The response of sub-adult savanna trees to six successive annual fires: an experimental field study on the role of fire season.
In mesic savannas worldwide, trees experience frequent fires, almost all set by humans. Management fires are set to reduce or enhance tree cover. Success depends greatly on responses of sub-adult trees to such fires. To date, the number of successive years that sub-adult trees can resprout nor the number of years that they must resist being top-killed by successive fires, nor the requisite height, have been reported. In a 6-year experimental field study in Guinean savannas of West Africa, we monitored annually the heights and responses of 1,765 permanently tagged sub-adult trees under annual fires set in three different periods of the long dry season: early-dry season (EDS), mid-dry season (MDS) and late-dry season (LDS). Annual MDS fires are the common local management protocols of Guinean savannas, although EDS fires are common in some of the savannas. Results showed that overall, the proportion of sub-adults that resisted being top-killed differed across fire seasons. Furthermore, resisting one fire gave a better chance of resisting the next. Only sub-adults that were able to resist direct damage for three successive EDS and MDS fires reached sufficient height to be recruited to the adult stage. Resistance height (avoiding topkill) was ~1 m for EDS and ~2 m for both MDS and LDS fires. Recruitment height (threshold for transition to adult stage) was ~3 m for EDS and ~ 3.3 m for MDS fires. No height was great enough for sub-adult trees to be recruited to adult stages in LDS fire. Synthesis and applications. The results of this novel field study showed clearly that successive early- and mi-dry season fires can enhance tree density and that successive late-dry season fires alone reduce tree density in Guinean savannas due to the effects of successive fires on sub-adult trees. The results suggest that a planned regime of these seasons of fire could be used to maintain the desired tree density in Guinean savannas and may inform fire management in other mesic savannas where goals are to increase or decrease tree densities. It also provides relevant information for comparative studies on the mechanisms of recruitment of sub-adult trees to an adult stage in all mesic savannas, a process that ultimately determines savanna physiognomy.