RESEARCH PAPER Differential and interactive effects of temperature and photoperiod on budburst and carbon reserves in two co-occurring Mediterranean oaks V. Sanz-Pe´ rez 1 , P. Castro-Dı´ez 1 & F. Valladares 2,3 1 Departamento de Ecologı´a, Universidad de Alcala´ , Madrid, Spain 2 Instituto de Recursos Naturales, Centro de Ciencias Medioambientales, Madrid, Spain 3 Departamento de Biologı´a y Geologı´a, Escuela Superior de Ciencias Experimentales y Tecnolo´ gicas, Universidad Rey Juan Carlos, Mo´ stoles, Spain INTRODUCTION Increasing temperatures in Europe and North America during recent decades have been related to earlier bud- burst in many plant species (Chmielewski & Ro¨tzer 2002). Because this advance in budburst may modify important ecosystem processes, such as plant–animal interactions and water and carbon cycles (Myneni et al. 1997; Churkina et al. 2005; Parmesan 2006), many tem- perature-based models have been developed to predict the timing of budburst (Cannell & Smith 1986; Murray et al. 1989; Kramer 1994; Ha¨nninen 1995). These models assume that the rate of development of buds is essentially determined by temperature, and that buds break when a certain number of ‘thermal units’ above a threshold tem- perature has been accumulated, after chilling require- ments have been fulfilled in winter (Hunter & Lechowicz 1992). Consequently, the higher the mean daily tempera- tures in spring, the faster the thermal units accumulate. Despite this general tendency, budburst responsiveness to temperature varies significantly across species due to different threshold temperatures and thermal unit Keywords Global warming; lipids; phenology; Quercus faginea; Quercus ilex; soluble sugars; starch; wood anatomy. Correspondence V. Sanz Pe´rez, Departamento de Ecologı´a, Universidad de Alcala´ , Alcala´ de Henares 28871 Madrid, Spain. E-mail: virginia.sanz@uah.es Editor J. Sparks Received: 22 December 2007; Accepted: 1 May 2008 doi:10.1111/j.1438-8677.2008.00119.x ABSTRACT Effects of temperature and photoperiod and their interactions on budburst and on the use of carbon reserves were examined in two Mediterranean oaks differing in wood anatomy and leaf habit. Seedlings of Quercus ilex subsp. ballota (evergreen and diffuse-porous wood) and Q. faginea (semi-deciduous and ring-porous wood) were grown under two temperatures (12 and 19 °C) and two photoperiods (10 and 16 h) in a factorial experiment. In the 16 h photoperiod at 19 °C, photosynthesis was suppressed in half of the seedlings by covering leaves with aluminium foil. The concentration of soluble sugars, starch and lipids in leaves, stems and roots was assessed before and after budburst. Under the 12 °C treatment (mean current temperature in early spring in the Iberian Peninsula), budburst in Q. faginea occurred earlier than in Q. ilex. Higher temperature promoted earlier budburst in both spe- cies, mostly under the 16 h photoperiod. This response was less pronounced in Q. faginea because its budburst was also controlled by photoperiod, and because this species needs to construct a new ring of xylem before budburst to supply its growth demands. Therefore, dates of budburst of the two spe- cies became closer to each other in the warmer treatment, which might alter competitive relations between the species with changing climate. While Q. ilex relied on carbon reserves for budburst, Q. faginea relied on both car- bon reserves and current photoassimilates. The different responses of the two Quercus species to temperature and photoperiod related more to xylem structure than to the source of carbon used for budburst. Plant Biology ISSN 1435-8603 142 Plant Biology 11 (2009) 142–151 ª 2008 German Botanical Society and The Royal Botanical Society of the Netherlands