How changes in spring and autumn phenology translate into growth-experimental evidence of asymmetric effects

Earlier leaf-out and later autumn leaf senescence under climate warming have been linked to increases in plant productivity and ecosystem carbon uptake. Yet, despite the potential implications of shifting phenology for plant carbon uptake, the degree to which phenological changes affect overall plant growth and the partitioning between above- and below-ground biomass remains unclear. Here we use a 3-year experiment to quantify changes in root and shoot growth of three woody plant species (two common European tree species, Fagus sylvatica and Quercus robur, and one shrub Lonicera xylosteum) under spring and autumn warming. In both tree species, the magnitude-and in Quercus even the direction-of the effects of growing-season length on growth depends on whether the warming happened in spring or in autumn. Each day earlier leaf-out in response to warming resulted in total biomass increases of 0.8%-2.5%, whereas delayed senescence led to reductions of 0.2%-2.1%. Advances in leaf-out also led to increased root-to-shoot biomass ratios because root growth was proportionally more stimulated than shoot growth. In the shrub species, earlier leaf-out had no effect, while delayed senescence led to increases in root, but not shoot, biomass. Synthesis. The strong asymmetry between growth responses to spring versus autumn phenology demonstrates that growing-season length per se is a weak indicator of individual-level tree productivity. The results further imply that phenological shifts are reshaping the functional balance between above- and below-ground growth, which is critical for quantifying forest carbon dynamics under climate change.