title: Altitudinal changes in temperature responses of net photosynthesis and dark respiration in tropical bryophytes.
abstract: OBJECTIVE
  There is a conspicuous increase of poikilohydric organisms (mosses, liverworts and macrolichens) with altitude in the tropics. This study addresses the hypothesis that the lack of bryophytes in the lowlands is due to high-temperature effects on the carbon balance. In particular, it is tested experimentally whether temperature responses of CO(2)-exchange rates would lead to higher respiratory carbon losses at night, relative to potential daily gains, in lowland compared with lower montane forests.
  
  
  METHODS
  Gas-exchange measurements were used to determine water-, light-, CO(2)- and temperature-response curves of net photosynthesis and dark respiration of 18 tropical bryophyte species from three altitudes (sea level, 500 m and 1200 m) in Panama.
  
  
  RESULTS
  Optimum temperatures of net photosynthesis were closely related to mean temperatures in the habitats in which the species grew at the different altitudes. The ratio of dark respiration to net photosynthesis at mean ambient night and day temperatures did not, as expected, decrease with altitude. Water-, light- and CO(2)-responses varied between species but not systematically with altitude.
  
  
  CONCLUSIONS
  Drivers other than temperature-dependent metabolic rates must be more important in explaining the altitudinal gradient in bryophyte abundance. This does not discard near-zero carbon balances as a major problem for lowland species, but the main effect of temperature probably lies in increasing evaporation rates, thus restricting the time available for photosynthetic carbon gain, rather than in increasing nightly respiration rates. Since optimum temperatures for photosynthesis were so fine tuned to habitat temperatures we analysed published temperature responses of bryophyte species worldwide and found the same pattern on the large scale as we found along the tropical mountain slope we studied.
authors.affiliation: Department of Biology and Environmental Sciences, Functional Ecology of Plants, University of Oldenburg, 26111 Oldenburg, Germany. sebastian.wagner@uni-oldenburg.de
authors.forename: Sebastian
authors.initials: S
authors.lastname: Wagner
authors.forename: Gerhard
authors.initials: G
authors.lastname: Zotz
authors.forename: Noris
authors.initials: N
authors.lastname: Salazar Allen
authors.forename: Maaike Y
authors.initials: MY
authors.lastname: Bader
mesh_terms: D000064:Acclimatization; D000531:Altitude; D044002:Bryophyta / Q000378:metabolism / Q000502:physiology* / Q000528:radiation effects; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D019069:Cell Respiration; D003624:Darkness; D017753:Ecosystem; D008027:Light; D016014:Linear Models; D009584:Nitrogen / Q000378:metabolism; D010176:Panama; D010788:Photosynthesis; D013696:Temperature; D014867:Water / Q000378:metabolism
chemical_list: D014867:Water; D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen