Original article Climate change effects in a semiarid grassland: Physiological responses to shifts in rain patterns M  onica Ladr  on de Guevara * , Roberto L  azaro, Eva Arnau-Rosal  en 1 , Francisco Domingo, Isabel Molina-Sanchis, Juan L. Mora 2 Departamento de Desertificaci on y Geoecología, Estaci on Experimental de Zonas  Aridas (CSIC), Carretera de Sacramento s/n, 04120, La Ca~ nada de San Urbano, Almería, Spain article info Article history: Received 30 June 2015 Received in revised form 9 August 2015 Accepted 13 August 2015 Available online xxx Keywords: Macrochloa tenacissima Stipa tenacissima Gas exchange Intrinsic water-use efficiency F v /F m Mediterranean abstract Several studies have predicted changes in precipitation patterns affecting both rain quantity and its temporal distribution for the Mediterranean Basin. This three-year study was performed to determine the physiological response of Macrochloa tenacissima, a dominant species in the western Mediterranean grasslands, to these changes. A rainfall manipulation experiment was therefore conducted to test the interaction of two factors: quantity (Q) and frequency (F) of rainfall, both at three levels (100%, 75% and 50% of natural rainfall). A mobile transparent polycarbonate rain-out shelter was designed to cover experimental plots of M. tenacissima when it rained, and then treatments were implemented by wa- tering. Reductions in Q and F caused seasonal down-regulation of net photosynthesis (A) and stomatal conductance (g s ), but the first variable showed greater resistance to change. At the annual scale, only the reductions of F had negative effects on A rates, but without causing significant changes to g s . The decrease in Q and F had opposite effects on intrinsic water-use efficiency (IWUE), enhancing and diminishing it, respectively. However, the response to Q was stronger, even exceeding the range of natural interannual variability. Rainfall Q and F reduction did not decrease F v /F m , as compared to ambient conditions. In conclusion, although the responses to the simulated rainfall patterns did not surpass the current seasonal oscillations of M. tenacissima's physiological parameters, they caused a down-regulation of its gas exchange and increased its water-use efficiency. © 2015 Elsevier Masson SAS. All rights reserved. 1. Introduction Although the predicted changes in rainfall are subject to higher uncertainty than temperature (De Castro et al., 2005), climate change models for the Mediterranean region of Europe have found robust signs indicating a decrease in both quantity and number of annual rainfall events. Changes in their size and seasonal distribution are also expected. This will lead to a greater concen- tration of events in winter, longer droughts in summer and an in- crease in extreme events (Giorgi and Lionello, 2008). The effects of annual rainfall amount on vegetation have been studied extensively (e.g., Le Hou erou and Hoste, 1977; Aronson and Shmida, 1992; Epstein et al., 1997; Knapp and Smith, 2001). However, evidence of the importance of other rainfall pattern components on ecosystem responses to climate, such as seasonal timing, frequency and intensity of precipitation, and drought length, has increased since the start of the century (e.g., Bates et al., 2006; Chou et al., 2008; Knapp et al., 2008). Rain manipulation experiments in hyperarid, arid, semiarid and dry-subhumid ecosystems (drylands) have been rarely imple- mented until recent years (e.g., Yahdjian and Sala, 2006; Thomey et al., 2011; Tielb€ orger et al., 2014). Nonetheless, this fact is not an indication of lack of interest, as these areas occupy ~41% of the terrestrial surface, contain ~46% of global carbon (C) reserves (Safriel and Adeel, 2005) and support ~50% of livestock Abbreviations: Q, rainfall quantity; F, rainfall frequency; A, net photosynthesis at leaf-scale; g s , stomatal conductance at leaf-scale; IWUE, intrinsic water-use effi- ciency; F v /F m , maximum photochemical efficiency of photosystem II; PAR, photo- synthetically active radiation; ITRV, inter-treatment response variability; ISRV, interseasonal response variability; IARV, interannual response variability. * Corresponding author. E-mail address: monigue28@yahoo.es (M. Ladr on de Guevara). 1 Current address: Departamento de Geografía. Universitat de Valencia, Av. Blasco Ib a~ nez 28, 46010, Valencia, Spain. 2 Current address: Departamento de Ciencias Agrarias y del Medio Natural. Universidad de Zaragoza, C/. Miguel Servet 177, 50013, Zaragoza, Spain. Contents lists available at ScienceDirect Acta Oecologica journal homepage: www.elsevier.com/locate/actoec http://dx.doi.org/10.1016/j.actao.2015.08.001 1146-609X/© 2015 Elsevier Masson SAS. All rights reserved. Acta Oecologica 69 (2015) 9e20