Leaf nitrogen resorption proficiency of seven shrubs across timberline ecotones in the Sergymla Mountains, Southeast Xizang, China

Abstract

Aims Our objectives were to disclose why evergreen shrubs, but not deciduous shrubs, dominate above timber- line 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 ni- trogen 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 Rhodo- dendron aganniphum var. schizopeplum than in deciduous shrubs. However, the leaf area-based N resorption pro- ficiency was relatively higher in deciduous shrubs due to their lower leaf mass per unit area. Although no signifi- cant 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 de- ciduous shrubs, N resorption proficiency in the evergreen shrub Rhododendron aganniphum var. schizopeplum is assumed to be more sensitive to future climate warming.