Bryophyte cover on trees as proxy for air humidity in the tropics

Climatic conditions are a prime candidate to explain local patterns of biodiversity and consequently there is great need of on-site climatic measurements. Among them, however, air humidity is notoriously difficult and time-consuming to measure, and it has been proposed that the epiphytic bryophyte cover can be used as an indicator of long-term air humidity conditions. Here we explore the utility of visually estimated epiphytic bryophyte cover on large canopy branches as a proxy for air humidity at 26 study sites in tropical forests where we measured microclimate for at least 12 months. Across all sites, bryophyte cover was weakly related to relative air humidity (R2 = 0.17), but when we separated highland (1800–3500 m elevation) from lowland (<1800 m) sites, relative air humidity showed significant and distinct relations to bryophyte cover (R2 = 0.36–0.62), whereas temperature was related to bryophyte cover only in the lowlands (R2 = 0.36). We conclude that epiphytic bryophyte cover can be used as a proxy for air humidity if temperature and elevation are taken into account within a circumscribed study region, but might not be applicable for comparisons across extensive elevational gradients or wide differences in temperature.