title: Inconsistent intraspecific pattern in leaf life span along nitrogen-supply gradient.
abstract: OBJECTIVE
  Leaf life span (LLS) has long been hypothesized to plastically increase with decreasing nitrogen (N) supply from soil to maximize N retention, carbon assimilation, and fitness; however, accumulating evidence shows no consistent trend. The apparent inconsistencies are explained by a recent model that assumes LLS has a hump-shaped quadratic response to the N-supply gradient. The available evidence mostly originates from comparisons of LLS at only two levels of N availability, and the hypothesis remains unanswered.
  
  
  METHODS
  We investigated LLS of two asteraceous forbs (Adenocaulon himalaicumandXanthium canadense) experimentally grown at eight levels of N supply, which covered a range of N supply in their natural habitats. We additionally conducted a literature search to retrieve studies reporting LLS response along an N-supply gradient.
  
  
  RESULTS
  The LLS of neither species showed a hump-shaped response along the N-supply gradient. Past studies examining the LLS of an aquatic forb and terrestrial shrubs and trees along the N-supply gradient (more than four levels of N supply) also refuted the hypothesis.
  
  
  CONCLUSIONS
  The LLS of a single species exhibited neither an increase nor a hump-shaped response to decreased N supply in a variety of life forms. Comparisons at only a few N levels are misleading with regard to LLS response to N supply.
authors.affiliation: Graduate School of Science and Engineering, Ibaraki University, Mito 310-0056 Japan shimpei.oikawa.dx@vc.ibaraki.ac.jp.|Institute for Global Change Adaptation Science, Ibaraki University, Mito 310-0056 Japan.
authors.forename: Shimpei
authors.initials: S
authors.lastname: Oikawa
authors.affiliation: Graduate School of Science and Engineering, Ibaraki University, Mito 310-0056 Japan.
authors.forename: Koya
authors.initials: K
authors.lastname: Suno
authors.affiliation: Tomakomai Experimental Forest, Field Science Centre for Northern Biosphere, Hokkaido University, Tomakomai 053-0035, Japan.
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Osada
mesh_terms: D019659:Asteraceae / Q000378:metabolism / Q000502:physiology; D002244:Carbon / Q000378:metabolism*; D017753:Ecosystem; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism*; D012987:Soil / Q000737:chemistry*; D013045:Species Specificity; D031210:Xanthium / Q000378:metabolism / Q000502:physiology
chemical_list: D012987:Soil; D002244:Carbon; D009584:Nitrogen
keywords: Adenocaulon himalaicum; Xanthium canadense; fertilization experiment; leaf longevity; leaf senescence; repeated measurement; soil fertility