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Offwell Wetland Survey

Plant Zonation along the East/West Line Transect.

The diagram below shows the ranges of twelve of the most important species occurring in the wetland, along the E/W transect line. The range is the total area in which the species is found.

The diagram was drawn from data taken from the continuous line transect survey, which noted every plant to touch the line along its entire length. Data was not used from the interrupted line transects, which recorded plants touching the line at either 1 or 2 metre intervals. This is because this type of transect will tend to underestimate the range of species. They are only recorded as being present if they happen to touch the line at the right point and are otherwise missed.

The blue bars at the bottom of the chart represent the changes in water depth along the line.

The chart clearly illustrates the plant zonation patterns in the wetland. It also illustrates how the boundaries of zones become blurred where certain species overlap and occur in more than one zone. Only two of the plants illustrated (Greater Spearwort and Bog Bean) were confined to one zone. The rest overlap and it becomes difficult to draw a distinct boundary between the zones. Increasing water depths, together with comparisons of the overall composition of the flora are the most reliable indicators of changes in zone.

The greater part of this transect runs through marsh, with a short stretch of dry land on the eastern edge. Here species such as Silver Birch and Willow are found. The Silver Birch does not extend into the marsh area as it is too wet, although it did occur at one point in the marsh. Here, one plant grew on a raised mound, which lifted it above the general water level. Willow however, is found right across the marsh zone, as it prefers the wetter conditions. Alder is another tree species which is tolerant of high soil water levels. This species occurred where water depth was less than 5 cms.

Water Mint, Yellow Iris, Marsh Bedstraw, Wood Clubrush, Reedmace and Branched Bur-reed all extended over most of the marsh zone. Reedmace was the last marsh species to die out in the swamp zone, as the water depth increased.

Greater Spearwort, Water Plantain and Bog Bean all grew in the swamp area, occurring in all but the deepest water of the stream channel.

 

EAST

WEST

Dry Land

Marsh

Swamp

Marsh

.

Greater Spearwort . . . . . . . . . . . . . . . . . . . .
Water Plantain . . . . . . . . . . . . . . . . . . . .
Yellow Iris . . . . . . . . . . . .. . .
Wood Clubrush . . . . . . . . . . .
Marsh Bedstraw . . . . . . . . . . . . . . . . . .
Water Mint . . . . . . . .
Willow . . . . . . . . . . . . . . . . . .
Bog Bean . . . . . . . . . . . . . . . . . . .
Alder . . . . . . . . . . . . . . . . . . . . . .
Reed Mace . . . . . . . . . . . . . . .
Branched Bur-reed . . . . . . . . . . . . . .
Silver Birch . . . . . . . . . . . . . . . . . . . . . .
Water depth 1-5cms . . . . . . . . . . . . . .
6-10cms . . . . . . . . . . . . . . . . . . .
11-15cms . . . . . . . . . . . . . . . . . . . .
16-20cms . . . . . . . . . . . . . . . . .. . . . .
21-25cms . . . . . . . . . . . . . . . . . . . . .
26-30cms . . . . . . . . . . . . . . . . . . . . . . .
31-35cms . . . . . . . . . . . . . . . . . . . . . . .
36-40cms . . . . . . . . . . . . . . . . . . . . . . .
41-45cms . . . .. . . . . . . . . . . . . . . . . . .
46-50cms . . . . . . . . . . . . . . . . . . . . . . .
51-55cms . . . . . . . . . . . . . . . . . . . . . . .
56-60cms . . . . . . . . . . . . . . . . . . . . . . .
 

The diagram above shows the ranges of the major species along the E/W line transect. However, it does not show how much of a particular species occurs at any one point. The range can include one small plant at either end, with a few in the middle or it may indicate a large stand,occupying a substantial area.

Within its range, each plant species will have an optimum zone where the environmental conditions suit it best. Its abundance will usually taper off at either end of its range, with maximum abundance occurring somewhere in between, in the optimum zone.

To access bar charts showing the changes in the percentage cover of each of the twelve major species charted above along their range, click here. The species are similarly colour-coded to aid comparison.


Plant Zonation diagram for the North/South Transect

 

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