Alternate stable states and restoration of intertidal systems

Transcriptie

1 Alternate stable states and restoration of intertidal systems Johan van de Koppel Netherlands Institute of Ecology (NIOO-KNAW) Centre for Estuarine and Marine Ecology

2 Netherlands Institute of Ecology Yerseke

3 This talk Restoration & alternate stable states Shallow lakes, for some background Salt marshes as a new case study Alternate stable states in non-pond ecosystems

4 Restoring ecosystems Pristine state X Degrade d state Human interference degrades ecosystem Restoration often involves reversal of interference Ecosystem may stay in degraded state despite of this reversal

5 Classical example: lake ecosystems

6 Classical example: lake ecosystems

7 Alternate stable states Increased phosphorous loading X

8 Positive feedback (a simplified view) Increased turbidity + + Decreased growth of submergend vegetation Increased algal growth + Increased phosphorous input From Scheffer et al, Nature 2001

9 Catastrophe fold From Scheffer et al, Nature 2001

10 Typical features Bistability: two states are stable Sudden shifts: small perturbation lead to dramatic changes Hysteresis: degradation route differs from recovery root Bimodality: state variables cluster around two values when measure repeatedly in time or space Positive feedback: changes accelerate

11 Salt Marshes Salt marshes as a case study Decline of marsh area in Westerschelde Restoration a big issue in Zeeland

12 Midden-Subatlanticum Early Roman Times 50 AD Tijdsinterval 250 jaar kaart 10 kaart 11 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Lagunes Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Archeologische vindplaatsen

13 Midden-Subatlanticum Late Roman Times Spatial ecology 350 AD Tijdsinterval 150 jaar kaart 13 kaart 12 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Lagunes Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Archeologische vindplaatsen 41

14 Midden-Subatlanticum Merovingian Time Spatial ecology 500 AD Tijdsinterval 150 jaar kaart 13 kaart 14 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Lagunes Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Archeologische vindplaatsen 43

15 Laat-Subatlanticum Middle Ages 1000 AD Spatial ecology Tijdsinterval 250 jaar kaart 15 kaart 16 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Inversieruggen in het schorrenlandschap Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Archeologische vindplaatsen 47

16 Laat-Subatlanticum Late Middle Ages 1250 AD Spatial ecology Tijdsinterval 250 jaar kaart 17 kaart 16 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Bedijkt gebied 49

17 Laat-Subatlanticum Modern Times 1530 AD Spatial ecology Tijdsinterval 280 jaar kaart 17 kaart 18 Noordzee, zeegaten en getijdegeulen Rivieren Strandwallen en duinen Getijdegebied: platen, slikken en schorren Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Bedijkt gebied 51

18 Laat-Subatlanticum Modern Times Spatial ecology 1950 AD Tijdsinterval 200 jaar kaart 19 kaart 20 Noordzee, zeegaten en getijdegeulen Rivieren, zeearmen, kanalen Strandwallen en duinen Getijdegebied: platen, slikken en schorren Kustveenmoeras Hogere gronden: Pleistoceen zand aan maaiveld Bedijkt gebied 55

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20 Problems with the Westerschelde

21 Eroding cliffs in salt marshes

22 Restoring the Westerschelde The effects of dredging have to be compensated (EU-directive) Measures: Retreat: ontpolderen Building dams to reduce wave & flow effects Effects are mixed: pioneer zone doesn t reestablish quickly

23 Feedback processes in salt marshes Research of Bregje van Wesenbeeck

24 Feedback processes on salt marshes Spartina + Higher elevation More nutrients Decreased erosion Increased sedimentation + Sediment

25 Predictions of a simple feedback model Spartina monoculture transition zone bare mudflat 10 Plant biomass Tidal Environmental inundation stress stress

26 How to test for alternate stable states Bimodality Equilibrium vs Biomass Biomass Threshold effects Biomass Threshold Bistability time time

27 Bimodality in salt-marsh vegetation

28 Biomodality in vegetation biomass A: marsh Hypothesis: plant biomass follows a biomodal distribution in the pioneer zone B: pioneer zone Biomass estimated using NDVI Frequency Figures: Biomass (deviation from average) vs frequency C: tidal flat Clear evidence for bimodality! NDVI 125 meter

29 Are there biomass thresholds? Hypothesis: The growth and survival of Spartina anglica (pioneer plant) increases with biomass 3 biomass categories, reflecting life-history stages: Seedlings Rhizomes Tussocks

30 Tussock development 100 A: Paulinapolder B: Krabbekreek % survival rhizomes seedlings tussocks 20 0 May Sep Jan May Sep Jan May Sep Jan May Sep Jan time (months) Van Wesenbeeck et al submitted

