LA PLANTE DANS L'éCOSYSTèME : UNE AUTRE éCOLOGIE VéGéTALE

摘要

The research conducted in Lamto savannah under Maxime Lamotte's direction, particularly in the International Biological Program (1968-1978), aimed at working out the energetic balance of this ecosystem. It adopted a then pioneering systemic approach involving standard quantitative methods. The evaluation of primary production was central in such a task. The basis of the method was to follow up different kinds of phytomasses. Plants speciesrnand organs were sorted out according to their condition (alive or dead) and to functional typologies of their phenological cycles. The collaboration with zoologists, who were studying the consumers and decomposers of phytomass, focused attention on the seasonal distribution of production. In the late 1960s and early 1970s, Lamto was one of the pioneering projects in which primary production estimates were based not only on above-ground biomass changes, but also on estimates of below-ground production. Furthermore, the methods of above-grounds estimations were improved in order to take full account of organ losses and turnover of plant material. For instance, additional studies of the clonal growth of grass species were developed. Such accurate estimates of the primary production placed the ecological surveys of the vegetation at Lamto at the onset of modern functional plant ecology.rnThe Lamto project was one of the first to demonstrate that the primary production of the savannah was higher than expected. Most of the production comes from grass species, especially from their underground parts. Analyses of the vertical and horizontal structure of vegetation were also undertaken from the beginning to assess another type of variability. These led to explanations of how the availability of plant material affects animal composition in the ecosystem. The results also contributed to explaining how and why vegetation changes occur along the climatic gradient in West Africa. Further work, mostly modelling, went on to associate structure, function, and dynamics.rnDespite the fact that Maxime Lamotte was an early pioneer of quantitative work, his vision of ecosystems was profoundly that of a naturalist, also taking into consideration the long-term and its evolutionary aspects. Therefore he encouraged various studies, from inventories of fauna and flora to landscape dynamics involving the ? ecocomplexe ? concept. He also encouraged studies in physical and human geography. Studies on interactions between the different biocenosis of savannah-forest mosaic were thus carried out. These particularly contributed to forging the perception - now well accepted - of fire as a "natural" disturbance and to elucidating the colonization dynamics of savannah by forest. Thus the imposed frame of quantifying the energy budget of the savannah, which might be regarded as limiting, did not prevent the use of various methods and concept in plant ecology work in Lamto; indeed, it yielded a wealth of novel results.%Les travaux menés sous la direction de Maxime Lamotte dans la savane de Lamto, notamment dans le Programme Biologique International (1968-1978), ont adopté une approche systémique et des méthodes quantitatives, alors pionnières, souvent innovantes tant dans le domaine de l'écologie animale que dans celui de l'écologie végétale. L'impulsion donnée par M. Lamotte était cependant animée par une sensibilité profondément naturaliste ainsi que par la conscience de l'importance du temps long.rnDans le domaine végétal, l'évaluation de la production primaire à partir du suivi des phytomasses était centrale pour établir le bilan énergétique de l'écosystème, elle s'est appuyée en particulier sur des typologies fonctionnelles d'espèces, le suivi des cycles phé-nologiques et de la croissance ainsi que sur l'analyse des structures spatiales. Il a ainsi été montré que la production primaire de la savane, plus élevée qu'on ne le pensait, mais très variable en fonction des saisons, est assurée principalement par sa strate herbacée, surtout sa partie souterraine. Ces données ont précisé l'influence de la disponibilité de la matière végétale sur la composition du peuplement animal ; elles ont aussi contribué à expliquer les changements de la végétation sur le gradient climatique ouest-africain. Les travaux ultérieurs de modélisation continueront à associer structure, fonction et dynamique.rnLes études écologiques ont également porté sur les interactions entre les différentes biocénoses de la mosa?que forêt/savane et associé le regard de géographes. Elles ont notamment contribué à une nouvelle perception du feu comme perturbation ? naturelle ? et à préciser les dynamiques du contact forêt/savane.