Diurnal and nocturnal transpiration behaviors and their responses to groundwater-table fluctuations and meteorological factors of Populus tomentosa in the North China Plain

Abstract

For theories of forest water utilization over day-night cycles, it is important to have a reliable information on plant transpiration behaviors during daytime and nighttime and their environmental controls. With the help of thermal dissipation probes, our study investigated the seasonal variations in the diurnal (Qday) and nocturnal stand water use (Qnight) of a Populus tomentosa plantation and explored their responses to groundwater table (GT) fluctuations and the meteorological factors. The results indicated that Qnight occurred on most nights during the growing season in the P. tomentosa plantation, but with a different seasonal dynamic from Qday. A distinct increase in the ratio of nocturnal to daily water use appeared at the beginning and the end of the growing season. During the experimental period, stem refilling accounted for 61% of Qnight, indicating that P. tomentosa was able to draw on stored water in the stem to support transpiration. The Qday and Qnight were positively and negatively related to GT, respectively. When the GT varied within 170 cm of the surface, the proportion of stem refilling accounting for Qnight increased with GT. The positive and negative correlations were found in meteorological factors with Qday and Qnight, respectively. Among the single meteorological factors except for ET0, vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) explained the highest variation in Qday in 2010 and 2011, respectively. In contrast, Qnight was dominantly affected by VPD in 2011. Nocturnal stand water-use could help P. tomentosa to recharge the consumption of water stored in the stem and overcome the seasonal drought. The compensation effect of groundwater on the nocturnal stand water-use of P. tomentosa could reduce its consumption of water stored in the stem. As the stand age of P. tomentosa plantation varied, the key meteorological drivers of both Qday and Qnight also changed. Our results enable better understanding of the water-use strategies of poplars in the North China Plain and can lead to refinement of current silvicultural techniques.