31 Stability?

32 Studying stability Hypothesis: Vegetation patches remain stable once the switched to the high-biomass state Testing: overlay analysis of remote sensing pictures covering an extensive period of salt marsh development

33 GIS study of salt-marsh development! GIS analysis by Daphne van der Wal, NIOO - CEME

34 Stability of vegetation patches A: B:

35 Lessons to be learned Salt marshes revealed bimodality and threshold effects, but no conclusive evidence for alternate stable states! Concept of alternate stable states is a simplification, can only explain ecosystem dynamics in part Local feedbacks vs large-scale feedbacks It s validity depends on definition of appropriate spatial and temporal scales Questions on whether alternate stable states exist or not will never be solved if we don t address the issue of spatial scale

36 Dynamics in space

37 Interactions between water & vegetation Tussock Gully Intertidal flat Feedback effect positive 0 negative biomass (grams) AB A Distance C B competition no competition Research of Bregje van Wesenbeeck distance classes => Scale-dependent feedback

38 Salt marsh Development

39 Modeling spatial feedback Explicit hydrodynamics: Delft 3D Linked to vegetation model Simulation for 30 years Includes scale-dependent feedback Tussock Gully Intertidal flat Feedback effect positive 0 negative Distance

40 Modeling spatial feedback Modeling by Stijn Temmerman

41 Feedbacks in space Spatial feedback is important in salt-marsh development & restoration => Can block development Positive feedback does much more that generate alternate stable states: important factor determining the spatial structure of a salt marsh What about the cliffs?

42 Spatial dynamics on a really large scale Spatial feedbacks may lead to complex ecosystem dynamics

43 Salt marsh: making a simple model Plants + + Silt/Clay Schematic cross-section through a salt marsh Wave erosion Flow erosion high Dune or Dike Flow erosion low Assumptions: Positive feedback between sediment erosion and plant growth & erosional losses Erosion of sediment due to (tidal) currents is high at the seaward edge (left), and low at the dike/dune edge of the salt marsh Wave erosion is more severe on sloping sediments

44 Modelling salt-marsh development ds dt Sedimentstion (I) and erosion (2 nd term) a I e max P + ( ) τ 1 α x a S Erosion dependent on slope ds d S dx S dp dt r 1 P K P S S + b c m P d P c + P ds dx P Plant growth Plant mortality Wave destruction dependent on slope

45 Simulated salt marsh development

46 Simulated salt marsh development Disturbance (eg, a storm)

47 Model results Complex dynamics of salt marshes can be explained by a simple feedback between sedimentation and plant growth Effects of positive feedback: Good: Buffering of physical gradient Bad: increased vulnerability: small disturbances lead to runaway collapse Van de Koppel, Van der Wal, Bakker and Herman, AmNat 2005

48 Empirical testing Prediction 1: Salt marshes become increasingly steep at their seaward edge Analysis of marsh cross-sections of a developing marsh confirms prediction (Jan Bakker, University of Groningen) Prediction 2: Cliff erosion and regrowth in front of the cliff can occur simultaneously GIS analysis of salt marshes in the Westerschelde confirms this prediction (Daphne van der Wal) Van de Koppel, Van der Wal, Bakker and Herman, AmNat 2005

49 Do salt marshes develop as predicted? Clay thickness was measured along 33 transect across the salt marsh Data: Group of Jan Bakker, RU Groningen

50 Positive feedback and salt marshes Clay layer thickness Data Harm van Wijnen & Jan Bakker, RU Groningen Clay layer thickness (cm) years 40 years years 10 years Distance from the alt-marsh edge

51 Positive feedback in a spatial context! Salt marsh becomes steeper with age Average slope of salt marsh y = 0002x R 2 = Data Harm van Wijnen & Jan Bakker, RU Groningen Age

52 Positive feedback in a spatial context! Data from Schiermonnikoog confirms that the salt marsh edge becomes steeper with age

53 GIS study of salt-marsh development! GIS analysis by Daphne van der Wal, NIOO - CEME Hellegat polder 1982

54 Simultaneous erosion and regrowth!

55 Salt marsh development and contributing forcing factors Vegetation change per unit area Are salt-marsh dynamics & structure only determined by forcing factors? Wave energy Tidal currents Proximity of channels Area (ha/ha) 0,6 0,4 0,2 0-0,2-0,4 HP TP PP ZG BL HP BH ZP Or is there intrinsic forcing? => self-organization? -0,6-0,8 Vegetation loss Vegetation expansion

56 Alternate stable states & salt marshes The concept of alternate stable states can only be used as an approximation of salt-marsh dynamics Positive feedback may generate dynamics that are far more complicated than that of alternate stable states models New concepts of the implications of positive feedback are required, that take into account space (unless you work in small ponds)