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Project: Salt-marsh - Unravelling interacting feedback loops that control non-linear salt-marsh dyn...

Project details

  
Acronym : Salt-marsh
Full project name : Unravelling interacting feedback loops that control non-linear salt-marsh dynamics: combining experiments and modeling
Initiating organisation : University of Groningen
Project leader : T.J. Bouma
Supporting organisation(s)NIOZ Royal Netherlands Institute for Sea Research, Yerseke
Financing : nwo
Project number : 83908320
Start date : Jan 01, 2009
End date : Dec 31, 2014
    

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Programme :ZKO - Carrying Capacity: Line 3 - National Programme Sea and Coastal Research (ZKO) - Carrying Capacity: Line 3 - Hypothesis-driven Research
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Description

Description :

Salt marshes are important coastal habitats in the Dutch Wadden Sea and Dutch estuaries, providing feeding grounds for migratory birds. Salt-marsh development is however increasingly threatened by sea-level rise and overgrazing by increasing goose populations. Research has mainly focused on the separate effects of sea-level rise or grazing, showing that salt marshes are well able to either follow sea-level rise or support large numbers of grazing birds. The implications of sea-level rise for the ability of salt-marshes to continue to support large populations of grazing birds are unknown thus far. Hence, we will study the interacting effect of sea-level rise and increased grazing for the resilience of marshes as important grazer habitats.
We hypothesize that two important salt marshes characteristics, being resistance to sea-level rise and hosting large numbers of migratory herbivorous geese, are intrinsically incompatible. Highly productive salt marshes with tall vegetation that reduce tidal currents can trap enough sediment to keep up with sea-level rise, but have little value as habitat for geese. Salt marshes that harbour large populations of geese have short-grazed vegetation, which may not reduce tidal currents enough to have significant sediment trapping. Hence, intensive grazing may reduce the capacity to compensate for sea-level rise.
The dynamics of salt marshes are determined by feedback processes between vegetation, hydrodynamics, sedimentation and herbivore grazing, which leads to strongly non-linear responses of salt marshes to change. Vegetation locally decreases the flow rate of water, increasing sedimentation which promotes plant growth due to an increased fertility and higher elevation, generating a positive feedback. Herbivores reduce vegetation biomass, which increases herbivore numbers as short swards are more nutritious to herbivores, again generating a positive feedback. Reduced vegetation biomass by grazing, however, lowers hydrodynamic friction, thereby decreasing sedimentation, which lowers plant productivity. Although the separate effects of these feedbacks have been studied, the interactive effects of grazing and sedimentation in determining the long-term resilience of salt-marsh ecosystems to sea-level rise and increasing herbivore numbers is mostly unknown.
In this proposal we aim at

  1. experimentally studying the feedback processes that exist between hydrodynamics, sedimentation, and herbivory, under different combinations of forcing factors (e.g., sea level, sediment and nutrient load, herbivore population size), and their dependence on spatial scale. We will extend existing databases on salt-marsh development in relation to grazing with spatially-explicit description of the effects of grazing on sedimentation and hydrodynamics, and perform manipulative experimental tests to unravel how grazing affects sedimentation by influencing sward biomass, sward structure, and by inducing spatial heterogeneity. We will address both aboveground grazing and destructive grubbing for roots.
  2. to incorporate the quantitative information on these (scale-dependent) feedback loops in a process-based, spatial-explicit Delft-3D hydrodynamic-sediment transport model, and use this model to 2A) develop predictive understanding of long-term, large-scale dynamics and resilience of salt marshes as important feeding grounds for migratory geese, 2B) develop predictive indicators of marsh developmental status and 2C) provide quantitative insight in possibilities of controlling large-scale and long-term sedimentation rates via canopy height by spatially explicit manipulation of grazing (e.g., increase the carrying capacity for geese by adding livestock grazing on too tall canopies that trap too much sediment, and thereby lure geese away from marshes at risk of drowning).


The insights in these non-linear dynamics and the (modeling) tools that will be derived by our study can be directly used to assess consequences of different management strategies on the compatibility of keeping up with sea-level rise and hosting large numbers of geese. Data will be made available via the web and annual workshop will be used to exchange knowledge with organizations responsible for salt-marsh management.

Source: ZKO