Abstract:

Base on data of one-year continuous measurements of stem sap flow density for Larix gmelinii by a TDP method and environmental factors of solar radiation, air temperature, soil temperature, air humidity and soil humidity, variations in the relations between the sap flow and environmental factors were investigated at different temporal scales(month, day and hour) paper. Stepwise regression analysis showed the major environmental factors influencing the stem sap flow were different under the different temporal scales. At the temporal scale of a month, soil temperature and soil moisture markedly affected stem sap flow density and one unit increase in soil temperature could induce an increase of 0.084-0.123 L·cm^-2 month^-1. At the temporal scale of a day, soil temperature, solar radiation and air temperature markedly affected stem sap flow, and a unit increase of of soil temperature the most significant factor among the three, could induce an increase of 1.9-2.7 mL·cm^-2 d^-1 in sap flow. As temporal scale become smaller to an hour, general high impacts were from solar radiation and air humidity although much complex patterns were found according to different season. On average, one unit increase in the two parameters could induce an increase of 1.239 mL·cm^-2 min^-1 and a decline of 0.056 6 mL·cm^-2 min^-1 in stem sap flow, respectively. With time scale from large to small, in general, factors most affecting stem sap flow changed from below-ground root-water-absorption related parameters of soil temperature and moisture to above-ground leaf transpiration related parameters of light and air humidity. Moreover, much less parameters (2) with higher R² (>89%) were found in the regression analysis at the large scale of month, while more parameters(≥4) with low R²(<0.76) were found at the small scale of hour. In the calculations of total tree or forests water consumption, accordingly, long-term monitoring of small amount of environmental parameters can be used to scaling-up with high precision, while such type of method may give large bias for short-term studies comparing with direct TDP method. Our findings would benefit the understanding stem sap flow features and its scale-up.

Key words: Larix gmelinii, soil temperature, soil humidity, air temperature, air humidity, solar radiation, stem sap flow density