Season-dependent phenological responses and implications for co-occurring plants and butterflies under climate anomalies and extreme weather events.

Despite extensive work on climate-driven phenological shifts, research has focused primarily on early-season events, single metrics and gradual shifts in mean climate, limiting insights into phenological responses across the full annual cycle. Using a 17-year dataset (2009-2025) from an old-growth forest, we examined how co-occurring plants and butterflies responded to climate anomalies and extreme weather events (EWEs) across multiple phenological metrics and seasons, and whether variation in their phenological overlap had ecological implications. Including EWEs substantially improved model performance, particularly for butterfly phenology, and revealed shifts opposite to those driven by climate anomalies, indicating distinct effects on phenology. Responses were also markedly season- and taxon-specific. When plants and butterflies diverged, the pattern varied between seasons: in spring, differences were primarily in magnitude, whereas in summer, they more often involved opposite directions (i.e. one taxon advanced while the other was delayed). However, these differential responses did not translate into changes in phenological overlap in response to climate anomalies or EWEs. Greater overlap was nonetheless associated with higher butterfly diversity, and this association was season-dependent. Our findings suggest that both phenological responses and their ecological consequences may depend strongly on seasonal context, highlighting the importance of considering seasonality and EWEs alongside mean climatic conditions when assessing phenological shifts under climate change.