Trésor Foundation Utrecht
Description of the vegetationThis text is a summary of a survey in the Trésor area, "The floristic composition and vegetation structure of the Trésor Reserve' (1998) by Renske C. Ek et al.
It was the second survey of the flora of this area.
The first survey: 1996 Herbarium Division, Utrecht University and Orstrom, Cayenne (Cremers et al.)
The 1996 survey took place mainly in the lower parts of the reserve, the 1998 survey covered the higher parts.
We can distinguish seven types of vegetation along the transection from the highest point at 267 m down to the lowest point in the wet savanna.
Vegetation type I: Mountaintop forestThis type of vegetation can be found in the higher regions of the Trésor reserve. The slope of the floor goes from being horizontal up to a 12º incline (21%) at the edge of the plateau and consists of a thin layer of fertile soil with a solid layer of laterite underneath. The availability of water is limited due to the thin upper layer and the drainage of water to the lower parts along the solid bottom layer. The dry season is a problem especially for the plants that root only in the thin top layer; that is why there is little or no undergrowth in this type of vegetation.
The forest is an "aged forest' with active regeneration; trees of all ages have been found, some very big, few middle-aged and many young trees and seedlings.
The crown is largely uneven and varies in height from 30 to 35 metres above the ground.
Vegetation type II: Forest on not too steep slopesThis type of forest can be found on hills with slopes varying from 4º to 18º (7-32.5%) without a solid laterite layer in the soil. However, we do find a partially impenetrable layer of ferriferous soil at a depth of 60 cm and this restricts the depth of the soil suitable for plants.
The availability of water is better than in vegetation type I but the solid layer restricts the possibilities to store water; a surplus of water runs over the solid layer to lower parts. As a result the lower parts can be more humid in the dry season than the higher parts.
This forest, too is an 'aged' type of forest with a crown at a height of approximately 40 m. Trees in all age categories have been found. The most abundant present are the families of the Chrysobalanaceae, the Myrtaceae, the Lecythidaceae, the Moraceae and the Myristicaceae.
A specimen of the Bocoa prouacensis with an estimated age of 1140 years can be found in this area.
Vegetation type III: Forest on steep slopes.The steep slopes were created by creeks that cut through the rock. The slopes are more than 18º (32,5%). There are many small waterfalls. The creeks are a continuous source of water. Compared with the surrounding forest, this type of vegetation gets less light, but the humidity is higher because of the creeks and waterfalls; there are many types of undergrowth and epiphytes. The rocks found here form a special kind of foundation for the vegetation.
The more humid areas near the creeks can be clearly distinguished from the less humid areas further away, both geomorphologically as well as taxonomically.
Vegetation type IV: Temporarily flooded forestThis type of vegetation surrounds the creeks in the lower regions of the Trésor reserve at an altitude of 5 to 10 meters. The slopes are never steeper than 4º (7%). The ground resembles the ground on the slopes except for the layer attainable by plants, which is deeper (120cm). The ground can store a lot of water and even in the dry season it will not dry up completely because of the existing creeks. In the rainy season this area can be temporarily flooded. The crown is rather low here (30m). Trees of all ages can be found, although young trees predominate.
Vegetation type V: Marsh forestMarsh forest originate in areas which are continuously flooded at altitudes of 5 meters and lower. Marsh forest is dominated by Symphonia glubulifera and a few other accompanying species.
This type of vegetation is found everywhere in the Guyanas and is hardly specific to this area.
Vegetation type VI: Isolated mountain forestAlthough the geomorphology of this type of vegetation resembles that of type IV, the compositon may differ because of the separation of these regions from the surrounding forest by savannas or marsh forest. Only a very thin layer of soil (40 cm) is suitable for plants, so there is only a very restricted possibility to store water. As these areas are higher than the surrounding forest or the savanna and because of the influence of wind and sunlight we can expect a higher degree of evaporation. In the dry season this may lead to less favourable circumstances than in the normal mountain forest. (This effect will be less if it is surrounded by swamps).
It is a mature forest with a crown at an average height of 35 - 40 m. There is an equal distribution among the classified sizes of these trees. Several very high trees (higher than 40m) have been found. And again we found a tree (Bocoa prouacensis) more than 700 years old.
Vegetation type VII: The wet savannaIn Trésor the wet savanna can be found in the lowest and flattest areas. The vegetation consists of low trees (lower than 10 metres) and grassy plains. It is a very open landscape. During the rainy season the plain is mostly flooded. In the dry season the ground can dry up as far down as 90 cm. Even during the rainy season the top layer can dry updue to the high extent of evaporation and limited water supply. That is why circumstances are unfavourable for most plants; only the species especially adapted as the Bactris campestris and the Polygala adenophora can survive here.