Decline in stomatal conductance is the primary reason for low photosynthesis in veteran pedunculate oak trees

Veteran trees are important elements in forests, urban and suburban areas and represent part of our cultural heritage. However, increasing age also brings a reduction in vitality. Information on tree physiological vitality can be gained by examining ecophysiological traits, such as photosynthesis, stomatal conductance, and leaf water potential. Here, we assess the effects of age on photosynthesis and water status of 600-year-old pedunculate oak (<i>Quercus robur</i>) by comparing them with neighbouring 25-year-old trees. While gas exchange measurements indicated lowered photosynthesis in old trees, maximum rates of Rubisco carboxylation and electron transport, along with day respiration, were similar to younger trees, suggesting that biochemical limitations to photosynthesis were not the reason. Moreover, while there was no difference in light-adapted and dark-adapted chlorophyll fluorescence between old and young trees, stomatal conductance (under unlimited soil water availability) was lower, indicating increased stomatal limitations to photosynthesis in veteran trees. On the other hand, high water potential during mild summer drought conditions indicated better access to soil water in old trees, while stomatal conductance in old trees was higher than in young trees at night. A reduced ability to open and close stomata may be a reason for the observed decline in veteran tree vitality, with a lowered ability to regulate stomatal conductance resulting in reduced carbon gain and unnecessarily high water loss.