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Climate extremes and predicted warming threaten Mediterranean Holocene firs forests refugia

  1. Juan C. Linaresb
  1. aInstituto Pirenaico de Ecología–Consejo Superior de Investigaciones Científicas, 50192 Zaragoza, Spain;
  2. bDepartamento Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Sevilla, Spain;
  3. cDipartimento Territorio e Sistemi Agro-Forestali, Università degli Studi di Padova, Legnaro 35020, Italy;
  4. dDepartamento Biologia Evolutiva, Ecologia i Ciències Ambientals, University of Barcelona, 08028 Barcelona, Spain;
  5. eFakulteti i Shkencave Pyjore, Universiteti Bujq?sor i Tiran?s, 1029 Tirana, Albania;
  6. fTree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, NY 10964;
  7. gInstitut Català de Ciències del Clima, 08005 Barcelona, Spain;
  8. hForest and Wood Technology Research Centre, 33936 Siero, Asturias, Spain;
  9. iDepartment of Engineering Science and Physics, College of Staten Island, City University of New York, Staten Island, NY 10314;
  10. jDepartment of Ecology and Ecosystem Management, Technische Universit?t München, 85354 Freising, Germany;
  11. kDipartimento Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, 27100 Pavia, Italy;
  12. lDepartment of Forestry and Natural Environment Management, Technological Educational Institute of Stereas Elladas, 36100 Karpenissi, Greece;
  13. mCentro de Estudios Avanzados de la Tierra, Universidad de Jaén, 23071 Jaén, Spain
  1. Edited by William H. Schlesinger, Cary Institute of Ecosystem Studies, Millbrook, NY, and approved September 29, 2017 (received for review May 16, 2017)


Climate extremes are major drivers of long-term forest growth trends, but we still lack appropriate knowledge to anticipate their effects. Here, we apply a conceptual framework to assess the vulnerability of Circum-Mediterranean Abies refugia in response to climate warming, droughts, and heat waves. Using a tree-ring network and a process-based model, we assess the future vulnerability of Mediterranean Abies forests. Models anticipate abrupt growth reductions for the late 21st century when climatic conditions will be analogous to the most severe dry/heat spells causing forest die-off in the past decades. However, growth would increase in moist refugia. Circum-Mediterranean fir forests currently subjected to warm and dry conditions will be the most vulnerable according to the climate model predictions for the late 21st century.


Warmer and drier climatic conditions are projected for the 21st century; however, the role played by extreme climatic events on forest vulnerability is still little understood. For example, more severe droughts and heat waves could threaten quaternary relict tree refugia such as Circum-Mediterranean fir forests (CMFF). Using tree-ring data and a process-based model, we characterized the major climate constraints of recent (1950–2010) CMFF growth to project their vulnerability to 21st-century climate. Simulations predict a 30% growth reduction in some fir species with the 2050s business-as-usual emission scenario, whereas growth would increase in moist refugia due to a longer and warmer growing season. Fir populations currently subjected to warm and dry conditions will be the most vulnerable in the late 21st century when climatic conditions will be analogous to the most severe dry/heat spells causing dieback in the late 20th century. Quantification of growth trends based on climate scenarios could allow defining vulnerability thresholds in tree populations. The presented predictions call for conservation strategies to safeguard relict tree populations and anticipate how many refugia could be threatened by 21st-century dry spells.


  • ?1To whom correspondence should be addressed. Email: rsanchez{at}upo.es.
  • Author contributions: R.S.-S., J.J.C., and J.C.L. designed research; R.S.-S., J.J.C., M.C., E.G., A.H., and J.C.L. performed research; R.S.-S., J.J.C., A.H., and J.C.L. contributed new reagents/analytic tools; R.S.-S., J.J.C., M.C., E.G., A.Q.A., L.A.-H., A.K., E.M.-S., P.N., A.P., E.P., E.T., J.A.C., and J.C.L. contributed to field data; R.S.-S., J.J.C., A.H., and J.C.L. analyzed data; and R.S.-S., J.J.C., M.C., E.G., A.Q.A., L.A.-H., A.H., A.K., E.M.-S., P.N., A.P., E.P., E.T., J.A.C., and J.C.L. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • Data deposition: The datasets reported in this paper have been deposited in the Pangaea repository, http://www.danielhellerman.com/10.1594/PANGAEA.882101.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1708109114/-/DCSupplemental.

Published under the PNAS license.

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