Sensitivity of leaf unfolding to experimental warming in three temperate tree species

Phenological events are sensitive to climate change, and a clear advancement of spring leaf phenology has been well documented. However, the observed phenological trends cannot be directly used to forecast future changes because the underlying mechanisms are still unclear. We conducted a two-year experimental warming experiment using one-meter-high saplings of birch (Betula pendula L.), oak (Quercus robur L.) and beech (Fagus sylvatica L.). The main objective of this study was to quantify the temperature sensitivity of leaf unfolding to six different warming levels. The results suggested that leaf unfolding dates were significantly advanced with warming for all species in both experimental years. However, different warming treatments elicited different leaf unfolding responses. At low and moderate warming (<+4 °C), the sensitivity of leaf unfolding to temperature (changes in days per °C) increased with warming in all three studied species. For birch, the temperature sensitivity continued to increase with additional warming. For oak and beech, in contrast, the temperature sensitivity dramatically decreased when warming exceeded +4 °C. Our findings confirm that continued and intense warming may not result in further advances of leaf unfolding in the late successional species oak and beech, if the warming exceeds a certain threshold leading to insufficient chilling temperatures for dormancy release.