Recurring weather extremes alter the flowering phenology of two common temperate shrubs

The aim of this study is to explore the effects of heavy rain and drought on the flowering phenology of two shrub species Genista tinctoria and Calluna vulgaris. We conducted a field experiment over five consecutive years in Central Europe, applying annually recurring extreme drought and heavy rain events on constructed shrubland communities and recorded the flowering status. Further, we correlated spring temperature and precipitation with the onset of flowering. Both species showed a response to extreme weather events: drought delayed the mid flowering date of Genista tinctoria in 3 of 5 years by about 1 month and in 1 year advanced the mid flowering date by 10 days, but did not affect the length of flowering. Mid flowering date of Calluna vulgaris was not affected by drought, but the length of flowering was extended in 2 years by 6 and 10 days. For C. vulgaris the closer the drought occurred to the time of flowering, the larger the impact on the flowering length. Heavy rainfall advanced mid flowering date and reduced the length of flowering of Genista tinctoria by about 2 months in 1 year. Mid flowering date of Calluna vulgaris was not affected by heavy rain, but the length of flowering was reduced in 1 year by 4 days. Our data suggest that extreme weather events, including alterations to the precipitation regime, induce phenological shifts of plant species of a substantial magnitude. Thus, the impacts of climate extremes on plant life cycles may be as influential as gradual warming. Particularly, the variability in the timing of precipitation events appears to have a greater influence on flowering dynamics than the magnitude of the precipitation.