title: Daytime depression in tree stem CO2 efflux rates: is it caused by low stem turgor pressure?
abstract: OBJECTIVE
  Daytime CO2 efflux rates (FCO2) from tree stems are often reported to be lower than expected from the exponential relationship between temperature and respiration. Explanations of daytime depression in FCO2 have focused on the possible role of internal CO2 transport in the xylem. However, another possible cause that has been overlooked is the daily dynamics of the water status in the living stem tissues and its influence on stem growth rate and thus respiration. The objective of this study was to assess the daily dynamics of stem water status and growth rate and to determine the extent to which they may be linked to daily variations in stem FCO2.
  
  
  METHODS
  FCO2 of young beech and oak stems were measured under controlled conditions. Relative stem turgor pressure (Psi(p)), obtained from simulations with the 'RCGro' model, was used as an indicator of the water status in the living stem tissues. Daily dynamics of stem growth were derived from Psi(p): growth was assumed to occur when Psi(p) exceeded a relative threshold value.
  
  
  RESULTS
  There was a strong correspondence between fluctuations in FCO2 and simulated Psi(p). The non-growth conditions during daytime coincided with depressions in FCO2. Moreover, FCO2 responded to changes in Psi(p) in the absence of growth, indicating also that maintenance processes were influenced by the water status in the living stem tissues.
  
  
  CONCLUSIONS
  Daytime depressions in stem FCO2 correlate with the daily dynamics of turgor, as a measure of the water status in the living stem tissues: it is suggested that water status of tree stems is a potentially important determinant of stem FCO2, as it influences the rate of growth and maintenance processes in the living tissues of the stem.
authors.affiliation: Applied Ecology and Environmental Biology, Ghent University, Coupure Links 653, Ghent, 9000, Belgium. an.saveyn@UGent.be
authors.forename: An
authors.initials: A
authors.lastname: Saveyn
authors.forename: Kathy
authors.initials: K
authors.lastname: Steppe
authors.forename: Raoul
authors.initials: R
authors.lastname: Lemeur
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D002940:Circadian Rhythm; D029964:Fagus / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D018547:Plant Stems / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D029963:Quercus / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism; D014197:Trees / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism*; D014867:Water / Q000378:metabolism; D052584:Xylem / Q000378:metabolism
chemical_list: D014867:Water; D002245:Carbon Dioxide