title: Optimal balance of water use efficiency and leaf construction cost with a link to the drought threshold of the desert steppe ecotone in northern China.
abstract: In arid environments, a high nitrogen content per leaf area (Narea) induced by drought can enhance water use efficiency (WUE) of photosynthesis, but may also lead to high leaf construction cost (CC). Our aim was to investigate how maximizing Narea could balance WUE and CC in an arid-adapted, widespread species along a rainfall gradient, and how such a process may be related to the drought threshold of the desert-steppe ecotone in northern China.
  
  
  
  Along rainfall gradients with a moisture index (MI) of 0·17-0·41 in northern China and the northern Tibetan Plateau, we measured leaf traits and stand variables including speciﬁc leaf area (SLA), nitrogen content relative to leaf mass and area (Nmass, Narea) and construction cost (CCmass, CCarea), δ(13)C (indicator of WUE), leaf area index (LAI) and foliage N-pool across populations of Artemisia ordosica
  
  
  
  In samples from northern China, a continuous increase of Narea with decreasing MI was achieved by a higher Nmass and constant SLA (reduced LAI and constant N-pool) in high-rainfall areas (MI > 0·29), but by a lower SLA and Nmass (reduced LAI and N-pool) in low-rainfall areas (MI ≤ 0·29). While δ(13)C, CCmass and CCarea continuously increased with decreasing MI, the low-rainfall group had higher Narea and δ(13)C at a given CCarea, compared with the high-rainfall group. Similar patterns were also found in additional data for the same species in the northern Tibetan Plateau. The observed drought threshold where MI = 0·29 corresponded well to the zonal boundary between typical and desert steppes in northern China.
  
  
  
  Our data indicated that below a climatic drought threshold, drought-resistant plants tend to maximize their intrinsic WUE through increased Narea at a given CCarea, which suggests a linkage between leaf functional traits and arid vegetation zonation.
authors.affiliation: Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China weihaixia@itpcas.ac.cn wubo@caf.ac.cn.
authors.forename: Haixia
authors.initials: H
authors.lastname: Wei
authors.affiliation: Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China.
authors.forename: Tianxiang
authors.initials: T
authors.lastname: Luo
authors.affiliation: Institute of Desertification Studies, Chinese Academy of Forestry, Beijing 100091, China weihaixia@itpcas.ac.cn wubo@caf.ac.cn.
authors.forename: Bo
authors.initials: B
authors.lastname: Wu
mesh_terms: D019306:Artemisia / Q000502:physiology*; D002681:China; D055864:Droughts; D004777:Environment; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology; D014867:Water / Q000502:physiology*
chemical_list: D014867:Water; D009584:Nitrogen
keywords: Artemisia ordosica; Carbon isotope; drought threshold; leaf area index; leaf trait relation; moisture index; sandy land; vegetation zonation