Response characteristics of stem sap flow of Torreya grandis to environmental factors in southern Anhui

Abstract:【Objective】Torreya grandis, a unique nut tree species in China, has high economic and ecological values. At present, it’s still unclear about the quantitative relationship between the distribution of T. grandis and the suitable environment. In this study, T. grandis was used to establish the quantitative relationship between the stem sap flow and the environmental factors. On this basis, we investigated whether the response characteristics of T. grandis stem sap flow to the environmental factors would be affected by different observation periods. It was further verified whether the response of the stem sap flow to the environmental factors under continuous 19 d high temperature was consistent with that in the observation period on annual scale. This study would provide a theoretical reference for the ecological locating monitoring in T. grandis economic forest.【Methods】The method of ecological positioning observation was used to record the changes of the sap flow density of T. grandis every 15 minutes based on thermal diffusion technology. A small automatic weather station was used to simultaneously obtain the data of environmental hydro-thermic factors. The data on the changes of the sap flow density of T. grandis and environmental factors from April 1, 2020 to March 31, 2021 were matched and divided based on the first observation period, the second observation period and the comprehensive observation period with year the scale. In every observation period, consecutive 19 d high temperature was selected as the continuous high temperature days of the observation period. The continuous high temperature days of the comprehensive observation period were the total continuous high temperature days of the two observation periods. The Pearson correlation analysis and stepwise regression analysis were carried out respectively in the first observation period, the second observation period and the comprehensive observation period, including the sap flow density and air temperature, daily maximum air temperature, air relative humidity, daily maximum air relative humidity, vapor pressure deficit, photosynthetically active radiation, soil temperature, soil water content, soil electrical conductivity. The correlation between sap flow density and different environmental factors, and the interpretation and quantitative relationship between the sap flow density and different environmental factors were found by the study. Then the Pearson correlation analysis and stepwise regression analysis between sap flow density and the aforesaid environmental factors in the high temperature days of the first and second observation periods as well as the comprehensive observation period were carried out. We compared the results of statistical analysis under the same period, analyzed the similarities and differences of the response characteristics of the sap flow density to the environmental factors between the high temperature days and the observation periods based on annual data. And we determined the feasibility of studying the response of the stem sap flow to the environmental factors with the data collected in high temperature days under the condition of limited experimental time.【Results】In two years of experiment, the different environmental factors in two observation periods based on the year scale showed relatively consistent periodic variation characteristics, and the sap flow density in different observation periods showed different variation characteristics. The sap flow density during the consecutive high temperature days in the first observation period showed significant correlation with the air relative humidity (r = −0.61), soil water content (r = 0.58) and soil electrical conductivity (r = 0.58). The sap flow density and the environmental factors during the consecutive high temperature days in the second observation period showed no significant correlation (p＞0.05). The sap flow density during the consecutive high temperature days in the comprehensive observation period showed significant correlation (p ≤ 0.01) with the soil water content (r = 0.85), soil electrical conductivity (r = 0.8), daily maximum air relative humidity (r = 0.62) and air temperature (r = 0.46). The correlations between the sap flow density and the environmental factors during the consecutive high temperature days were different from those of the observation period based on annual data. In the stepwise regression equation of the sap flow density and the environmental factors of the first observation period, four environmental factors, such as the soil water content, soil temperature, daily maximum air temperature and air relative humidity, were introduced one by one so as to explain the changes of 52% sap flow density. In the stepwise regression equation of the sap flow density and the environmental factors in the second observation period, only the soil water content was introduced into the model and the goodness of fit was 0.462. In the stepwise regression equation of the sap flow density and the environmental factors in the comprehensive observation period, the soil water content and soil electrical conductivity were introduced into the model one by one. When the soil electrical conductivity was introduced into the model, the goodness of fit increased from 0.519 to 0.526.【Conclusion】In various observation periods based on the annual scale, the responses of T. grandis stem sap flow to the environmental hydro-thermic factors were different. The sap flow density had a strong response to the soil-based hydro-thermic factors and the soil water content was most closely correlated to the changes of the sap flow density, so it could better explain the changes of stem sap flow. In the case of limited conditions or less observation costs, the locating monitoring on soil water content changes could quantitatively analyze the water changes of T. grandis. The method of 19 d continuous high temperature for studying the response of T. grandis sap flow density to the environmental factors had relatively short research time, so it could not accurately analyze the response characteristics of the stem sap flow to the environmental factors.