Meteorological conditions control the cessation rather than the beginning of wood formation in a sub-Mediterranean ring-porous oak

We studied the relation between xylem and leaf phenology, and how environmental factors can modify the growth rhythm of Quercus pyrenaica Willd. on the boundary of its northwestern distribution. For this, wood formation and leaf phenology were monitored in 2012 and 2013 at five sites along an altitudinal gradient. Ten trees were randomly sampled at each site every 10-12 days, their leaf phenological stage recorded, and a microcore extracted to study cambial activities. These events were related to each other, as well as to air temperature for 10-60 day running periods along the year, in order to identify the association between processes, and the most relevant time windows for the onset and cessation of tree growth. The first earlywood vessels began their expansion before budburst, and leaf unfolding occurred when at least the first row of earlywood vessels had undergone maturation. However, the temperature regime did not explain the beginning of cell maturation, whereas the cessation of cell enlargement was delayed by maximum temperature, and to a greater extent by thermal amplitude provided that water is available. Primary and secondary growth of Q. pyrenaica were clearly synchronized at all sites, confirming that the maturation of the first earlywood vessels is essential for crown development. This strong synchronicity suggests that temperature is not the main factor for the beginning of primary and secondary growth; on the contrary, the cessation of wood formation appears to be more dependent on temperature, though also modulated by precipitation.