Objective This paper explores the transpiration water consumption patterns of degraded and undegraded Populus simonii in the Bashang Area, Hebei Province of northern China, optimizes the allocation of ecological water use, so as to provide theoretical references for the restoration and reconstruction of degraded Populus simonii forests.

Method In this study, we selected undegraded and degraded Populus simonii as the research objects and used the thermal dissipation probe (TDP) method to determine the sap flow, and also monitored the meteorological factors to study sap flow characteristics of the two types of Populus simonii and their response mechanisms to environmental factors, as well as their sap flow and transpiration rates during the growing season (July, August and September).

Result (1) The relationship between daily changes of sap flow of degraded and undegraded Populus simonii was greater in sunny weather compared with rainy and cloudy days, with the largest difference observed on sunny days, and the initiation of sap flow of undegraded Populus simonii was always earlier than that of degraded Populus simonii. (2) The transpiration water consumption of undegraded Populus simonii was the highest during the growing season, and there was a significant difference in sap flow rates between the two species, with a 3 times difference in transpiration. (3) Temperature and photosynthetically active radiation (PAR) were the most important environmental factors affecting sap flow in Populus simonii, and undegraded Populus simonii was more sensitive to environmental factors.

Conclusion The low transpiration of degraded Populus simonii makes it difficult to obtain the water required to maintain normal physiology, resulting in physiological decline, and its sensitivity to changes in environmental factors is increased, which further weakens its ability to cope with future drought events. Therefore, it is recommended to renovate the degraded poplar stand.