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Temperature dependencies of stem dark respiration (R(d)) and light-driven bark photosynthesis (A(max)) of two temperate tree species (Fagus sylvatica and Betula pendula) were investigated to estimate their probable influence on stem carbon balance. Stem R(d) was found to increase exponentially with increasing temperatures, whereas A(max) levelled off or decreased at the highest temperatures chosen (35-40 degrees C). Accordingly, a linear relationship between respiratory and assimilatory metabolism was only found at moderate temperatures (10-30 degrees C) and the relationship between stem R(d) and A(max) clearly departed from linearity at chilling (5 degrees C) and at high temperatures (35-40 degrees C). As a result, the proportional internal C-refixation rate also decreased non-linearly with increasing temperature. Temperature response of photosystem II (PSII) photochemistry was also assessed. Bark photochemical yield (Delta F/F(m)') followed the same temperature pattern as bark CO(2) assimilation. Maximum quantum yield of PSII (F(v)/F(m)) decreased drastically at freezing temperatures (-5 degrees C), while from 30 to 40 degrees C only a marginal decrease in F(v)/F(m) was found. In in situ measurements during winter months, bark photosynthesis was found to be strongly reduced. Low temperature stress induced an active down-regulation of PSII efficiency as well as damage to PSII due to photoinhibition. All in all, the benefit of bark photosynthesis was negatively affected by low (&lt;5 degrees C) as well as high temperatures (&gt;30 degrees C). As the carbon balance of tree stems is defined by the difference between photosynthetic carbon gain and respiratory carbon loss, this might have important implications for accurate modelling of stem carbon balance.

Temperature dependency of bark photosynthesis in beech (Fagus sylvatica L.) and birch (Betula pendula Roth.) trees