锐齿栎水力结构和生长对降雨减少的响应

The responses of hydraulic architecture and growth of Quercus aliena to rainfall reduction

为探究气候变化背景下降雨减少对森林的影响,2013年在宝天曼锐齿栎天然次生林原位建立了3块降水减少(截雨)样地,研究降雨减少对锐齿栎水碳关系和生长的影响。结果表明:降雨减少后锐齿栎枝条水势显著低于对照,最低水势为(-1.36±0.11)MPa,但锐齿栎木质部栓塞88%的水势值为-3.19MPa,叶片气孔关闭时的水势值为-2.5MPa,故降雨减少在这一地区没有对锐齿栎水力结构造成严重的干扰。降雨减少后,锐齿栎的叶片、韧皮部和木质部的总非结构性碳浓度与对照没有显著差异。木质部导管密度和叶片气孔密度变大,而导管直径和气孔长度变小。在天气较为干旱时,降雨减少处理的锐齿栎气孔导度日变化呈"双峰"曲线,而在湿润天气时呈"单峰"曲线且中午峰值显著大于对照。降雨减少处理的锐齿栎木材密度、Huber值、比叶面积和胸径生长与对照没有显著差异。降雨减少后锐齿栎树木没有遭受水力失衡或碳饥饿的危害,生长也没有受到显著影响,但是水力输导系统发生了适应性调节。

In recent years, drought has caused increasing forest mortality at a global scale due to climate change, which draws wide attention. The main cause of drought-induced tree mortality is presumed to be out of the carbon-water balance in trees. This study investigated the effects of rainfall reduction on a secondary natural forest of Quercus aliena in Baotianman Mountains by imitating climate change. Three rainfall-intercepted plots were established in situ in a Q. aliena forest in Baotianman Mountain in April 2013, aiming to study the effects of reduced rainfall on the physiological relations of water-carbon, and growth of Q. aliena. The results showed that the predawn and midday water potential values of Q. aliena in the rainfall reduction plots were significantly lower than those of the control trees, and the lowest midday water potential value of rainfall-reduced Q. aliena was （-1.36±0.11）MPa. However, the water potential value which induced the xylem of Q. aliena lost 88% of hydraulic conductance was -3.19MPa, and the water potential value of Q. aliena at stomatal closure was -2.5MPa. Thus, rainfall reduction did not severely disturb the hydraulic architecture of Q. aliena in this region. After reduced rainfall, there were no significant differences in total nonstructural carbohydrates concentrations in leaves, phloem and xylem between rainfall-reduced trees and the control treatment. In summary, the rainfall-reduced Q. aliena trees did not show hydraulic failure or carbon starvation. However, the xylem vessel density and the leaf stomatal density of the rainfall reduction treated Q. aliena were significantly higher than those of control trees, and the xylem vessel diameter and the stomatal length were significantly smaller, which indicated that significant physiological and structural adaptations of rainfall-reduced Q. aliena trees occurred to cope with drought. During the relative drought, rainfall-intercepted Q. aliena showed a bimodal curve of the diurnal variation of stomatal conductance, while the control trees showed a unimodal curve. On the contrary, during the moist period, the diurnal dynamics of stomatal conductance of the rainfall-intercepted Q. aliena trees showed a monopeak curve similar to the control, and the peak value was significantly higher than that of control trees. There were no significant effects of reduced-rainfall on wood density, Huber value, specific leaf area and transversal growth of tree trunk of Q. aliena trees. Our results indicated that rainfall-reduced Q. aliena trees did not show hydraulic failure and carbon starvation, and the growth of Q. aliena was also not significantly influenced by reduced rainfall. However, their hydraulic structure showed adaptive adjustment to cope with more serious drought in future.