Summary We compared the water-use characteristics of co-occurring mature Quercus cerris L. and Quercus pubescens Willd. trees growing in resource-limited (mainly water) hilly habitats in Tuscany, Italy. The species differed in their distribu- tion along soil water gradients and in their access to, and use of, water, even though the study year was wetter than average, though with a summer drought. Compared with Q. cerris, Q. pubescens had greater access to soil water (less negative predawn water potentials) and a more conservative water-use strategy based on its relatively low stomatal conductance, high instantaneous water-use efficiency, less negative midday water potential and high soil-to-leaf hydraulic conductance. Quercus cerris had less conservative water-use characteristics than Q. pubescens, exhibiting relatively high stomatal conductance, low instantaneous water-use efficiency, more negative midday water potentials and low soil-to-leaf hydraulic conductance; however, Q. cerris had higher photosynthetic rates than Q. pubescens. Photosynthesis and stomatal conductance were positively correlated in both species. Although a strong corre- lation between ring widths and precipitation patterns was not found, some dry periods influenced ring-width growth. Quercus pubescens has always grown faster than Q. cerris, probably because of more efficient water use, although stand dynamics (driven by exogenous disturbance factors, including coppicing, browsing and competition) cannot be excluded. Ring-width variability, as well as tree-ring growth in dry years, which should be unaffected by stand dynamics, were higher in Q. pubescens than in Q. cerris. Moreover, Q. pubescens recov- ered completely after the drought in the seventies, even show- ing higher tree-ring growth than in the recent past, whereas Q. cerris showed a minor growth decline followed by a recov- ery to values comparable with those observed before the 1970s drought. Beginning in the early eighties, tree-ring growth de- creased in both species, though Q. pubescens showed consis- tently higher values than Q. cerris. These differences can be ex- plained by differences in water-use efficiency. Despite differ- ences between the species in water use and water status, the re- sults are consistent with the interpretation that both are drought tolerant, but that Q. pubescens is at an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with Q. cerris. Keywords: Mediterranean forests, oak, photosynthesis, soil-to-leaf hydraulic conductance, water potential. Introduction Quercus species are adapted to a wide range of soil water con- ditions (Abrams 1990, Dickson and Tomlinson 1996). They usually avoid the development of severely negative tissue wa- ter potentials by deep rooting and stomatal control of gas ex- change, and can tolerate more negative water potentials through osmotic adjustment and other morphological and physiological features (Rambal 1984, Stringer et al. 1989, Abrams 1990, Stone and Kalisz 1991, Bréda et al. 1993, Hamerlynck and Knapp 1996). Interspecific and ecotypic dif- ferences in water relations and gas exchange characteristics have been associated with occupation of sites differing in soil water availability (Wuenscher and Kozlowski 1971, Bahari et al. 1985, Abrams 1990, Kubiske and Abrams 1992, Hamer- lynck and Knapp 1996). Variations among deciduous Quercus species in foliage characteristics, the result of differences among habitats and consequently in selection pressures, are important, particularly in Mediterranean-type ecosystems subjected to erratic environmental constraints (Damesin et al. 1997, Damesin et al. 1998). Quercus cerris L. is a northern Euro-Mediterranean and rather drought-tolerant species that co-dominates with Q. pubescens Willd. large areas of the Ital- Leaf traits and tree rings suggest different water-use and carbon assimilation strategies by two co-occurring Quercus species in a Mediterranean mixed-forest stand in Tuscany, Italy ROBERTO TOGNETTI, 1–3 PAOLO CHERUBINI, 4 SUSANNA MARCHI 5 and ANTONIO RASCHI 2 1 EcoGeoFor Lab, Dipartimento di Scienze e Tecnologie per l’Ambiente e il Territorio (STAT), Università degli Studi del Molise, Contrada Fonte Lappone, I-86090 Pesche, Italy 2 Istituto di Biometeorologia (IBIMET), Consiglio Nazionale delle Ricerche, Via Caproni 8, I-50145 Firenze, Italy 3 Corresponding author (tognetti@unimol.it) 4 WSL - Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland 5 BioLabs, Scuola Superiore Sant’Anna, Viale Rinaldo Piaggio 34, I-56025 Pontedera, Italy Received January 4, 2007; accepted March 21, 2007; published online September 4, 2007 Tree Physiology 27, 1741–1751 © 2007 Heron Publishing—Victoria, Canada by guest on May 21, 2011 treephys.oxfordjournals.org Downloaded from