Changes Of Leaf Traits Of Betula Ermanii, A Timberline Tree Species, Along Altitudinal Gradient In Changbai Mountain

Alpine timberline, as the most foundational ecological boundary for tree species growth under extremely environmental conditions, has becoming a concern in global change research given its potential sensitivity to climate change. An increased understanding on formation mechanism of timberline and associated physio-ecological characteristics of plants is of great importance in forecasting vegetation dynamics under future climate change.Betula ermanii, is a dominant deciduous tree of subalpine regions in Changbai Mountain, where it prevails at the tree line.It is particularly vulnerable to climate change.To examine the leaf traits of B.ermanii how change along altititudinal grdient and the relationship between leaf traits and environmental factors, morphological, anatomical and physiological traits were tested at six different altitude on the northern slope of Changbai Mountain. The results showed that:1. Along an altitudinal gradient, temperature and precipitation and other factors changes correspondingly. The environment of B.ermanii can be characterised as low temperature, moist, strong ultraviolet radiation. In the distribution range of B.ermanii, July average temperature, growing season temperature, July maximum temperature and growing season maximum temperature decreased shaply with elevation. In contrast, growing season precipitation and UV-B radiation increased remarkably.2. Regression analysis showed that leaf morphological traits of B.ermanii were more sensitive to altitudinal gradient, becoming smaller and thicker leaves to adapt to high-altitude environment. Leaf mass per unit area (LMA) increased sharply along altitudinal gradient. At 1450 m, LMA was 29.78±0.6 g/m2, while at 2086 m it climbed up to 55.78±1.21 g/m2, 88 % higher than 1450 m. At the same time, leaf area (LA) and petiole length (PL) declined linearly with elevation. At 2086 m, LA and PL were 7.90±1.02 cm2 and 0.79±0.23 cm, lower 62 % and 52 % than 1450 m respectively. Leaf index (LI) had a slight decrease with elevation (R2 = 0.153, P <0.01).3. The leaves of B.ermanii were of typical bifacial. It had well- developed palisade tissue exhibiting xeromorphic characters. Leaf thickness (LT), palisade tissue thickness (PMT), spongy tissue thickness (SMT) and ratio of palisade to sponge (P/S) increased along altitudinal gradient. PMT increased significantly, at 2086 m, PMT was 23.74±1.02μm, 2.4 times than 1450 m. Upper epidermis thickness (UET) and lower epidermis thickness (LET) changed less obvious. Correlation analysis showed that LT had a significant positive correlation betwee PMT, SMT, LET. Furthermore, there was a significant positive correlation between PMT and SMT, SMT and LET.4. Chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl t) and carotenoid (Car) concentration of B.ermanii were non-linear wirh increasing altitude. All of them reached highest point from 1773 to 1862 m, and then decreased. Leaf nitrogen content per unit mass (Nm) was non-linear along altitudinal gradient. Nm increased gradually with elevation, and reached maximum, 33.78±1.87 mg/g, at 1862 m. And then Nm declined. Meanwhile, leaf nitrogen content per unit area (Na) increased linearly. At 1450m, Na was only 0.72±0.04 g/m2, roughly 46 % of 2086 m.5. Maximum net photosynthetic rate (Pmax) increased with the elevation, and reached to its highest point 13.31±1.54μmolCO2·m-2·s-1 at 2011m. And then it began to decline to 6.88±0.44μmolCO2·m-2·s-1 at 2086 m. At the same time, apparent quantum yield (AQY) had a similary change trend with Pmax. Dark respiration rate (Rd) increased with elevation, while photosynthetic nitrogen use efficiency (PNUE) decreased.6. High concentration of soluble sugar in leaves of B.ermanii was helpful to withstand low temperature. And accumulated non-structural carbohydrates (NSC) may be caused by low-temperature. Soluble sugar and non-structural carbohydrates (NSC) concentration of B.ermanii leaves increased with elevation, and both them reached to maximum at 1862 m, 11.58±1.36% dm and 13.25 % dm respectively. And then they declined with elevation, which were lower than the maximum 25 % and 20 % at treelines. Starch concentration of of B.ermanii leaves decareased along altitudinal gradient.It declined to 1.55±0.1 %dm, about 80% of maximum.7. Higher malondialdehyde (MDA) content of B.ermanii suggested it experienced stress injury. Due to a large amount of proline (Pro), flavonoid (Fla), B.ermanii can adapt to stress in habitat. MDA, Pro, Fla content responses to altitudinal gradient varied differently. MDA increased with elevation, and kept a relatively high level from1773 to 2011 m, and followed by a slight decrease. Pro increased overall, but up to 1862 m its rise slowed down. Fla declined, and dropped to the lowest point at 1862 m, and then increased rapidly with elevation.8. Plasticity index of Fla, Rd, PNUE, Na, PMT, LA, LMA, PL were relatively higher, suggesting these traits were more sensitive to the environment and played an important role in adapting to environment.9. The bioplot of Canonical Correspondence Analysis (CCA) revealed relationship between leaf traits of B.ermanii and environmental factors. LA, PL, PNUE and other traits were affected by growing season temperature. There was a close relationship between LMA, Na and UV-B radiation. In addition, PMT, Rd and LT were closely affected by growth season precipitation.From the results, it can be concluded that Betula ermanii respond to lowtemperature, humid and UV-B stress by developing many morphological, structural and physiological adaptations at the leaf level.