Climate-change effects on bryophyte carbon balances in the warm tropics: a rainforest experiment complemented by simulation modelling
Final Report Abstract
In the tropics, bryophytes (mosses and liverworts) can be very abundant in montane forests (sometimes called ‘mossy forests’), but in the lowlands they are generally scarce, both in biomass and diversity. This altitudinal pattern was already documented a century ago, but it is still unclear what limits bryophyte growth at the lower altitudes. Temperature and moisture are two factors thought to have a strong influence, which implies that climatic warming could pose further limits on bryophyte growth in the tropical lowlands, in a worse-case scenario turning these areas into ‘bryophyte deserts’. A plausible, though so far untested, physiological explanation for the lack of bryophytes in the warm tropics involves a combination of high respiratory carbon loss at night due to high temperatures and low photosynthetic carbon gain during the day due mainly to either low light intensities or fast drying. As a result of these high losses and low gains, poikilohydric organisms would have difficulties in obtaining a positive carbon balance. In this project we tested this hypothesis addressing: 1. instantaneous carbon exchange rates, 2. temperature acclimation of these rates, 2. integrated daily carbon balances, and 4. the alternative hypothesis that a lack of desiccation tolerance explains altitudinal patterns. 1. We found that optimum temperatures for photosynthesis closely matched ambient temperatures at the respective altitudes and that dark respiration and net photosynthesis rates at ambient night and day temperatures, respectively, did not differ between altitudes. So in contrast to our expectation, high respiration rates due to warm nights do not appear to be a distinguishing feature of the tropical lowland species compared to species from more bryophyte-rich lower montane areas. 2. Seeing that species are metabolically well adjusted to current local temperatures, an inability to acclimatize could lead to carbon balance problems under climatic warming and for montane species attempting to establish in the lowlands. We simulated warming by transplanting nine species to lower altitudes. Most transplanted samples died and showed no metabolic acclimation, but some samples in nearly all species fully recovered growth. This variable acclimation potential within species may allow populations to adapt and avoid displacement to higher altitudes under climatic warming. 3. Montane bryophyte species might also be excluded from the lowlands due to a lower desiccation tolerance, precipitation being more concentrated but less frequent in most lowland forests compared to montane forests. However, both montane and lowland species survived dry periods of ≥ 80 days, which by far exceeds the duration of dry periods in the wet lowland tropics. We can thus exclude that intolerance to long dry spells explains the absence of the tested montane species in the lowlands. We conclude that desiccation tolerance and temperature responses of carbon exchange rates cannot explain current bryophyte abundance patterns. To understand geographical patterns and to predict the faith of bryophytes under climatic warming we need to study longterm carbon balances. Though practically impossible to measure with sufficient resolution and repetition in the field, such balances can be estimated and explored with help of simulation models, which we have under construction. Our preliminary model confirms our expectation that the duration and timing of hydration are critically important for the carbon balance. For projections into the future, temperature acclimation of metabolic rates should also be included in such models but first needs to be studied further in the field, especially for tropical lowland species.
Publications
- 2013. Altitudinal changes in temperature responses of net photosynthesis and dark respiration in tropical bryophytes. Annals of Botany 111: 455-465
Wagner, S., G. Zotz, N. Salazar Allen & M.Y. Bader
(See online at https://doi.org/10.1093/aob/mcs267) - 2013. Desiccation tolerance cannot explain altitudinal distributions of tropical bryophytes. Journal of Bryology 35: 47-56
Bader, M.Y., Wagner, S., T. Reich, A.S. González González & G. Zotz
(See online at https://doi.org/10.1179/1743282012Y.0000000033) - 2013. The temperature acclimation potential of tropical bryophytes. Plant Biology
Wagner, S., G. Zotz & M.Y. Bader
(See online at https://doi.org/10.1111/plb.12037) - Current and future carbon balances of bryophytes in the warm tropics. International Association of Bryologists bi-annual conference (2013, London, UK)
Bader M.Y., S. Wagner and G. Zotz
