Tree roots lack dormancy and can advance budburst when warmed

The initiation of tree growth in spring is typically linked to leaf-out. Bud dormancy drives budburst timing while dormancy of tree roots has largely remained unexplored, although the latter can shape below-ground growth and carbon dynamics. As roots experience different temperatures from buds, their dormancy dynamics and growth timing can differ and need to be studied, in order to better understand above- and below-ground growth responses to climate warming.

We evaluated differences in dormancy dynamics between roots and buds in Fagus sylvatica and Populus nigra by quantifying the amount of warmth required to initiate above and below-ground growth from October to February. We furthermore carried out seven experiments, manipulating only the soil temperature prior to or during tree leaf-out to evaluate the potential of warmer roots to advance budburst timing. Soil temperature was manipulated via snow removal, heating buried wires, insulated pots, soil as well as chamber soil warming, using tree seedlings and adult trees of Fagus sylvatica as well as tree seedlings of Betula pendula.

Root dormancy was virtually absent in comparison to the much deeper and variable bud dormancy, with roots being able to start growing immediately after being exposed to warm temperatures during the winter. Furthermore, warmer soil temperature advanced budburst in the meta-analysis of all soil temperature manipulation experiments. Therefore, differences in root and bud dormancy dynamics and their interaction likely explain the non-synchronized below and above-ground growth periods, both processes requiring separate predictions under climate warming.