Is it Caused by Low Stem Turgor Pressure?
GROUND AND AIMS: 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 (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 p: growth was assumed to occur when p
