Measuring and modelling plant traits in floodplains of regulated rivers

In floodplains, the succession of vegetation towards its climax stadium can be reset, delayed or ceased by abiotic and biotic processes [1]. At century scale, vegetation in floodplains of free flowing rivers may mature into floodplain forests, but are eventually reset to bare sediment. Vegetation in floodplains of regulated rivers are not exposed to the fierce sedimentation and erosion powers of free flowing rivers. Therefore, the riparian vegetation along these rivers likely reaches the climax stadium if no measures are taken [2]. Moreover, unlike the floodplains of free flowing rivers, the floodplains of regulated rivers often need to fulfil several law-enforced ecosystem services, like providing water safety during high river discharges and as nature areas. Unfortunately, combining several ecosystem services is complex and puzzles floodplain managers [3]. Increasing the understanding of how abiotic and biotic processes (hereafter: filters, sensu Keddy [4]) shape floodplain vegetation patterns and influence succession rates, may facilitate floodplain management of regulated rivers. Plant traits are characteristics needed to establish a population under specific conditions and link filters and vegetation composition [5]. Moreover, plant traits can be a useful concept in revealing the dominant filters in shaping patterns, senescence and rejuvenation of floodplain vegetation. Specific traits or trait composition (i.e. a strategy) have been coupled to various riverine processes such as flooding and drought cycles (e.g. [6] [7]). For example, shorter time between floodings, together with the timing and longer duration of a flooding, steer towards species with shorter lifespan, while longer time between floodings and floods with shorter duration select for species with longer lifespan [6] [8]. Another example is Raunkiaer’s life forms [9], as those are good predictors of oxygen and drought stress [10]. However, as Reich [11] pointed out, the back bone of trait selection is the fast-slow continuum (a.o. the leaf economic spectrum (LES)). This continuum refers to the slow or fast use of all resources (nutrients, light and water), even though probably only one resource is limiting, and its alignment with either slow or fast metabolic rates (e.g. photosynthesis, respiration). For example, species thriving under low water availability have also low uptake rates of nutrients and low metabolic rates compared to species that do well in high competitive settings. Any additional filters to this slow-fast continuum selection on traits just modify those traits. In the floodplains of the regulated large rivers in the Netherlands, filters such as substrate type, and hence water and nutrient availability, flooding characteristics, and grazing pressure act on trait composition. Combining those filters with international literature on plant traits, resulted in the conceptual framework of those floodplains (Figure 1). The present field study was carried out to 1) test this theoretical framework, and 2) find out if the various filter-trait relations differed spatially. By combining field data and literature knowledge, a model is being developed to serve as a vegetation development tool in floodplains of regulated rivers.