Climate-change effects on bryophyte carbon balances in the warm tropics: a rainforest experiment complemented by simulation modelling

点击翻译按钮获取中文摘要

Climate change will affect plant growth and distributions globally, but accurate predictions are difficult. For tropical trees and crops, inconsistent responses to warming are reported, while for other plants such information is largely non-existent. Bryophytes (mosses and liverworts), an important component of tropical forests showing strong elevational gradients in abundance and diversity, were the focus of our DFG-funded project Global warming effects on tropical bryophytes: will negative carbon balances turn the tropical lowlands into a bryophyte desert? We addressed a temperature-centred hypothesis which links the paucity of bryophytes in the warm lowlands to difficulties in gaining a positive carbon balance due to high respiration rates in warm nights and low daily carbon gains. We also studied the effects of future warming, implied in this hypothesis, for montane bryophytes. Our most important findings were: 1. Temperature responses of photosynthesis and respiration appear well-adapted to current local conditions, so that CO2-exchange rates per se cannot explain the elevational bryophyte abundance pattern. 2. Montane bryophytes transplanted to lower, warmer elevations did not measurably acclimatize CO2-exchange, which was contrasted by long-term survival of samples from various species. Results on acclimation were thus not conclusive. 3. Desiccation tolerance was high in montane as well as lowland bryophytes and cannot explain elevational patterns and should not become limiting under mild climate change. We conclude that neither desiccation tolerance nor metabolic temperature responses can explain current bryophyte abundance patterns in the tropics. More likely, the timing and duration of moss hydration are the main distinction between lowland and montane habitats. However, our results do not allow projections beyond the current range of climates. Climate-change effects will depend on changes in hydration patterns and on acclimation, but information about these processes is lacking completely for lowland bryophytes. These issues are therefore the main foci in the proposed project. We aim to study climate-change effects on tropical lowland bryophytes with novel field- and model-experiments. In the field experiment, bryophytes are warmed and CO2-fertilized in open-top chambers in situ in the forest understorey for 18 months. We will determine changes in CO2-exchange rates, activity patterns, growth and survival. Simultaneously, a model will be developed, calculating carbon balances of bryophytes based environmental response curves, with measured or simulated microclimatic data as the input. The aims of the model are to better interpret the field data and to study the effects of changes in hydration patterns and other climatic changes on long-term carbon balances. Future uses of this model also include exploring explanations for the elevational gradients in the tropics and other biogeographical patterns in bryophyte abundance.