Contrasting responses of forest phenological guilds to complex floodplain change

Understanding the seasonal dynamics of the forest herb layer and the factors that influence it is essential for predicting how the forest ecosystem, including mutualistic interactions, will respond to global change. However, to date, no research has investigated how understorey phenological guilds respond to major environmental threats and forest interventions. We showed the marked changes in the phenological structure of floodplain forest plant communities over half a century at a multi-regional scale in Central Europe. Unlike previous studies focusing on individual species' responses to climate change, we examined shifts in species richness, frequency, and abundance between phenological guilds within the plant community. We examined the effects of temporal variation in climate, hydrology, soil conditions, and canopy structure, and assessed the effects of no-intervention and intervention management on phenological guild responses. Our results show a significant loss of local species richness and a decline in species frequency in the forest understorey vegetation, associated with species exhibiting summer phenology. In contrast, an increase in abundance and richness of spring flowering species was observed, attributed to high nutrient loads and reduced flooding, respectively. The abundance of spring species increased only in stands where canopy cover increased over time, probably due to the suppression of summer flowering species, allowing spring flowering species to spread on open bare ground. The community experienced a shift in maximum flowering plant richness from June to May. Despite management interventions such as clear-cutting and tree planting, our data show similar temporal trajectories in intervention and non-intervention forests, indicating that changes in phenological structure are largely independent of recent management activities.

Synthesis. We propose that a complex interplay of environmental factors, rather than climate change alone, is shaping shifts towards earlier phenological guilds in floodplain forests. This study highlights the importance of considering a comprehensive view of phenological guild dynamics and the impact of environmental factors on forest plant community structure in the face of global change.