RESPONSES OF PHENOLOGY AND GROWTH OF BETULA UTILIS AND ABIES FAXONIANA IN SUBALPINE TIMBERLINE ECOTONE TO SIMULATED GLOBAL WARMING, WESTERN SICHUAN, CHINA Effects of Local and Regional Processes on Herbaceous Plant Assemblages in the Betula utilis Forest Gaps of Taibai Mountain Effects of environmental factors on species distribution and diversity in an Abies fargesii-Betula utilis mixed forest Short-term gas exchange responses of Betula utilis to simulated global warming in a timberline ecotone, eastern Tibetan Plateau, China