The urban imprint on plant phenology

The modification of the surface radiation and energy balance in urban areas causes their temperatures to exceed those of the surrounding countryside(1). It has thus been suggested that urban environments may serve as field laboratories for studying the effects of a warming climate on biota in a space-for-time substitution(2–5). We investigated changes in the timing of plant phenology and temperature across study sites differing in the degree of urbanization using publicly available pan European data sets for the period 1981-2010(6,7). We found a significant advancement in leaf development, flowering and fruiting phenological phases with higher degrees of urbanization, while a significant delay was observed for leaf senescence phenological phases. Along with these phenological changes an increase of air temperature with higher degrees of urbanization was observed. This increase was largest during the periods of leaf development, flowering and fruiting and smallest during the period of leaf senescence. Based on these results we show that the apparent temperature sensitivity of phenological phases to urban warming is either significantly dampened (leaf development, flowering and fruiting) or reversed (leaf senescence) compared to the temperature sensitivity inferred from temporal changes in phenology and temperature. We conclude that gradients in urbanization represent a poor analog for the temporal changes in plant phenology, apparently due to confounding factors associated with urbanization.