Drought and mistletoe reduce growth and water-use efficiency of Scots pine

[EN] To what extent do mistletoes contribute to growth decline in drought-prone forests? Can the rising atmospheric CO2 concentrations offset the negative impacts of drought and mistletoe infestation on tree growth? Long-term data on growth and intrinsic water use efficiency (iWUE) may allow answering both questions. We used dendrochronology to assess long-term changes in radial growth and iWUE in Scots pine (Pinus sylvestris) trees severely infested by mistletoe (Viscum album) as compared to non-infested trees. The relationships among tree variables and mistletoe infestation were quantified using structural equation models. Linear mixed-effects models of basal area increment as a function of climate were fitted to severely infested and non-infested trees. Infested trees showed higher stem and crown diameters because they grew faster than non-infested trees in the past. Mistletoe infestation enhanced defoliation and reduced radial growth for more than 10years prior to sampling, while iWUE was significantly lower on severely infested trees only for the last 5years. Severely infested trees had higher growth responsiveness to drought stress than non-infested trees. Although infested and non-infested trees displayed similar rising iWUE temporal trends, the combined effect of drought stress and mistletoe infestation caused a reduction in growth and reversed the CO2-induced increase of iWUE in infested trees. We conclude that rising atmospheric CO2 concentrations cannot compensate for the impacts of drought and mistletoe on tree growth and iWUE. © 2013 Elsevier B.V.This study was supported by projects CGL2007-66066-C04-02/BOS, CGL2008-04847-C02-01 and CGL2011-26654 (Spanish Ministries of Science and Economy). JJC acknowledges funding by ARAID and collaborative efforts within the Globimed network (www.globimed.net). We thank the ICO and the Spanish Meteorological Agency for providing bird ringing and climate data, respectively. We thank J. Lichstein and two anonymous reviewers for their constructive comments on a previous version of the manuscript. We sincerely thank colleagues from the Forest Health Laboratory (Mora de Rubielos, Gobierno de Aragón) for their help and comments.Peer Reviewe