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Wetland Biotopes: Hydrologic conditions, vegetation zonation and succession. Keith Edwards B259 kredwards59@yahoo.com. Wetland Hydrology. Wetlands – transitional between terrestrial and open water ecosystems Are ecotonal systems Wetland biotopes – determined mostly by hydrology

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wetland biotopes hydrologic conditions vegetation zonation and succession

Wetland Biotopes: Hydrologic conditions, vegetation zonation and succession

Keith Edwards



wetland hydrology
Wetland Hydrology
  • Wetlands – transitional between terrestrial and open water ecosystems
  • Are ecotonal systems
  • Wetland biotopes – determined mostly by hydrology

- Source (precipitation, groundwater, surface water)

- Water Depth

- Flow rate and patterns

- Flooding frequency and duration (pulses)

  • Hydrology – probably most important factor for wetland establishment and maintenance


Modifies and determines






Allows specific


Mitsch and Gosselink, 2000


Hydrology: Influences chemical and physical traits of wetlands - which influences plant species composition

BUT: plants affect hydrology – ET, retarding flow rate


slavo ovice cw water velocity holcova 2006
Slavošovice CW Water Velocity – Holcova, 2006

4 August 2005 (48 hours after) 1 December 2005 (30 hours after)

5 August 2005 (72 hours after) 2 December 2005 (46 hours after)


7 August 2005 (120 hours after) 4 December (104 hours after)

9 August 2005 (168 hours after) 6 December (142 hours after)

hydrologic effects
Hydrologic Effects
  • Due to periodic flooding:

- wetlands lower species diversity than uplands

- waterlogged soils – low O2 levels – other chemical traits different from uplands

- requires plant adaptations

- only small % of all vascular plants have evolved such adaptations

- wetlands have unique flora with low diversity

Hydrologic conditions affect biotope type

- Longer flooding duration – lower species diversity

- Greater fluctuation in flooding / water level – more heterogeneous habitat – more niches available – more species able to establish (lake/pond shores, riparian)

- Flowing waters – increases erosion / sediment deposition rates – more heterogeneity

- Coastal wetlands – low diversity – double stress of flooding and salinity

Wetland hydroperiod – seasonal water level pattern – rise and fall of water in a wetland:

Examples: permanently flooded; seasonally flooded; saturated; temporarily flooded…

  • Degree of fluctuations – can help maintain particular biotopes – constancy of pattern year to year provides stability to wetland
  • Hydroperiod – integrates all inflows and outflows

- influenced by physical terrain and distance to other water bodies

Production – affected by openness of wetland systems to hydrological fluxes:

- systems with greater water flow through have higher NPP (but not always for marshes)

- generally higher flow through means greater nutrient and energy inputs

  • Within-wetland variation in water level fluctuations:

- produces environmental gradients that influence species distribution – plant zonation

Species change across space and time (succession)
  • Affected by:

- internal factors (competition, facilitation, peat accumulation, etc.) – autochthonous

- external factors (disturbance, hydrology, climate change, etc.) - allochthonous

plant distribution zonation
Plant distribution / zonation
  • Originally thought due to successional processes – example: mangroves
  • Now – numerous models of plant change / zonation in wetlands
hydrarch model
Hydrarch Model
  • Wetlands intermediate stage between purely aquatic to purely terrestrial climax community
  • Follows Clements’ ideas of succession and community development:

- Vegetation occurs as recognizable groups of species

- Autogenic processes most important

- Linear sequence of stages ending in a very stable, mature climax stage

Many wetlands are ecotonal – accumulate OM from plant matter and trap sediments – autochthonous factors
  • BUT – wetlands also subject to allochthonous factors

- Even if current upland stage on former wetlands does not mean change was due to autochthonous factors – must be tested

Now – hydrarch model considered to be only partly correct:

- OM accumulation self-limiting process – rate decreases as wetland becomes drier – result in drier wetland biotope but not necessarily an upland community

- External factors can affect hydrology

- Northern peatlands likely exception – more direct control over hydrology – but may not change over time into uplands (paludification)

- Lakes / ponds – some upland forests do occur on former wetlands – but again not clear if due to autochthonous factors – ex. INDU wetlands

indu ponds
INDU Ponds
  • Cowles (1899, 1901) and Shelford (1911) – first studied area
  • Ponds – formed by glacial action – of differing ages from inland to edge of Lake Michigan
  • Found young ponds deepest, dominated by aquatic plants
  • Older ponds – shallower with emergent vegetation
  • Oldest – little or no standing water – drier wetland systems or upland communities
  • Thought represented successional sequence
indu ponds24
INDU Ponds
  • Later studies – 1980’s, 1990’s
  • Wilcox and Simonen (1987):

