The response of sub-adult savanna trees to successive fires of various seasons

Abstract 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 six-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 fires of these savannas. Results showed that overall, the
proportion of sub-adults that resisted being top-killed differed across
fire seasons. Further, 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. 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.