ALTERNATIVE FIRE RESISTANCE STRATEGIES IN SAVANNA TREES

Bark properties (mainly thickness) are usually presented as the main e xplanation for tree survival in intense fires. Savanna fires are mild, frequent, and supposed to affect tree recruitment rather than adult s urvival: trunk profile and growth rate of young trees between two succ essive fires can also affect survival. These factors and fire severity were measured on a sample of 20 trees near the recruitment stage of t wo savanna species chosen for their contrasted fire resistance strateg ies (Crossopteryx febrifuga and Piliostigma thonningii). Crossopteryx has a higher intrinsic resistance to fire (bark properties) than Pilio stigma: a 20-mm-diameter stem of Crossopteryx survives exposure to 650 degrees C; while Piliostigma needs a diameter of at least 40 mm to su rvive. Crossopteryx has a thicker trunk than Piliostigma for two trees of the same height, the basal diameter of Crossopteryx will be 1.6 ti mes greater. Piliostigma grows 2.26 times faster than Crossopteryx bet ween two successive fires. The two species have different fire resista nce strategies: one relies on resistance of aboveground structures to fire, while the other relies on its ability to quickly re-build aboveg round structures. Crossopteryx is able to recruit in almost any fire c onditions while Piliostigma needs locally or temporarily milder fire c onditions. In savannas, lire resistance is a complex property which ca nnot be assessed simply by measuring only one of its components, such as bark thickness. Bark, properties, trunk profile and growth rate def ine strategies of fire resistance. Fire resistance may interact with c ompetition: we suggest that differences in fire resistance strategies have important effects on the structure and dynamics of savanna ecosys tems.