Climate-change effects on tropical lowland bryophytes – insights from a warming/CO2 experiment combined with carbon-balance modelling

Abstract Background and Aims Tropical regions may face drastic changes under climate change. Bryophytes, with already low biomass in tropical lowlands are likely to be severely affected. Their low biomass is hypothesized to result from a combination of low-light conditions in the rainforest understorey and under clouds, inactivity during daylight due to fast drying, and high respiration rates during warm nights. Warming is likely to further affect carbon balances, despite increased atmospheric CO2 and/or possible acclimatization to higher temperatures. Methods We conducted a climate-change simulation experiment in a Costa Rican rainforest understory and used the carbon-exchange model PoiCarb to simulate bryophyte carbon balances in the experimental conditions. Key Results Warming negatively affected the studied species, and increasing CO2 could not compensate sufficiently for this. No acclimation of CO2-exchange rates to increased temperature was detected. Warming increased the vapour pressure deficit in the chambers, probably aggravating the negative warming effects while not accurately reflecting future climate conditions. Modelling revealed that the reduced carbon balance under warming was primarily driven by drying, with higher respiration rates also contributing. Conclusions Warming is likely to challenge tropical-lowland bryophyte growth even more, threatening biodiversity through direct bryophyte species losses and cascading effects of decreased bryophyte abundance and diversity on other rainforest components.