## 2 Materials and Methods  
### Study Area and Sample Trees  
This study was carried out in the Saihanwula National Nature Reserve in Inner Mongolia (118 deg18 deg-118 deg55 deg east longitude, 43 deg59 deg-43 deg27 deg north latitude, 1000-1997 m above sea level) (Figure 1) in the southern section of the Greater Khingan Mountains, China. It is a dual transition zone from East Asian broad-leaved forest to cold-temperate conferous forest of the Greater Khingan Mountains and from grassland to forest, in a mid-temperate semi-arid climate zone. The soil at this study site is dark brown forest soil with a depth ranging from 30 to 80 cm, and the soil porosity is about 50%-60% [40]. The growing season usually lasts from early May to early October. The long-term average annual precipitation is 375 mm, mostly falling from June to September. The annual average temperature is 1.54 degC, and the highest monthly average temperature is 20.5 degC and the lowest is \(-\)23.2 degC (based on rainfall and temperature data of 1986-2015 from the Climatic Research Unit). The vegetation types of the Saihanwula National Nature Reserve are very complex. Its top is sub-alpine meadow vegetation. From high altitude to low altitude, the forest vegetation is distributed in turn, including evergreen coniferous forest _Picea meyeri_ Rehd. et Wils. and summer green coniferous forest _Larix principis rupprechtii_ [PERSON], which grow on the mountain shady slope of 1700-1800 m in the Saihanwula National Nature Reserve, with a canopy density of more than 80%, in addition to a developed moss layer with a coverage of 90% [40]. From the top of the mountain down, the coniferous forest transitions to summer green broad-leaved forest, including white birch (_Betula platyphyllla_ [PERSON]) and aspen (_Populus davidiana_ [PERSON]). _Betula platyphyllla_ forest has the largest distribution area, and its canopy closure is about 70%. Along the mountain valleys, there are _Populus cathayana_ [PERSON], _Ulmus laciniata_ [PERSON], and _Salix rorida_ [PERSON], etc., usually accompanied by mountain sparse forest vegetation, mainly _Ulmus pumila L., _Malus bacata_ [PERSON] and _Crataegus pinnatifida_ [PERSON] var. psilosa [PERSON]). _Ulmus macrocarpa_ [PERSON] is also distributed on the steeper shady slopes of the mountains, often mixed with _Armeniaca sibirica_ L. shrubs, with a canopy density of 60%. As the north is high and the south is low, it gradually transitions down to grassland and meadow vegetation. There are continuous rivers in the mountains and valleys, with willow forests and shrubs on both sides. Most of them have been reclaimed as planted forests. Crossing the valley to the sunny slope of the mountain, the most developed plants are _Quercus mongolica_ Fisch. ex [PERSON] and _Armeniaca sibirica_ L. shrubs [40]. Figure 1: Location of the study area in Northeast China, and the pictures of the _Betula platyphyllla_ Sukaczev Forest; equipment for measuring sap flow and the weather station. The experiment was carried out on a plot of 30 \(\times\) 30 m\({}^{2}\) (118\({}^{\circ}\)42\({}^{\prime}\)29.17\({}^{\prime\prime}\) E, 44\({}^{\circ}\)11\({}^{\prime}\)28.67\({}^{\prime\prime}\) N; 1387 m a.s.l. ; Figure 1), as a small long-term experimental forest. The altitude ranged from 1300 to 1400 m, located on a SW-NE facing slope with a slope gradient of 23\({}^{\circ}\). The major species of the forest stand is white birch, forming a relatively pure birch stand, with a stand density of 1100 ha\({}^{-1}\). The average tree diameter at breast height (DBH, cm) is 18.56 \(\pm\) 5.22 cm, and the average tree height is 13.40 \(\pm\) 4.34 m.  
All 115 white birch trees in this research plot were divided into four classes based on the tree diameter at breast height, i.e., small (DBH \(<\) 15 cm), medium1 (15 \(<\) DBH \(<\) 18 cm), medium2 (18 \(<\) DBH \(<\) 21), and large (DBH \(>\) 21 cm) classes. Trees from the small diameter class had an average tree DBH of 13.1 \(\pm\) 1.4 cm.