Beyond gradual warming: The surprising effects of sudden extreme weather events on flower phenology

Shifts in phenological events have recently been used as “fingerprints” of global warming. Even though this view has been documented in enormous continent-wide datasets, it neglects other facets of climate change which may attribute to these shifts. We argue that single, sudden extreme weather events have a major impact on phenological pattern. Here, we present data containing first evidence of shifted phenological response of plant species to a simulated 100-year extreme drought event, to extremely heavy rainfall and to recurrent soil freeze-thaw cycles in central Europe. Observed shifts did not generally follow one distinct pattern, but where differentiated by species. For example, drought enhanced mid flowering date for Holcus lanatus and delayed mid-flowering date for Calluna vulgaris. Interestingly, community composition further modified phenological response of individual species, e.g. delay of mid-flowering date of Calluna vulgaris by 9.3 days in communities composed of two grasses and two dwarf shrubs as compared to communities composed of two dwarf shrubs only. This modification of phenology by community composition was observed only after the drought event, not for control. Thus, the interaction between extreme weather events and community composition may play a crucial role in phenological shifts. We therefore propose that studies of altered phenology patterns related to climate change would profit from addressing the role of extreme weather events and community composition.