Background Drought and water shortage are important factors limiting the ecological construction in North China. Soil moisture is an important factor affecting tree stem sap, which determines the overall level of tree stem sap and its response sensitivity to environmental factors. The main factors of tree stem sap are different under different soil moisture conditions. The variations of tree stem sap and the response mechanism to meteorological factors under different soil moisture stress are important contents of understanding the adaptation mechanism of trees to environmental change and the water cycle mechanism.

Methods The representative plots of Acer truncatum and Pinus tabuliformis, two typical tree species in the low mountainous area of Beijing were selected. The thermal diffusion probe technique was used to monitor the yearly change of tree stem sap density of A. truncatum and P. tabuliformis. The time domain reflect system was set up in two plots to monitor soil moisture. The automatic weather station was used to monitor weather factors in real time. The tree stem sap density and environmental factors under different soil water stress were analyzed by regression analysis. The yearly change of tree stem sap density and its driving factors under different soil moisture conditions were investigated.

Results 1) The tree stem sap density of A. truncatum and P. tabuliformis demonstrated "unimodal" type yearly changes, and the number of days with soil moisture stress in 2022 accounted for 76.2% of the whole year. 2) The tree stem sap density of both A. truncatum and P. tabuliformis decreased significantly with the increase of moisture stress (P < 0.05). Soil relative water content had a substantial influence on tree stem sap density per unit of vapor pressure deficit (VPD) under soil moisture stress (P < 0.05), but not under no soil moisture stress. Under severe soil moisture stress, soil relative water content explained 62.2% and 54.3% of tree stem sap density per unit of VPD of A. truncatum and P. tabuliformis. 3) Multiple stepwise regression analysis showed that the dominant factor of tree stem sap density was meteorological factor under no soil moisture stress, and tree stem sap density was governed by meteorological factors and soil moisture under mild soil moisture stress, while the only dominant factor was soil moisture under severe soil moisture stress.

Conclusions We investigated the changes of tree stem sap and the dominant factor of the typical plantations in the low mountainous area of Beijing under different soil moisture stress. The results elucidate the response law of sap of typical artificial forests to soil moisture conditions in low mountain areas of Beijing. The results may provide technical guidance for the nurturing and management of artificial forests in this area.