title: Nitrogen and carbon source-sink relationships in trees at the Himalayan treelines compared with lower elevations.
abstract: No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.
authors.affiliation: Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland. maihe.li@wsl.ch
authors.forename: Mai-He
authors.initials: MH
authors.lastname: Li
authors.forename: Wen-Fa
authors.initials: WF
authors.lastname: Xiao
authors.forename: Peili
authors.initials: P
authors.lastname: Shi
authors.forename: San-Gen
authors.initials: SG
authors.lastname: Wang
authors.forename: Yong-De
authors.initials: YD
authors.lastname: Zhong
authors.forename: Xing-Liang
authors.initials: XL
authors.lastname: Liu
authors.forename: Xiao-Dan
authors.initials: XD
authors.lastname: Wang
authors.forename: Xiao-Hu
authors.initials: XH
authors.lastname: Cai
authors.forename: Zuo-Min
authors.initials: ZM
authors.lastname: Shi
mesh_terms: D028202:Abies / Q000254:growth & development / Q000378:metabolism; D000531:Altitude; D000704:Analysis of Variance; D002241:Carbohydrates / Q000096:biosynthesis; D002244:Carbon / Q000378:metabolism*; D002681:China; D017753:Ecosystem; D005843:Geography; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis; D028222:Picea / Q000254:growth & development / Q000378:metabolism; D014197:Trees / Q000254:growth & development / Q000378:metabolism*
chemical_list: D002241:Carbohydrates; D002244:Carbon; D009584:Nitrogen