Offwell Wetland Survey

Comparison of Species Lists from the Wetland Surveys
1991 and 2000

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Plant Species List 1991 and 2000

The wetland survey of 1991 was carried out a few months after completion of the restoration process. The 2000 survey was carried out nearly ten years after restoration had been completed. A number of points emerge from comparing the two species lists generated by these surveys. These are detailed below.

The 1991 survey had the sole purpose of listing all the plant species which were present in the wetland at that time. It was a walking survey which noted all the different species present over a short period in July. In contrast, the 2000 survey was a full ecological survey carried out in September.

It is important to remember that although restoration was finished by 1991, rhododendron clearance actually started in this area in 1988. The Wetland was actually flooded in March 1989 to create a transition from open water to dry land. Therefore by the time of the 1991 survey, there was significant natural regeneration particularly where it was wetter. Prior to the clearance rhododendron dominated.

The later timing of the 2000 survey (September rather than July) means that the number of plant species may have been slightly under-recorded. This is due to the fact that the foliage and flowers of early-flowering species may already have died back so that the plants had become unrecognisable.

It is important to remember that a species list is merely a record of what species are present at a particular time. It does not give any information on how much of the species is present at the time. One small plant of a species, or an entire stand of a species, covering a substantial area, each give rise to an equal record in the list.

1.   Plant species diversity in the wetlands was comparable in 1991 and 2000. The 1991 survey found 92 species of flowering plants, ferns and mosses, while the 2000 survey found 85 species. This is an 8 to 9-fold increase over the number of plant species in the area prior to restoration. This can be inferred from a survey of another area in the Woodland Education Centre similarly dominated by Rhododendron (The Woodland Project).

This survey found only 12 plant species where the Rhododendron grew. Of these, only three species occurred under the rhododendron canopy. These were all mosses and they were only found in less dense regions, where light levels were slightly higher. All the other species recorded were tree species which had managed to struggle above the level of the rhododendron canopy before it completely closed over. On the death of these trees, there would be no replacement because light levels under the established canopy are too low to permit seedling development.

The related increase in animal diversity is likely to have been of a many times greater order of magnitude than that observed for plant species. This is because each plant will have a number of its own particular associated animal species.

2.    Fifty one species were common to both surveys.  These were either species characteristic of wet woodlands, which would be found in the drier margins of the wetlands, or wetland species. Both types would have colonized the area rapidly, either from the seedbank in the soil or from plant fragments or seeds dispersing from other nearby areas. The seedbank in the soil would have contained an array of seeds from potentially suitable species because the area had been both wet woodland and a wetland in years gone by. wpeC6.jpg (213442 bytes) wpeC7.jpg (51483 bytes)

Wet Woodland Species

Enchanter's Nightshade Yellow Pimpernel

3. Differences between the surveys.  

  • Compositae  
    A number of species found in the 1991 survey have since died out in the wetlands. It is very noticeable that several of the species recorded in 1991, but not in 2000, belong to the Dandelion family, the Compositae. These are classic quick-colonising (what a gardener might term "weed") species, which produce large numbers of easily wind-dispersed small seeds. These rapidly colonise areas of bare or disturbed ground such as that presented by the wetland area soon after restoration. Later on, as other plants begin to establish in the area, these early colonisers are out-competed and will die out. Marsh Thistle and Fleabane which are characteristic of damp, marshy areas still remain.

  • Habitat Changeover. In addition to seeds dispersing into the area, clearance and disturbance of the wetland area soils would have stimulated much of the dormant seed bank in the soil. This would include species characteristic of the different types of habitat to be found in the area at different times in the past. This would have included species characteristic of woodlands as well as wetlands. Only those species which are suited to the new wetland habitat would persist, with the rest gradually dying out. Examples of species such as these are, Slender St. John's Wort, Dog's Mercury, Clover and Silverweed.



Wetland Survey Contents