Tropical bryophytes greatly increase in abundance and diversity with altitude, but the reasons for this are unclear. Wagner et al. study temperature responses of net photosynthesis (NP) and dark respiration (RD) in 18 species along an altitudinal gradient up to 1200 m above sea level in Panama. Contrary to previous suggestions, they find that optimum temperatures of NP are fine-tuned to mean habitat temperatures, and RD increases with altitude. Previously published studies show the same pattern of results, and they conclude that the scarcity of these poikilohydric autotrophs in tropical lowlands is probably due to increasing evaporation rates at lower altitudes restricting the time available for photosynthetic carbon gain, rather than because of increasing nightly respiration rates.