Plants living on the edge: colonization processes of aquatic and riparian vegetation along restored lowland streams

Abstract

Understanding species distributions and patterns in plant diversity is a central goal in ecology. Two contrasting concepts occur in this field, explaining species distributions by species requirements and tolerances to environmental conditions (known as ‘environmental filtering’), or by patterns in the arrival of seeds and viable plant fragments (known as ‘dispersal filtering’). Quantitative information on their relative importance, as well as on the different mechanisms underlying the filtering processes is however generally lacking, but critical for ecosystem conservation and restoration. For vegetation of streams and their riparian zones it is particularly unclear which of the filtering processes dominates. These ecosystems have been heavily degraded worldwide due to human interventions like damming and channelization, and often show only marginal ecological improvement after restoration. More innovative restoration measures, in which construction of narrower and shallower channels was combined with a meandering watercourse and wider stream valleys, seem more promising for ecological success. This thesis aimed at identifying the most important drivers of plant species distribution and diversity patterns along lowland streams and their riparian zones, and assessing ecological responses to innovative restoration. Natural seed arrival patterns along the riparian gradients of restored stream reaches were monitored with seed traps (dispersal filtering). This was combined with field experiments on recruitment of introduced seeds and seedlings along these gradients (environmental filtering), to subsequently compare the influence of both processes on adult distribution patterns in the naturally developing vegetation. Additionally, the aquatic and riparian vegetation at restored reaches was compared to unrestored (still channelized) reaches. Deposited seed numbers and species richness were significantly higher in flooded seed traps than in non-flooded seed traps, demonstrating the importance of restored flooding regimes for riparian plant colonization. Experimental introduction of seeds and seedlings showed that the hydrological gradient acted as a strong environmental filter on germination, seedling survival and seedling growth, through imposing stress (by inundation) at low elevations and resource limitation (water shortage) at higher elevations. This induced a wet-dry gradient in the vegetation, reflecting the preferred habitat conditions as adult plants. Strikingly, patterns in seed arrival foreshadowed this gradient; seeds of species with adult optima at wetter conditions dominated seed arrival at low elevations while seeds of species with drier optima arrived higher up. These results demonstrated that environmental filtering during early recruitment stages, but also non-random dispersal, are important drivers of early successional riparian vegetation zonation and biodiversity patterns. The innovative restoration promoted a higher diversity of riparian plant species, particularly at the channel margins and the land-water interface. The higher flow velocities at the restored reaches resulted in a shift to more typically lotic (running water) in-stream plant communities, indicated by less floating-leaved species and more species with growth forms trailing on the water surface. Widening of the floodplain enlarged hydrological wetland niches resulting in a higher species diversity of typical wetland plants. For future stream restoration projects, combinations of reduced channel dimensions, restored meandering of the watercourse, and a widened but gradually sloping stream valley were shown to be essential.