Emergent properties of bio-physical self-organization in streams

Zelforganisatie, de vorming van ruimtelijke patronen door ecologische interacties, is een wijdverbreid fenomeen in natuurlijke ecosystemen. Theoretische studies laten zien dat zelforganisatie een belangrijk regulerend proces in ecosystemen is. Het kan ecosystemen bijvoorbeeld meer bestendig maken tegen verstoringen. Het onderzoeken van zulke bijkomende, spontaan verschijnende gevolgen van zelforganisatie helpt ons in het beheren en beschermen van de natuur tegen klimaatverandering en menselijke invloeden. Voor dit proefschrift heb ik ruimtelijke zelforganisatie van vegetatie in rivieren onderzocht. In het bijzonder heb ik mij gericht op de interacties tussen waterstroming en onderwater groeiende vegetatie en op de gevolgen van deze interacties voor rivierstroming en biodiversiteit. Mijn onderzoek toont aan dat vegetatie voor het rivierecosysteem als buffer werkt tegen veranderende hydrologische condities. Zulke hydrologische condities kunnen uiteenlopen van zwakke stromingscondities tot harde stromingen (overstromingen). Tegelijkertijd ondersteunt onderwater groeiende vegetatie de biodiversiteit in de rivier. Door diversiteit aan leefgebieden in stand te houden, worden condities gecreëerd die gunstig zijn voor andere soorten. Zo biedt dit zelforganisatieproces een natuurlijke oplossing voor stromingsregulatie. Deze bevindingen staan in contrast met het huidige rivierbeheer waarbij vegetatie wordt verwijderd, omdat verondersteld wordt dat begroeiing het risico op rivieroverstroming verhoogt. Terwijl verwacht wordt dat wereldwijde klimaatverandering en menselijk ingrijpen in rivieren de hydrologische extremen zullen doen toenemen, laat deze studie zien hoe zelfgeorganiseerde rivierecosystemen zich kunnen aanpassen om geschikte stromingscondities te handhaven en tegelijkertijd een hoge aquatische biodiversiteit te ondersteunen

Indications of dynamic effects on scaling relationships between channel sinuosity and vegetation patch size across a salt marsh platform

Salt marshes are important coastal areas that consist of a vegetated intertidal marsh platform and a drainage network of tidal channels. How salt marshes and their drainage networks develop is not fully understood, but it has been shown that the biogeomorphic interactions and feedbacks between vegetation development and channel formation play an important role. We examined the relationships among tidal channel sinuosity, marsh roughness, vegetation type (pioneer, Elymus athericus or Phragmites australis), and patch size at different spatial scales using a high-resolution vegetation map (derived from aerial photography) and lower-resolution satellite imagery processed with linear spectral mixture analysis. The patch-size distribution in all vegetation types corresponded to a power law, suggesting the presence of self-organizational processes. While small vegetation patches are more dominant in pioneer vegetation, they were present in all vegetation types. The largest patch size is restricted to E. athericus. We observed an inverse logarithmic relationship between channel sinuosity and vegetation patch size in all vegetation types. The fact that this relationship is observed in both pioneer and later successional stages suggests that after the establishment of a drainage network in the dynamic pioneer stages of salt marsh development, the later stages of salt marsh succession largely inherit the meandering pattern of the early successional stages. Our study confirms recent evidence that no significant changes in the specific features of tidal channel networks (e.g., channel width, drainage density, and efficiency) take place during the later stages of salt marsh development

Self-organization of river vegetation leads to emergent buffering of river flows and water levels

Global climate change is expected to impact hydrodynamic conditions in stream ecosystems. There is limited understanding of how stream ecosystems interact and possibly adapt to novel hydrodynamic conditions. Combining mathematical modelling with field data, we demonstrate that bio-physical feedback between plant growth and flow redistribution triggers spatial self-organization of in-channel vegetation that buffers for changed hydrological conditions. The interplay of vegetation growth and hydrodynamics results in a spatial separation of the stream into densely vegetated, low-flow zones divided by unvegetated channels of higher flow velocities. This self-organization process decouples both local flow velocities and water levels from the forcing effect of changing stream discharge. Field data from two lowland, baseflow-dominated streams support model predictions and highlight two important stream-level emergent properties: vegetation controls flow conveyance in fast-flowing channels throughout the annual growth cycle, and this buffering of discharge variations maintains water depths and wetted habitat for the stream community. Our results provide important evidence of how plant-driven self-organization allows stream ecosystems to adapt to changing hydrological conditions, maintaining suitable hydrodynamic conditions to support high biodiversit

