Soil water affects transpiration response to rainfall and vapor pressure deficit in poplar plantation

Influences of environmental factors on transpiration are interactive. Sensitivities of transpiration responses to both evaporative demand and rainfall under contrasting soil water conditions constitute the physiological basis of the drought tolerance of trees. Such knowledge is practically significant for plantation management, especially for irrigation management. We therefore conducted a 6-year study on the transpiration of a poplar plantation in temperate China to elucidate the existence and pattern of the influence of the soil water over stand transpiration responses to (1) vapor pressure deficit (VPD), the major indicator of air dryness and (2) the rainfall, in terms of total amount and event size. The results showed that the response of plantation transpiration (E (c) ) to VPD was conditioned by soil moisture. There was a significant difference in the frequency distribution of maximum sap flux under contrasting soil relative extractable water. E (c) after rainfall of different sizes varied under similar VPD. The increasing occurrences of only large rainfall events led to enhanced total E (c) during the growing season, but prolonged rainless intervals did not lead to a continuous decrease of E (c) , suggesting appreciable supplements from the soil water were present to sustain transpiration. In addition, the balance of soil water between replenishment and extraction also conditioned the influence of rainfall over subsequent E (c) during the respective rainless intervals. Based on the E (c) responses to VPD and rainfall under different soil moisture levels, irrigation that directly replenishes the deep soil layers in order to alleviate water stress on transpiration during the small-rain event-dominated growing season is an effective and water-saving approach to guarantee trees survival during drought period.