- same plant species sequence found (youngest to oldest)

- studied pollen and macrofossil record in sediment cores of one pond – whether similar sequence over time

- older than 150 years BP – diverse grouping of aquatic, emergent and floating species

- 150 years BP to now – rapid species change due to European settlement and industrialization

Singer (1996):

- pollen and macrofossil record in pond

- compared to regional terrestrial pollen record – record of long-term regional climate change

- Pond vegetation mirrored changes in terrestrial pollen record

- Concluded – changes in pond vegetation due more to regional climate changes than autochthonous factors

Plant species changes in wetlands:

- interaction between autochthonous and allochthonous factors

  • Successional theory – lakes and wetlands considered to be “immature” ecosystems – higher production than respiration rates
  • However – wetlands are detrital-based systems – is trait of “mature” ecosystems
  • Wetlands have traits of both “immature” and “mature” systems
trade offs
  • Grime’s CSR model – plants grouped by life history traits, and response to stress and disturbance
  • Predict where species fall along stress and disturbance gradients:

- Menges and Waller (1983) plant distribution in riparian wetlands related to flooding frequency and plant physiological traits

Basis – plants require same types of resources:

- Light

- Water

- Nutrients

- Space

  • Fundamental niches of species overlap at rich end of nutrient gradient

Some species better

competitors – others

better able to tolerate








Trade-off in allocation

of fixed carbon



Better competitors –

found in nutrient-rich, less

stressful areas

  • Actual distribution of plants along gradient – realized niche







Poorer competitors – better

stress tolerators – found in

poorer quality habitats


Real wetlands – need to test that competition / stress

tolerance main factors influencing plant distribution

centrifugal organization model
Centrifugal Organization Model
  • Expanded CSR model
  • Wisheu and Keddy (1992):

- Multiple gradients in wetlands

- Benign ends of all gradients – core habitats – dominated by better competitors – lower species diversity

- More adverse conditions – dominated by species with

adaptations to particular stress / disturbance

- Species diversity – hump-backed curve – highest in mid parts of gradients – but biomass greater in core habitat

Model predictions:

- Rare species restricted to peripheral habitats

- Moore et al (1989) - North American wetlands – found rare species were restricted to infertile, peripheral sites

- Eutrophication – shift species composition towards that of core habitat – favor more competitive species – removal of peripheral habitats – decreased species diversity

assembly rules environmental sieve models
Assembly Rules / Environmental Sieve Models
  • Assembly Rules (functional guilds):

- Plant species placed into groups based on functional traits

- Keddy (2000) – clustered 43 wetland species into 7 groups – fall along life history and growth forms in relation to light levels

- Traits used to predict presence and response of plants to an important environmental factor (filter)

- Filters – environmental conditions that restrict species pool

- Can predict which plant traits prevent establishment / presence of particular species

- Predict changes in species composition as environment changes

Environmental Sieve Model (van der Valk, 1981):

- Gleasonian approach – continuum model

- Developed to explain plant composition in prairie potholes under flooded and drained conditions

- Prairie potholes – shallow depressional wetlands in mid- continental North America – typified by fluctuating water levels

- Model – three life history traits;

- life span (annual, short-lived or long-lived perennials)

- propagule longevity – how long reproductive parts viable and available (persistent, forming a seed bank or short-lived but easily dispersed)

- plant establishment requirements (require exposed soil – Type 1; can have standing water – type 2)


12 possible

life history types

Example: Phragmites – is a VD-1 (long-lived perennial, V;

easily dispersed, short-lived seeds, D;

requires exposed soil to germinate -1)

- As environmental sieves change – other life history types favored:

- long-term flooded wetlands – few or no annuals

- S species favored – form seed banks

- long-lived perennials persist

- Sieve model:

- qualitative model- predicts species presence but not amount

- ignores autogenic factors

- Easily incorporate other environmental factors:

- include fire frequency to predict change in species composition of depressional swamps in southeast US

- swamps frequently drawn down – then more susceptible to fire

- high fire frequency – results in grass / sedge meadows

- intermediate fire frequency – hardwoods (maple) killed – end result is cypress (Taxodium ascendens) savannas

- low fire frequency – mixed hardwood / cypress swamp