Project 1. The Altitudinal Gradient of Tropical Bryophytes
Unraveling the Causes and Projecting Climate Change Effects
Sebastian Wagner (PhD student), Maaike Bader, Gerhard Zotz
Bryophytes (mosses and liverworts) and lichens in the warm tropics have been suggested to exist at the edge of their physiological abilities, because they lose a large proportion of their daily carbon gain to nightly respiration, principally due to the high temperatures. A rise in temperature may therefore force bryophytes uphill and cause their complete disappearance from the lowland tropics, where they already have low abundances compared to higher altitudes. In this project, funded by the German Research Foundation (DFG) from 2009 to 2013, we used the working title "Will negative carbon balances turn the tropical lowlands into a bryophyte desert?". We aimed to explain the observed altitudinal gradient in bryophyte abundance in the tropics and to predict the effects of climatic warming. We used an experimental approach, using transplantation and laboratory experiments and gas exchange and fluorescence measurements, to determine carbon balances, acclimatization potentials and desiccation tolerance of a range of tropical lowland and montane bryophyte species in Panama.
We found that, contrary to our expectation, the temperature responses of photosynthesis and dark respiration were well suited to ambient temperatures, even in the warm lowlands. There was no acclimatization of photosynthesis or respiration of montane bryophytes to lowland conditions in transplanted samples, according to our gas exchange measurements, and mortality was high. However, a few samples of nearly all species survived the hotter conditions for at least 21 months, indicating acclimatization and a potential for the species to adapt to climatic warming. Temperature effects on metabolic rates thus appear to be less important for the altitudinal distribution of tropical bryophytes than previously thought, but temperature is probably still a crucial factor as it determines how fast mosses dry out. This, in turn, determines the time available for photosynthesis and the frequency of wet-dry cycles, both of which are important for the carbon balance. This is confirmed by a carbon balance model that is still under further construction. We also tested whether prolongued desiccation could explain the absence of montane species in the lowlands or could become a problem in future dryer conditions. However, we found that dry periods can be survived for much longer than 'necessary' in the lowlands and under moderate warming/drying scenarios, both in lowland and in montane bryophyte species. Future research should focus on 1) climate change effects on hydration patterns, activity times and resulting carbon balances, and 2) the acclimatization potential of lowland species to higher temperatures, using active experimental warming.
Publications related to this project can be found here.
A typical mossy tree trunk in the montane cloud forest of Fortuna (western Panama, 1200 m, featuring also a young bromeliad), and a typical moss-free tree trunk (covered only with crustose lichens) in the lowland rain forest on Barro Colorado Island (Panama Canal, 25 m).
Project 2. The ecology of tropical montane vascular and non-vascular epiphyte communities
Diana Gomez (PhD student), Maaike Bader, Gerhard Zotz