Section 6.08 Conclusion
La modélisation est un exercice complexe et
risqué. La diversité des modèles et les divergences
d'approches montrent que le sujet reste largement ouvert. Aussi, faut-il dans
le choix d'un modèle, considérer la maîtrise des
paramètres d'entrée et la possibilité de produire des
résultats vraisemblables et conformes à la réalité
de terrain. A l'heure actuelle, la grande majorité des modèles
sont des `boîtes noires', difficiles à pénétrer et
d'appropriation très contraignante. Le modèle CASS utilisé
dans cette partie du travail, a l'avantage d'être transparent et dispose
d'un paramétrage adapté pour les savanes et ses processus.
Les résultats obtenus montrent qu'au niveau des
écosystèmes de savanes d'importantes modifications peuvent
subvenir soit à cause de facteurs naturels ou humains. En l'absence de
perturbations humaines, les formations végétales
dégradées ont tendance à se reconstituer. Le but de
l'aménagement forestier est justement d'équilibrer les
prélèvements avec la capacité des
écosystèmes à se reconstituer. Les facteurs humains
influent sur les stocks de biomasse différemment selon la nature de la
perturbation et de son amplitude. Nous avons testé les
conséquences de plusieurs activités humaines (feu, collecte de
bois, pâturage, abandon/jachère, plantation etc.). Il
apparaît selon les résultats des simulations que les
écosystèmes de savane sont très sensibles (réaction
rapide après le passage d'un feu ou des coupes à blanc) et les
délais de reconstitution peuvent parfois prendre beaucoup de temps (au
moins 25 ans). Le suivi dans la modélisation des autres
réservoirs de carbone (litière et carbone du sol) montre que les
modifications de la végétation influent directement sur ces
réservoirs, même si les modifications sont plus lentes que celles
de la biomasse sur pied. Ainsi, tout effort de conservation des formations
ligneuses a un effet d'entraînement sur l'amélioration des stocks
de carbone des autres réservoirs.
Toutefois, pour introduire les variations entre les
différentes formations de savane, dont le concept est unifié dans
le modèle, nous avons procédé à la modification des
paramètres de base (PPN et allocation feuilles, branches, racines). Les
simulations proposées dans cette partie sont par conséquent des
scénarios vraisemblables selon les activités en cours dans les
savanes. Cependant, les dynamiques futures du climat, les changements de
précipitations, de température, de concentration de CO2 etc.,
peuvent affecter significativement la fixation du CO2 atmosphérique et
les processus de décomposition des plantes entraînant une
libération de GES. Ces nouvelles conditions écologiques sont en
prendre en compte dans le paramétrage du modèle.
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