3.1. General ecology of wetland vegetation

We will first review some basic knowledge on the ecology of wetlands, as well as the main constraints on plants and their adaptations, and how this knowledge can be used to manage wetlands. Then, we will present the questions raised by reedbed management in Prespa, and the results of our field studies and experiments.

The general ecology of wetland vegetation

Wetland values and vegetation management

Wetlands are one of the most important types of ecosystem for human populations, producing many ecological services, but paradoxically one of the most impacted by global change, including climate change and the intensification of human activities. Vegetation is a very important component of the functioning of a wetland ecosystem and thus contributes to many ecological services, such as regulating the climate, delivering fresh water, primary production and providing habitat for wildlife, amongst others. In the context of global change, the management of vegetation is often needed, in order to restore, maintain or increase these valuable services. The management of vegetation requires an understanding of the functioning of wetlands and of the most important drivers of vegetation structure and species composition. However, an ecological knowledge of wetland vegetation is not sufficient on its own in addressing the wide diversity of ecological situations in the field; therefore, practical management is usually a combination of empirical knowledge and an experimental, in situ approach. 

What are wetlands?

Wetlands are a transition ecosystem, with indistinct borders between the terrestrial and aquatic habitats they contain. The definition of wetlands varies, but most converge in the importance of submergence (or saturation) of soil by water, with two fundamental consequences: hydric soil, characterised by a reduction of ions, and vegetation dominated by plant species adapted to flood (or soil saturation). Wetlands are diverse, along with their geological, geomorphological and historical context, their hydrology (duration, timing and depth of flooding), water chemistry and different types of human disturbance. 

Wetland vegetation ecology

Wetland plant species constitute an ecological, but taxonomically heterogeneous, group, including various growth forms corresponding to different adaptations to flood-induced stresses, and species from very different plant families and classes. A broad definition of this group could be: “plants that are adapted to flood conditions”. Flooding has important consequences for the physical environment of plants (and animals, too). Light intensity decreases exponentially with depth and becomes an important limiting factor for photosynthesis.  Concentrations of dissolved gases are usually low in water and decrease rapidly when the temperature increases; this has important consequences for oxygen, which is needed for the respiration of most plants and animals. Oxygen is produced by green plants (through photosynthesis) during daylight, but this production stops during the night and can be quickly consumed by respiration. Similarly, the concentration of CO₂ in the water is low and limits photosynthesis. Submerged pollination is difficult because of the high viscosity of water and low survival of the pollen grains. In sediment, the availability of oxygen is even lower than in the water, leading to an accumulation of toxic compounds and the loss of nitrogen through denitrification. More generally, the lack of oxygen in sediment has multiple consequences for the availability of nutrients to plants. 

Adaptations to flooding in plants growth types

Plant adaptations to flooding result in different growth forms along the flooding gradients (relating to water depth, flood duration and frequency), often with a clear zonation pattern. 

Helophytes (such as Phragmites australis, common reed) have an erect growth form, with leaves and flowering organs maintained outside the water, thus avoiding the difficulties of living in water. They cope with the lack of oxygen in sediment through the active transfer of oxygen to the root system, and their pollination is aerial. 

Floating-leaved submerged plants (e.g. Nymphaea alba, white water lily) have moved their stomata to the upper side of their leaves (in contrast with other plants), facilitating gas exchange (CO₂ and O₂) with air instead of the water. Their flowers are above the water surface and their pollination is also aerial. 

Submerged plants have strongly modified leaves, which are much dissected, in order to maximise their contact with water and optimise gas exchange underwater. These leaves are also able to take up some nutrients from the water column (depending on species and the relative concentrations of ions in water and sediment), where they are in a more available form. Some submerged plants have acquired the capacity to use dissolved carbonates (CO3--), in addition to CO₂, as a substrate for photosynthesis, which allows a higher productivity (in neutral or basic conditions) and gives a strong competitive advantage.

Among these latter groups some species can reproduce under water, which is the most complete stage of adaptation. In this group are algae and a few angiosperms (e.g.: Callitriche sp., Ceratophyllum sp., Myriophyllum sp., Potamogeton sp., Vallisneria spiralis). 

Further Reading

White, G., Self, M & S. Blyth. 2011. Bringing reedbeds to Life: Creating and managing reedbeds for wildlife. Royal Society for the Protection of Birds.
https://www.rspb.org.uk/globalassets/downloads/documents/conservation-projects/bringing-reedbeds-back-to-life/bringing-reed-beds-to-life-report.pdf