A semi-detailed landscape-based vegetation map (scale: 1: 37,500) is presented for the 13 km2 Lesser Antillean steep volcanic island of Saba, Netherlands Caribbean. The map is based on a total of 49 vegetation plots that were sampled in 1999 using a stratified random sampling design and analysed using TWINSPAN cluster analysis. Three hundred and fourteen (314) plant species, representing 56% of the total known flora (565 species), were recorded in the sample plots. The principal lower sections of the island possess a tropical savannah climate whereas the upper slopes reaching a maximum altitude of 870 m can best be characterized as a tropical rainforest climate.
A total of two main and nine different sub-landscape types were distinguished based on geology, geomorphology and nine distinguished vegetation types. In Saba, sharp contrasts in soil, geomorphology and climatic factors are found on a small spatial scale and this meant that compared to the other islands of the Dutch Caribbean there is little mixing and merging of vegetation types at the landscape vegetation level. Consequently, vegetation type translates relatively directly into landscape vegetation units. Aside from important contrasts in vegetation that correspond to what is known about differences in soil and climate, our study also shows that large vegetation changes have taken place on the island since the survey by STOFFERS, five decades earlier. These largely appear to be due to three major forces: a) hurricane impacts; b) natural succession made possible due to diminished agricultural activity and; c) invasive plants and plant pest species.
The most recent hurricane, hurricane Georges, which struck the island one year before this study, clearly caused much damage to the vegetation, especially high on Mount Scenery. As a consequence, the elfin woodland vegetation has virtually disappeared, while remnant sections have been radically altered. Based on studies elsewhere in the region, the elfin woodland can be expected to take very long (if at all) to gradually recover. The impact of various hurricanes in the last 60 years has clearly caused major disturbance of the vegetation throwing it back into earlier stages of succession. The development of the “Tree fern brake” into “Pioneer forest” vegetation must be seen as a positive change where a secondary community had entered a higher stage in the sequence of succession. The virtual disappearance of the formerly prominent secondary shrub communities like Miconia thickets, Piper dilatatum thickets and Leucaena thickets can also be seen as likely evidence of natural successional forces thanks to diminished agriculture and woodcutting. Invasive species was the third major force of change that clearly appears to have been active on Saba in recent decades. The lasting impacts of insect invaders which have decimated formerly prominent Opuntia (cactus) and Tabebuia (tree) populations testify to the impact of invasive species as a major driver of recent vegetation changes on Saba.
Our field data show that most wilderness areas of Saba remain strongly affected by roaming grazing goats even though the contribution of goats to the local island economy is negligible. Goat dung or traces of grazing were recorded in or adjacent to 46% of the sample plots. Grazing by exotic mammals reduces the resilience of natural vegetation types and interferes with natural succession. Highest livestock densities and impacts seem to be in the more vulnerable coastal arid zones along the western and southern sections of the island with poor soil conditions and more open and shrubby vegetation. The development of ‘Dry evergreen woodland’ under similar conditions on the more remote, windy and salt spray-affected, but less-grazed, northern sectors of the island, suggest that those disturbed areas of the southern and western coastal zones should have potential for woodland recovery if and when goat grazing is reduced. Therefore, a key priority for terrestrial conservation in Saba should be to reduce feral grazer densities to allow vegetation recovery and reduce vulnerability to erosion. We suggest the use of pilot demonstration projects for grazer exclusion as a useful way to help build stronger arguments and public support for tackling the roaming goat problem in Saba.
Satellite image-based mapping of tropical forests is vital to conservation planning. Standard methods for automated image classification, however, limit classification detail in complex tropical landscapes. In this study, we test an approach to Landsat image interpretation on four islands of the Lesser Antilles, including Grenada and St. Kitts, Nevis and St. Eustatius, testing a more detailed classification than earlier work in the latter three islands. Secondly, we estimate the extents of land cover and protected forest by formation for five islands and ask how land cover has changed over the second half of the 20th century. The image interpretation approach combines image mosaics and ancillary geographic data, classifying the resulting set of raster data with decision tree software. Cloud-free image mosaics for one or two seasons were created by applying regression tree normalization to scene dates that could fill cloudy areas in a base scene. Such mosaics are also known as cloud-filled, cloud-minimized or cloud-cleared imagery, mosaics, or composites. The approach accurately distinguished several classes that more standard methods would confuse; the seamless mosaics aided reference data collection; and the multiseason imagery allowed us to separate drought deciduous forests and woodlands from semi-deciduous ones. Cultivated land areas declined 60 to 100 percent from about 1945 to 2000 on several islands. Meanwhile, forest cover has increased 50 to 950%. This trend will likely continue where sugar cane cultivation has dominated. Like the island of Puerto Rico, most higher-elevation forest formations are protected in formal or informal reserves. Also similarly, lowland forests, which are drier forest types on these islands, are not well represented in reserves. Former cultivated lands in lowland areas could provide lands for new reserves of drier forest types. The land-use history of these islands may provide insight for planners in countries currently considering lowland forest clearing for agriculture.