Quelques exemples de problèmes posés à l’analyste et au modélisateur par la complexité de la forêt tropicale humide
Authors: Pascal Jean-pierre 
Published: November 2022

The different levels of complexity of tropical wet evergreen forests pose several kinds of problems to the data analyst and modeller. First of all, some examples are given of the difficulty in conducting samplings appropriate to the hypotheses to be tested, because of the heterogeneity of the formation. The heterogeneity, both structural and floristical, could be the result of variations in the topography, the nature and properties of the soil, conditions of drainage, exposure, etc., as well as the history of the forest. This heterogeneity may be expressed as variations in the floristic composition or in the proportion of the species, or even in variations in the height and vertical structure of the stand. Experience has shown that it is not always easy to demonstrate these variations through statistical tests, whereas they are sometimes foreseeable in the field. This is also true for the study of spatial distribution of species where the results could differ greatly depending on the scale at which the stand is studied.

Problems concerning space and time scales linked to the forest dynamics are dealt with in the second part. Under the same word - chablis or gap - are, in fact, grouped phenomena of varying amplitude and nature, involving different closing mechanisms. From the simple evolution of an eco-unit, beginning from an elementary chablis, to the large openings caused by multiple and recurring tree falls, all the intermediates are encountered, creating zones in the forest evolving at different speeds and with different processes.

Other mechanisms which also play a role in the forest dynamics are described. The most obvious is the susbstitution, which can be defined as the replacement of a canopy tree by another younger one growing in its immediate vicinity. This young tree will occupy the canopy after the standing death of the older tree, without creating a chablis, Le., without causing a large opening. Another more complex mechanism is seen when the chablis are rare and the canopy trees have a long life. The stand is then organised into circular functional units around dominant trees. On the standing death of the dominant tree, a younger one in its periphery takes over and, over a period of time, a new belt of trees will be organized around it, slightly shifted from the preceding one.

A third kind of problems concerns the studies on the structure of tree populations. As a general rule, in the wet evergreen forests, few species are common and many are uncommon or rare. Hence, the majority of the species have too few individuals for their populations to be easily studied. The inconvenience is minor for establishing global models, for example of production, but it becomes quite difficult for studies on the structure and the dynamics of particular tree species populations.

Lastly, growth analysis show that, except for pioneer species, it is most often difficult and sometimes impossible to attribute a growth curve type to a given species. In fact, different shapes are encountered for the same species according to the individuals : no growth over a long period, a growth phase followed by a stoppage or the contrary, regular growth, etc. This phenomenon constitutes an important obstacle for modelling.