in Himalayan streams: testing a distribution model in a highly heterogeneous environment

1. Aquatic bryophytes were sampled from 108 streams spanning over 3000 m of altitude in four regions of Nepal. Richness, cover and community composition were related to physicochemistry using multiple regression, DECORANA ordination and TWINSPAN. The performance of a hierarchically scaled descriptive model, developed in New Zealand for predicting bryophyte distribution, was examined in this highly heterogeneous Himalayan region. 2. Community composition and cover varied highly significantly with altitude, streambed stability and alkalinity, with evidence of effects of riparian land use on bryophyte cover. Cover was greatest in streams at low to middle altitudes with steep slopes (> 15°), high stability and low conductivity (< 60 μS cm^-1), where communities were dominated by two Isopterygium spp., two Philonotis spp., Mnium punctatum and Lejeuneaceae. 3. Richness, by contrast, increased significantly but weakly at high altitude and moderate stability, where streams were dominated by Eurynchium praelongum, Rhynchostegium spp., Fissidens grandifrons and Hygroamblystegium spp. Richness and cover were lowest in unstable streams at the lowest altitude, where no single taxon was consistently most abundant. 4. Although these results were similar to those in the descriptive model developed for bryophytes in New Zealand, subtle differences were apparent. Substrate size, although influencing the presence of bryophytes in New Zealand streams, appeared to be unimportant in Nepal. By contrast, streambed stability was more important in Nepal than New Zealand, perhaps reflecting pronounced monsoonal floods, and subsequent increased frequency of bed movement in the former. A suggested habitat template indicates that large plant size and vegetative reproduction may be responsible for the widespread distribution of some species, even into unstable Himalayan streams.