Early dynamics in plant community trait responses to a novel, more extreme hydrological gradient

For temperate regions such as Northern Europe, predicted climate change patterns include an increase in winter precipitation causing increased risk of flooding, whereas periods of droughts will become more frequent in summer. The aim of this study is to explore variations in plant functional trait distributions along a hydrological gradient spanning from recurrent drought events to recurrent flooding—mimicking future precipitation patterns. The experiment was conducted in a controlled grassland experiment over a period of 3 years. A novel and more extreme hydrological regime was achieved by manipulating the flow of a nearby stream thereby creating a continuous hydrological gradient from flooding during winter to drought during summer. Plant community responses were recorded along this hydrological gradient. Community-weighted trait distribution changes along the gradient were described using null models. Six functional traits were considered: seed mass, leaf dry matter content (LDMC), leaf area, leaf thickness, specific leaf area (SLA) and height. Over time, responses in plant functional traits changed at the community level. Over the study period consistent changes occurred in the mean trait value of several traits. Communities in relatively dry plots became dominated by species with water-conserving life strategies, represented by high seed mass and thick leaves. In contrast, disturbance-resistant species (high leaf dry matter content) became dominant in flooded plots, indicating that persistence to flooding was the most important factor controlling the functional structure in those communities. Furthermore, a high abundance of small-seeded species in flooded plots likely indicates higher frequency of species with higher dispersal ability through hydrochory. In conclusion, plant traits are useful for predicting responses to climate change, but abrupt and extreme climate event may cause unexpected responses because they have no analog to previously more stable conditions. We suggest that traits related to dispersal and resistance to disturbance are useful in describing responses to flooding and that these traits should be included in future investigations of plant community responses to extreme hydrological events.