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S. Shanmugam and M. Barnsley
adopted using the M4 motorway as the eastern boundary which is present in the 1991
and 1994 photographs but not in the 1962 and 1971 photographs. Each parcel of a
given habitat is thus a closed polygon in a coverage. The vector coverage for a given
year comprises a number of different layers, where each layer contains polygons
representing the parcels of a given habitat. The attributes of each polygon in any given
coverage include its area, perimeter, habitat type and hydrotype (Fig. 3).
Vector GIS and Dune-Landscape Succession Analysis
Overlays were produced of the individual habitat maps for each year considered in this
study. The resultant change vectors present a vivid picture describing the type of
successional changes that took place between 1962 and 1994. The changes are
reflected not only in terms of an increase or decrease in the areal extent of individual
habitats, but also in terms of partial or complete change of each habitat parcel into
another stage in the successional sequence.
To understand the nature of landscape/habitat succession, each habitat was
compiled as a separate layer (i.e., vector coverage) and overlays were constructed
between corresponding layers from different years. A total of 156 vector coverages,
representing all the habitat / landscape types for the four years in consideration, were
generated. Figure 4 is an example of a vector coverage containing the eight habitat/
landscape types. The areal extent of each of the habitats / landscape was computed for
each of the years studied. The results confirm that there has been a general greening of
the dune system since 1962. In other words, there has not only been a substantial
decrease in the areal extent of open sand / mobile dune, but also a reduction in the
generation of dune slacks and an increase in the biomass on existing slacks. The most
likely explanation of this is that there has been a decrease in the mobility of the dunes,
as a consequence of over-stabilisation.
A quantification of the change brought about in the dunal system over the
years, described qualitatively above, is given in Figure 5. This shows that about 20%
(approximately 82 hectares) of the study area within the NNR was covered by mobile
sand during the early sixties, but that this had reduced to a mere 1.5% (6.5 hectares) by
1994. Similarly, the areal extent of the partially-vegetated sand/dunes (PVD) reduced
from 26% in 1962 to 2.7% by 1994. On the other hand, the area of moderately
vegetated dunes (MVD) increased from 18.7% to 56.8% over the same period of time.
In the case of the dune slacks, the partially vegetated slacks have reduced in
extent from ~12% to 1.6%: this is a result of their transition into more densely
vegetated slacks (0.24% in 1962 to 5.65% in 1994). The slacks also have a tendency to
transform into woodlands, the area of which has increased eight-fold. Table 3 provides
S. Shanmugam and M. Barnsley
adopted using the M4 motorway as the eastern boundary which is present in the 1991
and 1994 photographs but not in the 1962 and 1971 photographs. Each parcel of a
given habitat is thus a closed polygon in a coverage. The vector coverage for a given
year comprises a number of different layers, where each layer contains polygons
representing the parcels of a given habitat. The attributes of each polygon in any given
coverage include its area, perimeter, habitat type and hydrotype (Fig. 3).
Vector GIS and Dune-Landscape Succession Analysis
Overlays were produced of the individual habitat maps for each year considered in this
study. The resultant change vectors present a vivid picture describing the type of
successional changes that took place between 1962 and 1994. The changes are
reflected not only in terms of an increase or decrease in the areal extent of individual
habitats, but also in terms of partial or complete change of each habitat parcel into
another stage in the successional sequence.
To understand the nature of landscape/habitat succession, each habitat was
compiled as a separate layer (i.e., vector coverage) and overlays were constructed
between corresponding layers from different years. A total of 156 vector coverages,
representing all the habitat / landscape types for the four years in consideration, were
generated. Figure 4 is an example of a vector coverage containing the eight habitat/
landscape types. The areal extent of each of the habitats / landscape was computed for
each of the years studied. The results confirm that there has been a general greening of
the dune system since 1962. In other words, there has not only been a substantial
decrease in the areal extent of open sand / mobile dune, but also a reduction in the
generation of dune slacks and an increase in the biomass on existing slacks. The most
likely explanation of this is that there has been a decrease in the mobility of the dunes,
as a consequence of over-stabilisation.
A quantification of the change brought about in the dunal system over the
years, described qualitatively above, is given in Figure 5. This shows that about 20%
(approximately 82 hectares) of the study area within the NNR was covered by mobile
sand during the early sixties, but that this had reduced to a mere 1.5% (6.5 hectares) by
1994. Similarly, the areal extent of the partially-vegetated sand/dunes (PVD) reduced
from 26% in 1962 to 2.7% by 1994. On the other hand, the area of moderately
vegetated dunes (MVD) increased from 18.7% to 56.8% over the same period of time.
In the case of the dune slacks, the partially vegetated slacks have reduced in
extent from ~12% to 1.6%: this is a result of their transition into more densely
vegetated slacks (0.24% in 1962 to 5.65% in 1994). The slacks also have a tendency to
transform into woodlands, the area of which has increased eight-fold. Table 3 provides