Turbulence-mediated facilitation of resource uptake in patchy stream macrophytes

Many landscapes are characterized by a patchy, rather than homogeneous, distribution of vegetation. Often this patchiness is composed of single-species patches with contrasting traits, interacting with each other. To date, it is unknown whether patches of different species affect each other’s uptake of resources by altering hydrodynamic conditions, and how this depends on their spatial patch configuration. Patches of two contrasting aquatic macrophyte species (i.e., dense canopy-forming Callitriche and sparse canopy-forming Groenlandia) were grown together in a racetrack flume and placed in different patch configurations. We measured 15NH4+ uptake rates and hydrodynamic properties along the centerline and the lateral edge of both patches. When the species with a taller, denser canopy (Callitriche) was located upstream of the shorter, sparser species (Groenlandia), it generated turbulence in its wake that enhanced nutrient uptake for the sparser Groenlandia. At the same time, Callitriche benefited from being located at a leading edge where it was exposed to higher mean velocity, as its canopy was too dense for turbulence to penetrate from upstream. Consistent with this, we found that ammonium uptake rates depended on turbulence level for the sparse Groenlandia and on mean flow velocity for the dense Callitriche, but Total Kinetic Energy was the best descriptor of uptake rates for both species. By influencing turbulence, macrophyte species interact with each other through facilitation of resource uptake. Hence, heterogeneity due to multi-specific spatial patchiness has crucial implications for both species interactions and aquatic ecosystem functions, such as nitrogen retention

How to build vegetation patches in hydraulic studies: a hydrodynamic-ecological perspective on a biological object

Vegetation in freshwater and coastal ecosystems modifies flows, retains sediment, protects banks and shorelines from erosion. Hydraulic laboratory studies with live vegetation or artificial plant mimics, or numerical models with abstracted patches, are often used to quantify the effects of vegetation on water flow and sedimentation. However, the choice of plant and patch characteristics is often not supported by field observations of patch dimensions, density or spacing between consecutive patches. The discrepancy between plants in natural conditions and in flume experiments or numerical studies may affect the relevance of these findings for natural ecosystems. In this study, we provide guidelines for building realistic vegetation patches in hydraulic studies. We collected data on four species of fully submerged freshwater aquatic macrophytes that can grow into well-defined patches. We considered three relevant levels: individual plants (inside patches), isolated patches and multiple neighbouring patches. At the plant level, we observed significant differences in biomechanical traits (Young’s modulus, flexural stiffness), resulting in stem Cauchy numbers ranging from 85.25 to 325.84, and leaf Cauchy numbers from 163.81 to 2003.97. At the patch level, we found significant relationships between patch length, width and height, showing covariation among different patch characteristics. The relationships among patch dimensions differed significantly among sampling sites for three of the four species, suggesting high intraspecific variability in patch sizes. By providing a first set of guidelines for choosing correct and ecologically relevant plant characteristics, this dataset aims to improve our understanding of the complex processes occurring inside and around submerged vegetated patches

Self-organization of river vegetation leads to emergent buffering of river flows and water levels

Global climate change is expected to impact hydrodynamic conditions in stream ecosystems. There is limited understanding of how stream ecosystems interact and possibly adapt to novel hydrodynamic conditions. Combining mathematical modelling with field data, we demonstrate that bio-physical feedback between plant growth and flow redistribution triggers spatial self-organization of in-channel vegetation that buffers for changed hydrological conditions. The interplay of vegetation growth and hydrodynamics results in a spatial separation of the stream into densely vegetated, low-flow zones divided by unvegetated channels of higher flow velocities. This self-organization process decouples both local flow velocities and water levels from the forcing effect of changing stream discharge. Field data from two lowland, baseflow-dominated streams support model predictions and highlight two important stream-level emergent properties: vegetation controls flow conveyance in fast-flowing channels throughout the annual growth cycle, and this buffering of discharge variations maintains water depths and wetted habitat for the stream community. Our results provide important evidence of how plant-driven self-organization allows stream ecosystems to adapt to changing hydrological conditions, maintaining suitable hydrodynamic conditions to support high biodiversity

Data presented in the paper “Flow-divergence feedbacks control propagule retention by in-stream vegetation: the importance of spatial patterns for facilitation”

The research objective was to investigate the main factors affecting the ability of patches of the submerged macrophyte Callitriche platycarpa to influence the dispersal of other aquatic plant species, by trapping vegetative fragments. Hence, we tested the role of propagule traits, spatial patch configuration and hydrodynamic forcing on the number of fragments trapped in a flume laboratory experiment. Moreover, we tested the role of submerged vegetation cover and structure on fragment retention through a field release experiment. The data include a 4-week mesocosm monitoring of vegetative fragment buoyancy, and the number of fragments trapped by submerged vegetation in both flume and field releases