Differential warming at crown scale impacts walnut primary growth onset and secondary growth rate.

Trees are exposed to significant spatio-temporal thermal variations, which can induce intra-crown discrepancies in the onset and dynamics of primary and secondary growth. In recent decades, an increase in late winter and early spring temperatures has been observed, potentially accelerating bud break, cambial activation, and their coordination. Intra-crown temperature heterogeneities could lead to asymmetric tree shapes unless there is a compensatory mechanism at the crown level. An original warming experiment was conducted on young Juglans regia trees in a greenhouse. The average temperature difference during the day between warmed and control parts from February to August was 4 °C. The warming treatment advanced the date of budbreak significantly, by up to 14 d. Warming did not alter secondary growth resumption but increased growth rates, leading to higher xylem cell production (by 2-fold) and to an increase in radial increment (+80% compared with control). Meristem resumptions were asynchronous without coordination in response to temperature. Buds on warmed branches began to swell 2 weeks prior to cambial division, which was 1 week earlier than on control branches. A difference in carbon and water remobilization at the end of bud ecodormancy was noted under warming. Overall, our results argue for a lack of compensatory mechanisms at the crown scale, which may lead to significant changes in tree architecture in response to intra-crown temperature heterogeneities.