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Tree Physiology Advance Access originally published online on December 5, 2008
Tree Physiology 2009 29(2):249-259; doi:10.1093/treephys/tpn023
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Summer drought impedes beech seedling performance more in a sub-Mediterranean forest understory than in small gaps

T. Matthew Robson1,2, Jesús Rodríguez-Calcerrada3, David Sánchez-Gómez1,3 and Ismael Aranda1,2

1 Instituto Nacional de Investigaciones Agrarias y Tecnologías Agroalimentarias, Centro de Investigación Forestal, Carretera a Coruña Km 7.5, E-28040 Madrid, Spain
2 Corresponding author (robson{at}inia.es; aranda{at}inia.es)
3 Unidad de Anatomía, Fisiología y Genética Forestal, Escuela Técnica Superior de Ingenieros de Montes, Universidad Politécnica de Madrid, Unidad Mixta INIA-UPM Ciudad Universitaria s/n, E-28040 Madrid, Spain


  Abstract

Refugia of mixed beech forest persist in the central mountains of the Iberian Peninsula at the south-western limit of European beech (Fagus sylvatica L.) distribution. The lack of beech regeneration is a concern in this region that has experienced reduced rainfall and higher temperatures over the past 30 years. Beech is considered especially susceptible to climate change because of its conservative shade-tolerant growth strategy; hence seedling responses to drought stress in gaps and in the understory are of particular interest. During the summer of 2007, a watering treatment raised the soil water content by up to 5% in gap and understory plots of beech seedlings in a mixed beech forest. Root-collar diameter was increased by our watering treatment in understory seedlings. Neither drought-avoidance through stomatal closure nor physiological drought-tolerance mechanisms were able to mitigate the effects of water stress in the understory seedlings, whereas osmotic adjustment enhanced the ability of the gap seedlings to tolerate water stress. Overall, high photosynthetic rates in the gaps, despite the photoinhibitory effects of high radiation, allowed gap seedlings to survive and grow better than the understory seedlings irrespective of water availability. Our results indicate that further intensification of summer drought, predicted for the Iberian Peninsula, will hinder the establishment of a beech seedling bank in the understory because of the conflicting seedling trait responses to simultaneously withstand water stress and to tolerate shade.

Keywords: chlorophyll fluorescence, climate change, Fagus sylvatica, functional traits, P–V curves, water-use efficiency, 13C

Received June 10, 2008; Accepted October 2, 2008


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