Phenology across scales: an intercontinental analysis of budburst in temperate tree populations

The seasonality of development (phenology) of vegetation is sensitive to temperature. It is one of the most prominent biological markers of current global warming. The budburst period is of particular interest because the budburst date is decisive for the development and survival of deciduous trees. It reflects a trade-off between the need to maximize the growth period and the risks associated with late frost. Our study analyses the intra-community variability (ICV) of budburst dates acquired over 107 site-years in temperate deciduous forests located in the USA (67 site-years) and Europe (40 site-years) using phenological cameras. The average date of budburst shows a virtually identical sensitivity to temperature in American and European forests. The annual ICV of budburst was not significantly different in both continents (with an average value of 3 days, computed as the standard deviation of budburst across the community), despite a lower species richness in European forests (2 species on average) than in American forests (4.5 species on average). Earlier budburst and lower temperatures increased the ICV, which could reach up to 10 days. We suggest that the ecological consequences of the ICV of budburst should be investigated further. We show that over a growing season, the earliest trees of the community absorbed on average 10% more radiation than the latest trees (no difference across continents). This corresponds to a photosynthesis difference of 120 gC m^-2 yr^-1, the impacts of which in terms of individual growth, nutrient and water acquisition and/or exposure to water stress should be further investigated.