RESPONSES OF PHENOLOGY AND GROWTH OF BETULA UTILIS AND ABIES FAXONIANA IN SUBALPINE TIMBERLINE ECOTONE TO SIMULATED GLOBAL WARMING, WESTERN SICHUAN, CHINA Short-term gas exchange responses of Betula utilis to simulated global warming in a timberline ecotone, eastern Tibetan Plateau, China Freezing damage in early spring induced by simulated global warming: a case study in Abies faxoniana