Concordance of long-term shifts with climate warming varies among phenological events and herbaceous species

Many temperate herbs now flower earlier than a few decades ago. Little is known about other phenological events, despite the importance of life history integration for plant fitness. This study addresses the hypothesis that temporal shifts of multiple phenological events in herbs are concordant with temporal changes in weather. Explicitly showing that changes in timing of annual life cycle events are correlated with changes in weather-predicting variables provides support for the hypothesis that a phenological shift is concordant with climate change. Observations of six phenological events and five phenophases were made year-round for 25 yr for herb species in a deciduous forest fragment, Trelease Woods in Illinois, USA. Dates for 43 species were analyzed by linear mixed-effects models for events and phenophases and were compared to weather data from a nearby station. For early species, emergence was delayed by 1.5 d/decade, while end expansion advanced by 3.8 d/decade and begin dormancy advanced by 2.5 d/decade. For late species, end expansion advanced by 6.7 d/decade, while begin senescence delayed by 17.7 d/decade. Begin flowering and end flowering advanced similarly for both seasonal groups, at 3.8 to 4.2 d/decade. Some events showed no temporal change. Species differed greatly in the degree or direction of change, related to seasonality of event or length of phase. Overall, for a given species, most events are advancing (68.4%) and most durations are shortening (74.4%). In 12 of 13 cases, inter-annual variation in event date was predicted by a temperature-event–season combination variable, but in only six cases did both event date shift and weather variable warm through time. This finding supports the hypothesis that climate change is associated with changes in some, but not all, phenological events. This first long-term, multi-phase study of a community of temperate herb species indicates little temporal coherence of responses of multiple phases. Changes in date are event specific, phase specific, and species specific. This complexity of responses among species and uneven responses within a species’ integrated annual cycle events has implications for evolutionary responses and more immediate interactions among plant, animal, and microbe species in this community.