Plant Zonation in Wetlands

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Anything more than a cursory glance at a wetland will reveal that the plants are not indiscriminately placed. They are arranged in distinct, although often overlapping zones. These zones, moving from dry land to open water, encompass:
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Little or no standing water.

Increasing water depth

Open water
Dry Land Marsh Plants Swamp Plants Rooted Floating
Plants		 Free Floating Plants Submerged Plants
Adaptations Adaptations Adaptations Adaptations Adaptations Adaptations


This overall zonation is called a hydrosere. A similar succession of plant species can be observed in any wetland when moving from dry land to open water. The hydrosere reflects differences in the degree of adaptation to aquatic life of different plant species. For example, in the Offwell wetlands, Rhododendron, which prefers drier conditions, is found in the dry land areas at the back of the wetlands. Yellow Iris grows best in wet, but not totally submerged conditions, whereas Canadian Pondweed is found completely submerged in open water. For an illustration of this click here.

The succession of different plant species can also be observed over a period of time in individual areas of a wetland. The plants established in a particular region will continually affect their immediate environment, subtly altering the local environmental conditions. This happens through processes such as the trapping of silt in roots and old leaf bases. This raises the level of the land, making it drier and gradually transforming it into an area more suited to a different set of plant species.

Wetland Development

Open water must be shallow enough for submerged plants to become established. If it is too deep the light penetrating to the bottom is insufficient for plant growth. Plants such as Canadian Pondweed and Water Milfoil colonise these shallow open water regions at depths of up to 3 metres. As time passes, silt becomes trapped among the roots and old leaf bases of these plants, gradually raising the level of the bottom. This allows rooted plants with floating leaves to become established. These include plants such as Water Lilies. These can only become established where the water is shallow enough to allow the leaves to reach the surface. Water Lilies usually grow where the water is 0.5 - 2.0 metres deep, although Yellow Water Lilies have been recorded from depths of up to 3.6m. Other floating leaved plants such as Broad-leaved Pondweed are generally found in water of moderate depth, from 1 - 2 m deep.
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wpe45.jpg (21320 bytes) As they become established, the floating leaved plants gradually shade out the underlying submerged plants. Stands of Water Lilies can form a complete floating carpet of leaves on the water surface, producing a layer sturdy enough for Moorhens to run over.

The roots and leaf bases of these plants also trap additional silt, further raising the level of the underlying sediment. This allows emergent swamp plants such as Yellow Iris and Branched Bur-reed to invade and colonise the area.
wpeC3.jpg (37494 bytes) These species die back in winter and the old leaves and tough fibrous rhizomes  provide an extraordinarily efficient trapping area for sediment entering the wetland in floodwater or in rainfall run-off.

This raises the land level sufficiently for marsh plants to become established. This continual succession process may extend over several years until eventually dry land exists where once there was open water and wetlands.

Trees such as Alder and Willow will colonise areas which are drying out, transforming them into wet woodland or Carr.

 

Offwell Wetland Line Transect Diagrams

Plant Adaptations to Aquatic Life

Wetland Survey Contents