Concomitant dendrometer and leaf patch pressure probe measurements reveal the effect of microclimate and soil moisture on diurnal stem water and leaf turgor variations in young oak trees

Tree water relations and their dependence on microclimate and soil moisture were studied over several months in young oaks (Quercus robur L.) subjected in large lysimeter-based open top chambers to environments with a controlled soil water supply. Automated single point dendrometers and the recently developed leaf patch clamp pressure (LPCP) probe were used for monitoring water-related stem radius variations (Delta W) and turgor-dependent leaf patch pressures (P-p). Both parameters showed distinct diurnal patterns with sharp negative and positive peaking of Delta W and P-p, respectively, after solar noon and recovery to initial levels in the evening. During the day, varying solar radiation was responsible for short time fluctuations of P-p in the range of minutes to hours reflecting feedback regulation of leaf turgor by sunlight driven stomatal movements. At longer timescales, i.e. days to months, atmospheric vapour pressure deficit (VPD) and soil water content (SWC) were the main determinants of Delta W and P-p. Daily minimum and maximum values of Delta W and P-p decreased and increased, respectively, with increasing VPD or decreasing SWC and recovery of Delta W and P-p in the evening was impeded by low SWC. In well-watered oaks, daily positive peaking of P-p preceded daily negative peaking of Delta W; these time lags gradually increased with increasing soil drought, suggesting hydraulic uncoupling of stem and leaves.