Shifting sands and changing plants

Excavating dredged sand on the edge of the Alvor Estuary

Human beings are so widespread and their actions so large scale that almost all environments have already been affected by them. Human impact is everywhere. But the effects of human impact are far from straightforward: plants and animals respond in complex ways to man-made changes and careful, long-term, monitoring is needed to understand them. One example concerns studies undertaken by A Rocha Portugal on how the sand dune flora of the Alvor Estuary is recovering from massive disturbance ten years ago.

Offshore dredging produces onshore changes

In an operation lasting several years up to 1994, sand dredged from the Alvor Estuary was deposited on neighbouring sand dunes, obliterating large areas of natural vegetation. This sand extraction also involved the use of heavy machinery that compacted large areas of the dunes. Since 1994, much of this dredged sand has been removed. The result is that a mosaic of three main vegetation types on the Alvor Dunes can now be recognised: an area of natural dunes and two disturbed areas.

The natural dunes are those areas covered by undulating, ecologically mature sand dunes. This area - the "Mature Dunes" - is assumed to have a low fertility, because it receives few nutrients and loses other nutrients through leaching. Two separate mature areas were examined, one further from the sea ("Mature 1"') and the other closer to the shore ("Mature 2").
The first type of disturbed zone - the "Sand Piles" - is an area where the dredged sand was deposited and has not been removed. Because this sand is relatively rich in plant nutrients, it is more fertile than the "Mature Dunes."
The second type of disturbed zone - the "Excavated Areas" - was buried by dredged sand but then subsequently exposed again as the overlying sand was removed. The fertility of the "Excavated Areas" is variable but is generally moderately high. Aiding their fertility is the way that some nutrients from the overlying dredged sediment have been washed down into them; hampering their fertility is the fact that these sands have been highly compacted by the heavy mechanical equipment.

An earlier study by Jeanette Hall in 1995 had shown marked differences in vegetation composition between the sand pile and natural areas. In July 2002, Richard Gunton returned to these sites to study and compare the direction in which plant succession was continuing within the different areas. For consistency, the study used the methods applied earlier, based on random one-metre square quadrats within each area. The study allowed an accurate estimation of species diversity, the frequency of different species in each area, the maximum height of the vegetation, and the amount of bare ground present.

Results

The results (summarised in the table below) showed marked differences in the plant communities between the different areas: multivariate analysis highlighted the degree to which variation among the areas was greater than within any one of them.

SITE:	Mature Dunes I	Mature Dunes II	Sand Piles	Excavated Areas
Total number of species found	29	29	37	41
Index of Plant Diversity	11.7	14.6	15.6	19.9
Average vegetation height (cm)	92	42	38	53

In general terms the two types of disturbed zones - the "Sand Piles" and the "Excavated Areas" - show both a higher total number of species and a greater index of plant diversity than the two undisturbed plots. By looking at the species composition it is clear that this is due to the presence of weedy species that would not normally occur in dune floras, and reflects a higher fertility in these areas than in the mature sand-dune habitats. On the other hand, many of the specialised sand dune plants are missing in the disturbed areas. Other differences were apparent, for example the greater amount of bare ground in the "Mature Dunes," and the shorter nature of the vegetation on the sand pile. Further work planned is to look more closely at how the substrates of the disturbed areas can be manipulated to encourage the re-formation of natural dune vegetation in ways which are also compatible with the needs of local breeding colonies of Little Terns and Kentish Plovers. With further dredging operations planned in the near future, it is hoped that the lessons learnt can be applied to minimise the impacts of such works.

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For further information on this project, contact portugal@arocha.org