Warming Delays But Grazing Advances Leaf Senescence of Alpine Plants

Abstract

Leaf senescence is the final stage of the life cycle of leaves and is critical to plants’ fitness as well as to ecosystem carbon cycling. To date, most understanding about the responses of leaf senescence to environmental changes has derived from research in forests, but the topic has been relatively neglected in natural grasslands. We conducted a 3-year manipulative asymmetric warming with moderate grazing experiment to explore the responses of leaf senescence of five main alpine plants on the Qinghai-Tibetan Plateau. We found that warming prolonged leaf longevity through earlier leaf-out and later leaf senescence for all plants, and grazing prolonged it through a greater advance in leaf-out than first leaf coloration for all plants. Warming did not affect leaf N content or nitrogen resorption efficiency (NRE) , but grazing increased N content for G. straminea and P. nivea and decreased NRE for K. humilis , P. anserine and P. nivea . There were positive relationships between leaf-out and leaf senescence, and positive relationships between NRE from old leaves and leaf senescence for three out of five plant species. Therefore, our results indicated that earlier leaf-out did not completely result in earlier leaf senescence, and delayed leaf coloring increased NRE from old leaves for some plants measured under warming and grazing. Our study suggested that alpine plants may develop strategies to adapt to environmental change to assimilate more carbon through prolonged leaf longevity rather than increased NRE through earlier leaf coloring under grazing in the alpine meadow.

Keywords: Leaf-out, Leaf longevity, Leaf senescence, Nitrogen resorption efficiency, Chlorophyll content, Alpine plants