We studied natural populations of sessile oaks (Quercus petraea (Matt.) Liebl.) growing in two neighboring valleys (Ossau and Luz, denoted O and L, respectively) on the northern side of the Pyrenees in France (from 43°15′N, 00°44′W to 42°53′N, 00°06′E). In each valley, populations grew at elevations varying between 100 and 1,600 m a.s.l., corresponding to a temperature gradient of 6.9 °C between the populations at the lowest and highest elevations [108 (link)]. Functional traits related to phenology, morphology and physiology of these populations have been extensively monitored in situ and ex situ (i.e. in common gardens) over the last 15 years, to quantify the respective contributions of phenotypic plasticity and genetic variation to their within and between population variation [13 (link), 31 (link), 70 (link), 109 (link)]. In addition, reciprocal transplant experiments conducted at a smaller scale confirmed the co-gradient variation [6 (link)] of the timing of bud burst along elevation, thus enhancing local adaptation [80 (link)]. A general overview of the populations sampled in this study is provided in Additional File 2 Table S4 and Fig. 5A.

Sampling buds from sessile oak trees in the Pyrenees. (A): Location of the three selected populations along two elevation clines (Ossau and Luz valleys). Population ID follows that of Additional file 2 Table S4. (B): Shotgun to harvest terminal branches on three canopee, (C): Bud storage in liquid nitrogen, (D): Leaf unfolding observation with binoculars

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