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Offwell Wetland Survey

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  • 1.     The flora of the wetland area is now dominated by plant species characteristic of wetlands in England.

    Where previously little but Rhododendron grew (Before), there now exists a wide range of plant species characteristic of wetlands. These include Yellow Iris, Water Mint, Branched Bur-reed and Reedmace, amongst many others. A total of 85 species were recorded in the course of this survey. Species List   This represents an eight to nine-fold increase in the plant diversity of the area.
  • 2.     There is a clear transition in the wetland area from open water, through swamp and marsh to dry land.

    Plant species are zoned accordingly within these four distinctive areas. Species preferring drier soil conditions, such as Silver Birch and Bramble tend to be restricted to the dry land margins of the wetland. Marsh species such as Yellow Iris and Water Mint thrive in the wetter marsh areas, while Branched Bur-reed and Bog Bean dominate in the swamp zones with permanent standing water. Canadian Pondweed grows only in the open water areas.

    For bar charts demonstrating the distribution of a few selected plant species which clearly illustrate the extremes of zonation, click here.

    This plant zonation is also clearly visible when looking at the Line Transect diagrams drawn for the wetland area.

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Line transect diagrams
These illustrate the changes in vegetation along the lines.

 

  • 3.     The zones within the wetland are distinct but overlap. Some plants may occur in more than one zone and the boundaries between zones are often blurred.

    The differing ranges of some of the main plant species in the wetland in relation to water level, have been illustrated in plant zonation diagrams. The range of a species is the region over which it occurs in a particular habitat. By plotting the ranges of different species along the transect lines, plant zonation in the wetland area is clearly illustrated. The overlapping of species between zones is also apparent. There is one plant zonation diagram for each transect through the wetland.

 

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Plant zonation diagrams
illustrate the range of different species in relation to water level.

    

  • 4.     Individual plant species are not uniformly distributed along their ranges in the wetland.

    Access a series of bar charts which show the changes in the % cover of all the major plant species shown in 3. above, along their ranges. The changes in % cover are illustrated in relation to water depth. The species are similarly colour-coded to aid comparison between the plant zonation diagrams and the bar charts. This data is derived from the belt transects.

 

  • 5.   The six most dominant species in each of the major zones within the wetland are shown in the table below.

Dry Land

Marsh

Swamp (South)

Swamp(West)

Open Water

Rhododendron  58% Wood Clubrush 37% Yellow Iris 46% Bog Bean 67% Canadian
Pondweed

18%

Willow 22% Yellow Iris 28% Duckweed 31% Water Mint 32%
Silver Birch 18% Willow 19% Bur-reed 26% Wood Clubrush 25%
Bramble 16% Water Mint 15% Water Mint 23% Greater Spearw. 18%
Scots Pine 10% Soft Rush 15% Willow 8% Bur-reed 6%
Soft Rush 5% Eurhynchium praelongum 12% Canadian Pondweed 7% Reedmace 5%
 
  • The numbered percentages next to each species are the mean % cover of the species in the respective zones.

    The figures for the dry land and marsh areas are derived from the amalgamated mean values from both the N/S and E/W transects. The figures for the two swamp areas (southern and western) are each derived from the single transects crossing these areas. The open water data comes from the N/S transect, which was the only one to cross open water. Diagram

    Those with a head for figures may note that the % cover figures in several of the columns add up to well over 100%. This is because the vegetation in the wetland is layered vertically. One plant may completely overlap another so that the total percentage cover in a single quadrat may be several hundred percent.

    The dry land areas of the wetland are dominated by tree species, which because of their size naturally cover large areas. Rhododendron is still the dominant species in this region. The marsh area adjoining the dry land zone, where conditions are too wet to suit the Rhododendron, prevents it from encroaching further. However, it is easy to see that if the wetland were allowed to silt up to any extent, the Rhododendron would once more rapidly cover the area.

    Rhododendron is a non-native species which is often extremely invasive. Another such species, Japanese Knot Weed, also occurs in the wetland area (species list). Unlike Rhododendron, this species thrives in wet conditions. It therefore has to be controlled and removed from the wetland as far as possible at frequent intervals.

    The table illustrates the point already made, that different species overlap zones within the wetland. Willow is one of the six dominant species in three of the zones. Yellow Iris and Water Mint are also dominant in more than one zone.

    The tallest layer in the marsh area is made up of young Willow trees. Under and between these small trees (approximately 2 -3metres in height), Yellow Iris and Wood Clubrush dominate in a layer slightly over 1 metre high. Water Mint twines in amongst their stems, gaining support from the adjacent vegetation. Surprisingly, one of the dominant species in this region in the moss, Eurhynchium praelongum. This is predominantly a wet woodland species, although it has a marsh form. This clothes the bases of the plant stems in much of the marsh area.

    The two swamp zones are very different to each other, with different dominant species. Only Water Mint and Branched Bur-reed are common to both. The Branched Bur-reed is much more common in the southern marsh area than in the western. The difference between them is likely to be due to the fact that the western swamp area has the stream flowing into it. There is therefore constant current and high water depths.

    The only plant in the open water zone picked up by the belt transect was Canadian Pondweed. It should also be noted that the floating Common Duckweed tended to be confined to the swamp areas. This is because the emergent plant stems provide barriers to its movement. On the open water, it drifts and is carried off by minor currents and wind to collect in marginal areas.

  • 6.     Animal biodiversity must have increased to an even greater extent than plant biodiversity since restoration of the wetland.

    Each plant species will have usually have several different invertebrate species associated with it. Therefore for every extra plant species there will usually also be one or more animal species added to the fauna of the wetland more.

    To access a partial animal species list for the open water area, comprising twelve species, click here.

    A very general description of the wetland fauna can be found here.

    Links to information on some wetland species are also provided here.

    Dragonfly species list for the wetlands.

 

Wetland Survey Contents

 

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