Abstract: Under the background of global climate change, regional precipitation pattern is changing, with consequences on forest ecosystem productivity and hydrological processes. To investigate the effects of rainfall reduction on the characteristics of transpiration water use and their responses to environmental factors in trees, we applied rainfall reduction treatment to a plantation of black locust (Robinia pseudoacacia), a major afforestation species, in a sub-humid region of Loess Plateau. Transparent waterproof panels were laid out in the inter-rows of the rainfall reduction plot to reduce the rainfall input by 47%. Stem sap flow was measured by Granier’s thermal dissipation probes. Environmental factors including solar radiation, air temperature, relative humidity, rainfall events and soil moisture were monitored simultaneously. The response characteristics of stem sap flow to rainfall reduction and seasonal soil moisture changes were analyzed. The results showed that rain reduction significantly reduced the norma-lized sap flux density of black locust trees. After three years of treatment, normalized sap flux density in the rain reduction treatment was significantly lower than that in the control. The diurnal peak time of the normalized sap flux density in the rain reduction plots was earlier than that in the control, indicating that stomatal adjustment occurred earlier and that the time lag between sap flow and meteorological factors was changed. The fitting equation parameters of normalized sap flux density response to transpiration driving factors were significantly different between both treatments, indicating that the sensitivity of black locust response to meteorological factors in the treatment of rain reduction was weakened. These results suggest that precipitation changes not only affect forest soil water availability, but also affect the transpiration characteristics of black locust trees and their responses to environmental factors.

Key words: Robinia pseudoacacia, soil moisture, stem sap flow, transpiration