title: Increased water use efficiency does not prevent growth decline ofPinus canariensisin a semi-arid treeline ecotone in Tenerife, Canary Islands (Spain).
abstract: CONCLUSIONS
  Intrinsic water-use efficiency ofPinus canariensis(SweetexSpreng.) growing at a semi-arid treeline has increased during the past 37 years. Tree-ring width by contrast has declined, likely caused by reduced stomatal conductance due to increasing aridity.
  
  
  BACKGROUND
  Rising atmospheric CO2concentration (Ca) has been related to tree growth enhancement accompanied by increasing intrinsic water-use-efficiency (iWUE). Nevertheless, the extent of risingCaon long-term changes in iWUE and growth has remained poorly understood to date in Mediterranean treeline ecosystems.
  
  
  OBJECTIVE
  This study aimed to examine radial growth and physiological responses ofP. canariensisin relation to risingCaand increasing aridity at treeline in Tenerife, Canary Islands, Spain.
  
  
  METHODS
  We evaluated temporal changes in secondary growth (tree-ring width; TRW) and tree ring stable C isotope signature for assessing iWUE from 1975 through 2011.
  
  
  RESULTS
  Precipitation was the main factor controlling secondary growth. Over the last 36 yearsP. canariensisshowed a decline in TRW at enhanced iWUE, likely caused by reduced stomatal conductance due to increasing aridity.
  
  
  CONCLUSIONS
  Our results indicate that increasing aridity has overridden the potential CO2fertilization on tree growth ofP. canariensisat its upper distribution limit.
authors.affiliation: Department of Botany, Ecology and Plant Physiology, Universidad de La Laguna (ULL), C/Astrofísico Francisco Sánchez s/n, E-38207 La Laguna, Tenerife, Spain.
authors.forename: Patricia
authors.initials: P
authors.lastname: Brito
authors.affiliation: Ecophysiology of Plants, Department of Ecology and Ecosystem Management, Technische Universität München, Von-Carlowitz-Platz 2, 85354 Freising, Germany.
authors.forename: Thorsten E E
authors.initials: TE
authors.lastname: Grams
authors.affiliation: Ecophysiology of Plants, Department of Ecology and Ecosystem Management, Technische Universität München, Von-Carlowitz-Platz 2, 85354 Freising, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Matysssek
authors.affiliation: Department of Botany, Ecology and Plant Physiology, Universidad de La Laguna (ULL), C/Astrofísico Francisco Sánchez s/n, E-38207 La Laguna, Tenerife, Spain.
authors.forename: Maria S
authors.initials: MS
authors.lastname: Jimenez
authors.affiliation: Department of Botany, Ecology and Plant Physiology, Universidad de La Laguna (ULL), C/Astrofísico Francisco Sánchez s/n, E-38207 La Laguna, Tenerife, Spain.
authors.forename: Agueda M
authors.initials: AM
authors.lastname: Gonzalez-Rodríguez
authors.affiliation: Institute of Botany, Leopold-Franzens-Universität Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria.
authors.forename: Walter
authors.initials: W
authors.lastname: Oberhuber
authors.affiliation: Department of Alpine Timberline Ecophysiology, Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Rennweg 1, A-6020 Innsbruck, Austria.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Wieser
keywords: Canary Island Pine; Climate change; Mediterranean climate; drought; stable carbon isotope; treeline