Flowering and leaf phenology are more variable and stronger associated to functional traits in herbaceous compared to tree species

Phenology of plants is changing rapidly due to global change which may have a large impact on many interactions in ecosystems. So far studies on the phenology of herbaceous species are largely underrepresented in phenology research which is mainly focused on trees and shrubs. In this study we investigated how different phenological stages during the annual life cycle differ between herbaceous species and trees and how they interact with functional traits. To answer these questions, we recorded multiple generative and vegetative phenological stages of 21 herbaceous and 19 tree species in the Botanical Garden of Jena in 2021. We further measured several functional traits for which relationships to flowering phenology have been found in comparable studies. We found evidence that the studied herbaceous species in comparison to the trees showed a larger variability in phenology indicated by a larger within compared to between growth form standard deviation for all phenological stages. This suggests that herbaceous species occupy more temporal niches to avoid competition. We also found that vegetative height is more strongly associated with the phenology of herbaceous species than with trees. For other functional traits, we were able to confirm known relationships between functional traits and phenology across both growth forms, whereby leaf dry matter content was particularly associated with phenological stages. This work is an explicit example of how useful phenological observations in botanical gardens can be to study and understand the complex relationships between plant phenology and functional traits. Further research is needed to investigate avenues to use widely available trait data to predict species-specific phenology.