Diversity of plant assemblages dampens the variability of the growing season phenology of wetland ecosystems

Background: The functioning of ecosystems is highly variable through space and time. Climatic and edaphic factors are forcing ecological communities to converge, whereas the diversity of plant assemblages dampens these effects by allowing communities’ dynamics to diverge. This study evaluated whether the growing season phenology of plant communities within wetland ecosystems is determined by the climatic/edaphic factors of contrasted regions, by the species richness of plant communities, or by the diversity of plant assemblages. From 2013 to 2016, we monitored the phenology and floristic composition of 118 plant communities across five wetland ecosystems distributed along a gradient of edaphic and climatic conditions in the Province of Quebec, Canada. Results: Growing season phenology of wetlands was driven by differences among plant assemblage within ecosystems, and not by the species richness of each individual community (<1% of the explained variation). Variation in the growing season length of wetlands reflected a balance between the effects of climatic and edaphic factors on green-up dates and the dampening effects of plant assemblage diversity on green-down dates. Conclusions: The latter dampening effect may be particularly important in the context of increasing anthropogenic activities, which are predicted to impair the ability of wetlands to adapt to fluctuating environmental conditions. Our findings suggest that stakeholders should not necessarily consider local monospecific plant communities of lower conservation value to the global functioning of wetland ecosystems.