Vegetation mapping of the Guadiana Estuary

The Guadiana Estuary has an extremely important natural heritage. The salt-marsh vegetation of the Estuary makes up part of this richness and carries out an essential role in the functioning and healthy status of this ecosystem. As part of the objective of the Interreg-funded “MegaSIG” collaboration between Spain and Portugal (www.megasig.org) to create a tool for environmental management and education, description and mapping of the vegetation of the estuarine zone was undertaken by A Rocha

The vegetation map was developed on a GIS platform (ArcView 8.3) through photo-interpretation of georeferenced aerial photographs at a scale of 1:5000, integrating data already available from the study area. The classification of the vegetation was based on the scheme adopted by the European Habitats Directive, given that a large proportion of the area in question is a Natura 2000 site covered by this legislation. Field visits allowed for the verification of the vegetation types present, and to collect other data such as the presence of non-native species, evidence of disturbance and threats. From the field observations and published records, an inventory of plant species was compiled and a library of photographs of plants species and communities was developed.

Figure 1 Vegetation map of the Guadiana Estuary

An area of 4605 ha of estuarine habitats was mapped (Figure 1). The inventory of plant species contains 420 species, including some of conservation importance. Examples are Cistanche phelypaea (Orobanchaceae), a parasitic plant with large flowering spikes of yellow flowers (Figure 2), and Limonium algarvense (Plumbaginaceae), a more delicate plant, endemic to the Iberian Peninsula and Morocco and one of more than 100 species of this genus present in Spain and Portugal. A large part of the area is dominated by plants of the family Chenopodiaceae and other ‘halophytes’, in particular the succulent shrubs Arthrocnemum macrostachyum and Sarcocornia. A typical zonation was found in places, from the colonies of the Cord-grass Spartina maritima, frequently submersed in tidal waters at the edge of the creeks, passing through the Sarcocornias and Sea Purslane Haliminone portulacoides in the mid-marsh, to the plants of the upper marsh such as Arthrocnemum, Shrubby Seablite Suaeda vera and Shrubby Sea Lavender Limoniastrum monopetalum, which also form a distinct community on top of the banks of the salt pans and fish farms. The areas of each of these communities were: Spartina maritima 2.7ha, Sarcocornias 559.1 ha, Arthrocnemum 203.1ha, Suaeda vera 51.2ha and Limoniastrum monopetalum 0.4ha. However, it also became clear that a large proportion of the marshes are affected by the presence of the highly competitive exotic grass Spartina densiflora.

Spartina densiflora is a South American cordgrass that has become established in the salt marshes of SW Europe, as well as parts of NW Africa and SW North America. It was introduced to Iberia as early as the 16th century, and has become invasive in the estuaries of the Odiel and Guadiana. In the former, it has invaded 90% of the tidal marshes. It has a wide salinity range tolerance, and high rates of both vegetative and sexual reproduction. In the Guadiana study area Spartina densiflora was recorded as present on more than 1000 ha of the salt marshes. This represents 22% of the total salt marsh area (23% of the 644 polygons mapped). Some 168 ha consisted of Spartina swards of between 70 and 100% cover of this one species. The vegetation communities most affected were channel margins and middle marshes with Sarcocornia perennis subsp. alpini and Halimione portulacoides, lying above the colonies of Spartina maritima which have a lower elevational tolerance. The ecological restoration works at the Castro Marim/ V.R.S.A Nature Reserve appear to have facilitated the invasion of the cordgrass, as has also been experienced in the Guadalquivir marshes.

Figure 2 Cistanche phelypaea

The high competitive ability of Spartina densiflora has allowed it to dominate large areas of salt marsh vegetation. Other studies have shown the reduced plant species diversity of such areas, and this was again observed in the current investigation. Significant areas consisted of an almost 100% sward of the species. Exotic plant species generally do not support a rich native fauna, and whilst not investigated in this current study, one could expect this to be the case here. The species has a significant impact on the biodiversity of the Guadiana salt marshes, and represents one of the most important conservation challenges for this area.

At the same time, this grass may be providing two important ecosystem services, as highlighted for the Odiel Marshes National Park. The grass has been shown to stabilise shorelines in the face of considerable erosion pressures, and also to be highly effective at cleaning inter-tidal sediments contaminated with heavy metals.

It is recommended that control measures for Spartina densiflora concentrate on preventing further expansion, rather than eliminating it from areas where it has invaded extensively. Priority marsh areas with minimal connectivity to the rest of the estuary system, should be monitored for the presence of the Spartina, and any established foci should be eliminated by physical removal whilst still of small extent.

The GIS map developed in this study provides a useful tool for monitoring change in the condition of the salt marshes of the Guadiana Estuary in the future, and specifically to monitor, and plan control measures for, the invasive Cordgrass Spartina densiflora.

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Project leader: Will Simonson. For more information contact portugal@arocha.org

Partners: University of the Algarve, University of Huelva and Instituto de Recursos Naturales e Agrobiologia de Sevilla, funded under the Programme Interreg IIIA. Reserva Natural do Sapal de Castro Marim.