在湿润的青藏高原东南部,为什么常绿灌木广泛占据高海拔的林线过渡带及以上的高山带,而落叶灌木只能零星分布?未来气候变暖对该区不同功能群物种的影响是否相同?通过测定西藏东南部色季拉山林线过渡带7种灌木凋落叶的氮含量,比较了极端高海拔地区灌木不同表达单位的叶氮回收潜力在不同功能群间的差异,以及不同海拔、不同坡向间的差异,试图从养分限制的角度为解答上述科学问题提供基础数据。研究结果表明:1)从基于单位质量叶氮含量(N_(mass))的叶氮回收潜力来看,常绿灌木裂毛雪山杜鹃(薄毛海绵杜鹃)(Rhododendron aganniphum var. schizopeplum)显著高于其他6种落叶灌木,但由于受比叶重的影响,基于单位面积叶氮含量(N_(area))的叶氮回收潜力则表现为落叶灌木总体较高;2)落叶灌木山生柳(Salix ori-trepha)和拉萨小檗(Berberis hemsleyana)的叶氮回收潜力在不同海拔或不同坡向间均无显著差异,但裂毛雪山杜鹃基于N_(mass)的叶氮回收潜力在高海拔地段明显偏高。在极端高海拔的林线过渡带,通过降低凋落叶中的氮含量(增加叶氮回收潜力)以达到高效的养分利用可能是常绿灌木裂毛雪山杜鹃适应高寒胁迫环境的重要策略。与落叶灌木相比,常绿灌木裂毛雪山杜鹃叶氮回收潜力对未来气候变暖可能更敏感。
英文摘要:
Aims Our objectives were to disclose why evergreen shrubs, but not deciduous shrubs, dominate above timberline in humid southeastern Qinghai-Xizang Plateau, and to test if different functional types converge in response to the warming climate from aspect of nitrogen limitation. Methods Based on investigations of nitrogen concentration in senesced leaves of seven shrubs across timberline ecotones in the Sergymla Mountains, Southeast Xizang, we analyzed differences in leaf mass- and area-based nitrogen resorption proficiency among different functional types (evergreen vs. deciduous), altitudes and aspects at 4 200-4 400 m a.s.l. Important findings Leaf mass-based nitrogen resorption proficiency was higher in the evergreen shrub Rhododendron aganniphum var. schizopeplum than in deciduous shrubs. However, the leaf area-based N resorption proficiency was relatively higher in deciduous shrubs due to their lower leaf mass per unit area. Although no significant difference in the resorption proficiency was found between altitudes or aspects for the deciduous shrubs of Salix oritrepha and Berberis hemsleyana, leaf mass-based N resorption proficiency was higher at higher altitude for the evergreen shrub Rhododendron aganniphum var. schizopeplum. Decreasing N concentration in senesced leaves, i.e., increasing resorption proficiency, which can improve N use efficiency, is an important strategy for the evergreen shrub to cope with the stressful alpine environment across timberline ecotones. Compared with the deciduous shrubs, N resorption proficiency in the evergreen shrub Rhododendron aganniphum var. schizopeplum is assumed to be more sensitive to future climate warming.