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Temperature is widely considered to be a major factor controlling the phenology of boreal and temperate tree species (Sarvas, 1972, 1974; Schwartz, 2003). Several studies have documented changes in the phenology of trees such as advancement of leaf unfolding and flowering of 2–3 d per decade on average during the last 50 yr (Keeling et al., 1996; Menzel, 2000; Penuelas et al., 2002; Walther et al., 2002; and see the meta-analysis of Root et al., 2003; Gordo & Sanz, 2005; Menzel et al., 2006; Richardson et al., 2006), and delay of leaf senescence of 1 or 2 d during the same period (Myneni et al., 1997; Menzel & Fabian, 1999; Menzel et al., 2001). These changes have been attributed to the temperature increase that many regions have experienced in the last decades because of global warming (Menzel et al., 2006). Such changes in the growing period of trees have important consequences on carbon cycling (Chapin et al., 2002) and on the Earth’s climate system because of the feedback between vegetation and atmosphere (Betts et al., 1997; de Noblet, 2000). Changes in the plant reproductive period also have important consequences on the reproductive success of populations, and thus on their dynamics (Sherry et al., 2007). Moreover, changes in growth or reproductive phenology have major consequences on species interactions, either positive or negative which affect the dynamics of communities (Edwards & Richardson, 2004; Cattadori et al., 2005; Sherry et al., 2007). Therefore, it is imperative to precisely assess the impact of the on-going and future climate change on species phenology. However, to achieve this task one crucial question arises: Can we use the trends observed in the last decades as a predictive tool to forecast phenological changes for the future? (Primack et al., 2009). So far, these trends appear linear with an average global warming of 0.6°C, but will they be conserved with a mean global warming of 3°C as projected for the middle of this century in some climate scenarios? (IPCC, 2007). To answer this question we need to consider the response of individuals to warming through an experimental approach, using various levels of warming to explore how their phenology will respond in the long term.
Pioneer studies have consisted of transfer experiments, showing that phenology – and especially leaf bud burst – will be affected by future climate change (Beuker, 1994). However, the temperature increase in these experiments was often not realistic regarding the climate predictions for the future. During the last decade, several experimental works have been conducted to study the impact of global climate change on species phenology. Most of these studies have been conducted on herbaceous species (Sherry et al., 2007) and the intraspecific variability of response has rarely been studied (Franks et al., 2007; Doi et al., 2010). Some other experiments tested the effect of an increasing CO2 concentration or an increasing temperature, or both, on tree species phenology. In most cases, phenology was not affected by increasing CO2 concentration (Guak et al., 1998; Norby et al., 2003; Korner et al., 2005; Asshoff et al., 2006; Kilpelainen et al., 2006), but it was affected by increasing temperature. Leaf unfolding date was often advanced by increasing temperature (Repo et al., 1996; Guak et al., 1998; Arft et al., 1999; Hollister et al., 2005; Kilpelainen et al., 2006), but some studies showed contrasting effects (Norby et al., 2003) or no effect (Jones et al., 1997). However, the response of leaf senescence was highly variable, from strong delay (Norby et al., 2003) to advancement (Kilpelainen et al., 2006), or absence of response (Jones et al., 1997; Arft et al., 1999). Recently, some authors have also tested the effect of increasing rainfall on herbaceous plant reproductive phenology (Sherry et al., 2007) and found no significant effect.
Field studies that experimentally simulated warming have mainly used open-top chambers, either closed (Kennedy, 1995b; Kilpelainen et al., 2006; Walker et al., 2006), or not (Repo et al., 1996; Jones et al., 1997; Arft et al., 1999; Hollister et al., 2005), and night screens (Van Wijk et al., 2003; Prieto et al., 2009). In open-top chambers warming is achieved through high transmittance of solar radiation into the chamber. In night-screen systems, temperature is increased only during the night by reflectance of the infrared radiation emitted by the surface. However, in such passive temperature-enhancing systems, unwanted confounding ecological effects may occur, such as strong changes in moisture, nonconstant warming, different diurnal and night warming, site disturbance, wind exclusion and downregulation of photosynthesis (Kennedy, 1995a; Marion et al., 1997; Ainsworth & Long, 2005; Kimball, 2005). Much fewer studies have used free-air temperature increase (FATI) systems, to overcome these problems (Price & Waser, 1998; Saavedra et al., 2003; Loveys et al., 2005). Such experimental efforts remain scarce, have focused only on temperature increase (except Sherry et al., 2007), and were not designed to study vegetative pheno-logy. Furthermore, these experiments usually concern annual plants, and thus do not explore the long-term impact of changing climatic conditions on plants.
In this study we aimed to determine whether projected levels of warming for the coming decades will lead to linear changes in the phenology of tree species or if more complex responses can emerge. To this end, we focused on three European oak species: common oak (Quercus robur), pubescent oak (Quercus pubescens) and holm oak (Quercus ilex). These species were chosen because they have contrasting geographical ranges: common oak is a temperate–boreal species, holm oak is a Mediterranean species and pubescent oak has an intermediate distribution (see the Supporting Information, Fig. S1). They have different leaf habit (common oak and pubescent oak are deciduous while holm oak is evergreen) and different tolerance to drought stress (Rameau et al., 1989, 2008), are congeneric and they have a high economic importance in Europe. To our knowledge, of these three species, long-term phenological records are only available for common oak (Cannell et al., 1999; Menzel et al., 2001; Ahas et al., 2002), which showed a mean advancement of 3.3 d per decade in leaf unfolding date.
We monitored the phenology (leaf unfolding and leaf colouring dates) of individuals from different populations of each species in a nonintrusive climate change field experiment using FATI systems and precipitation reduction systems. We sought to answer the following questions: Are the measured changes in phenology linear with temperature increase and consistent with those currently recorded in natural populations? Do the changes in phenology vary among species and populations (i.e. at both interspecific and intraspecific levels)? Is the survival of individuals affected by changes in temperature and precipitation?
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Fig. S1 Geographical distribution of Quercus robur, Quercus pubescens and Quercus ilex, and the locations of the populations sampled.
Fig. S2 Variation of soil water content at 15 cm and 30 cm depth in the plots in 2002–2004, according to rainfall exclusion and warming treatments.
Table S1 Mean date of leafing and leaf senescence for each year and species, according to populations, rainfall treatment and warming treatment.
Table S2 Analysis of variance of the total leaf area and of the specific leaf area.
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