The following summary of the natural environment of the study area is largely based on The vegetation of the Sierra Parima, Venezuela-Brazil: some results or recent exploration (Huber et al., 1984).
There are no meteorological records for the
Homoxi region. However, the general
trend in Roraima is for higher rainfall and a less pronounced dry season[1]
the further one travels to the west.
Thus there is a transition from precipitation of 1500mm east of Boa
Vista to over 2000mm in the upper Uraricoera and Surucucus basin, and a
corresponding fall in the number of dry months from 4-5 to 2 or less. Homoxi, which lies to the south of
Surucucus, probably falls into the submesothermic ombrophilous climatic type
which, according to Steyermark et al. (1995), is also experienced in the
Orinoco basin a short distance to the south.[2] This climate is typical of the Guayanan
uplands between 500 and 1200m elevation and is characterised by mean annual
temperatures of 18-24ÆC, rainfall in excess of 2000mm, and fewer than two dry months (i.e.
months with less than 50mm mean precipitation).
The principal vegetation type in the Homoxi region is tall evergreen submontane rainforest on terra firme. The following extract, written by Sir Ghillean Prance and extracted from Huber et al. (1984), provides a useful introduction:
This
forest type which covers much of the region... occurs from about 600-1000m
altitude. It is quite similar in
species composition to the adjacent Guayanan-Amazonian lowland forest on terra
firme. I agree with RADAMBRASIL (1975)
that the region above 600m is distinct from the lowland forest. The difference is mainly in the physiognomy
rather thasn in species composition. In
the submontane forest the trees are not as tall, the trunks tend to be thinner,
the canopy is of rather uniform height, but emergent trees are frequent. There appears to be more clumping of
individuals of various common species than in the foresat at lower
altitudes. This zone extends to about
1000m in the Parime range where it changes to the more humid cloud forest...
Some
of the commonest species encountered in this forest include Cedrelinga
catenaeformis, Clathrotropis macrocarpa, Elizabetha sp., Swartzia sp., Ormosia
steyermarkii (Leguminosae); Cariniana micrantha and Eschweilera odora
(Lecythidaceae); Virola elongata (Myristicaceae), Coussapoa ferruginea
(Cecropiaceae); Miconia puncrata and M. multispicata (Melastomataceae); Mahurea
palustris (Clusiaceae); Aspidosperma sandwithianum and Macoubea guianensis
(Apocynaceae); and Alchorneopsis floribunda (Euphorbiaceae). A common palm in this formation is
Iriartella setigera...
Even
at higher altitudes, the river margins have a distinct flora with many of the
common Amazonian riverine species, creating a vegetation type different from
the adjacent terra firme. For example,
Macrolobium acaciaefolium, Inga nobilis, I. ingoides, and Pithecellobium
divaricatum (Leguminosae), Mouriri myrtifolia (Melastomataceae), Homalium
racemosum (Flacourtiaceae), and Guarea glabra and G. trichilioides (Meliaceae)
are all typical of this formation. In
this region, water is always available and flash floods occur sporadically, but
forests characterized by long-term inundation do not occur at the alrtitudes of
the Parima region.
Other vegetation types known to occur within the Sierra Parima region include montane cloud forest, summit and outcrop floras, white sand formations (similar to the Amazonian caatingas of the Upper Rio Negro and the campinas of the Lower Rio Negro), and savanna formations. These savanna patches tend to occur on the summits of hills (e.g the Serra de Surucucus to the north of Homoxi) and it is thought that the occurrence of at least some of them (e.g. the ‘fern savannas dominated by Pteridium) have been influenced by the land management practices of indigenous peoples of the region.
Roraima lies on the southern flank of the Guiana Shield, which is an ancient complex of crystalline rocks. These are mainly granites, schists and gneisses of Pre-Cambrian age, which are locally overlain by Proterozoic sediments of the Roraima Formation (McGregor & Eden, 1998).
The principal geological units in the southern part of the Brazilian Serra Parima (within which the Homoxi region lies) are the Complexo Guianense and the Granito Surucucu. The Complexo Guianense is the crystalline basement composed primarily of acidic granitic and gneissic rocks of the Lower Precambrian. The Granito Surucucu is made up of alkaline granites forming part of a batholithic structure that stretches from the Rio Auarís to the headwaters of the Parima and Mucajaí. These are the result of an intrusion approximately 1500 million years ago.
In addition, Projeto Radam (RADAMBRASIL, 1975) demonstrated the presence in the region of isolated stacks and volcanic pipes composed of gabbros and
ultrabasites. These were estimated to be of Proterozoic or Archean age and were christened the Parima-Tapuruquara magmatic event.
Soils on the Guayana Shield are generally poor in nutrients (especially phosphorus, calcium and nitrogen) on account of the low mineral content of the parent rocks and the high weathering rates experienced. They tend also to have low Cation Exchange Capacity (CEC) and to be relatively acidic. The subsoils often contain high concentrations of toxic aluminium compounds, which can have a significant effect on the nutrient balance of the vegetation growing on them. In general it is only in areas of basic or intermediate rocks that favourable nutrient conditions occur (Steyermark et al., 1995).
The soils of the Serra Parima and surrounding areas have not been studied in depth. However, RADAMBRASIL (1975) showed that in general the soils of the region can be divided into the following two units.
· Uplands with shallow (mainly dystrophic) lithosols and red-yellow podzols with low fertility and high levels of aluminium.
· Lowlands with red-yellow podzols mixed with lithosols and diastrophic red-yellow latosols.
[1] December to February.
[2] Nevertheless it may share some affinity with the climatic pattern experienced at Sierra Parima - a weather station on the Venezuelan side of the border. The weather at Sierra Parima is characterised by a lower rainfall than might otherwise be expected for the region, with a more marked dry season and occasion periods of relative drought.