Landscapes Forest and Global Change - ESA - Escola Superior ...

M.T.C. Rodrigues & V. Silva 2010. Landscape changes in a watershed in the southwest of Portugal
410
2. Methodology
2.1 Study area
The area selected for this study corresponds to the watershed of Ribeiro do Canas, a sub-basin
of the Sado River Basin (Fig. 1). It is small a watershed (5335 ha) that corresponds to about
0.8% of the Sado Basin. The climate is mediterranean with average annual temperature being
approximately 16.3 °C and annual precipitation around 575 mm.
According to the biogeographic map of Costa et al. (2002) the territory in analysis is included in
the Mediterranean West Iberian Province (Lusitan-Extremadurean Subprovince, Marianic-
Monchiquensean Sector and Baixo Alentejano-Monchiquense Subsector). The head of the
natural potential climatophillous vegetation series consists of the association Pyro
bourgaeanae-Quercetum rotundifoliae that is, sclerophyllous forests dominated by Holm Oak
(Quercus rotundifolia). The regressive succession consists of three stages. The first substitution
stage of the forests (Hyacinthoido hispanicae-Quercetum cocciferae) is dominated by Kermes
Oak (Quercus coccifera); the next regression stage consists of shrublands Retameto
sphaerocarpae-Cytisetum bourgaei; a scrub community dominated by Gum Rockrose (Cistus
ladanifer), the association Genisto hirsutae-Cistetum ladaniferi, represents the maximum
degradation stage (Costa et al. 1998).
The changes that occurred in part of the study area, namely alongside Ribeiro do Canas, can be
observed on aerial photographs from 1947 (Fig. 2A) and 2004/05 (Fig. 2B). In 1947 even the
most steep terrain was then cultivated with cereals. Only some scattered ork oak treas were left.
It can also be observed that at that time riparian vegetation had been largely destroyed. In
2004/05 a remarkable recovery not only of riparian vegetation but also of vegetation occupying
the slopes can be noticed. An increase in biomass and structural diversity of plant communities
is clearly evident. Nowadays the progressive succession has led to the appearance of tall shrub
communites dominated by Kermes Oak, that is to say, the vegetation has almost reached the
climax stage.
2.2 Methods
Land cover of the study area was characterized for four time periods: 1895, 1963, 1990 and
2004/05. Land cover was obtained by digitizing existing maps in analogical format for 1895
(Carta Agrícola) and 1963 (Carta Agrícola e Florestal) and color orthophotomaps for 2004/05.
Land cover for 1990 already existed in the form of a polygon shapefile (Carta de Ocupação do
Solo). Eight Land Cover classes were considered: Agriculture, Forest, Semi-natural vegetation,
Bare soil, Water, Urban, Eucalypt plantations and Olive stands and Orchards. However, the last
five classes had areal percentage values always fewer than 4% during the time period of
analysis and will not be presented in this paper.
A Digital Elevation Model was produced which allowed the determination of slope, solar
radiation and topographic wetness index (Beven and Kirby 1979). The information from these
biophysical parameters was intersected with the information of land cover. All the analysis were
performed in ArcGis 9.2 (ESRI) except the computation of solar radiation which was performed
in ArcView 3.2 (ESRI) using the extension Solar Analyst (Fu and Rich 2002).
3. Results
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.