The response of sub‐adult savanna trees to six successive annual fires: An experimental field study on the role of fire season

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 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.

Different effects of fire age and fire recurrence on grass and woody plant chemistry in Kafue National Park, Zambia

“…Reducing fires, increasing the abundance of grazers, and/or decreasing the abundance of herbivores result in bush encroachment with completely different ecological processes than the ones prevailing in the original savanna system (e.g., Hibbard et al, 2001; Scholes & Archer, 1997; Skarpe, 1990; Staver et al, 2009). While the drivers for bush encroachment (in part due to the suppression of fire) is being studied intensively (e.g., Joubert et al, 2012; Stolter et al, 2018), the anthropogenic increase of fire and its highly variable seasonal effects on tree survival under different ambient conditions have not yet received similar attention (N'Dri et al, 2022).…”

Multispecies allometric equations for shrubs and trees biomass prediction in a Guinean savanna (West Africa)

“…When drying up, the grass layer provides abundant fuel for fires burning annually through the reserve, usually in mid-January. In the Lamto savannas, and generally in all the wet savanna ecosystems, regular burning of vegetation exerts a stabilizing effect on the habitat by preventing massive tree invasion (Monnier 1968;Menaut and Abbadie 2006;N'Dri et al 2022).…”

“…In tropical savannas, fire and herbivory induce rapid changes in vegetation structure affecting the ratio between above-and belowground tree parts (Frost et al 1986;Sankaran et al 2005;Staver et al 2009;Tomlinson et al 2012;Scogings and Sankaran 2019). In wet savannas, fire is recognized as the main factor controlling the growth dynamic of woody plants (Sankaran et al 2005;Bond 2008;Staver et al 2011;N'Dri et al 2022) and should promote a higher RSR compared to dry savannas (Gignoux et al 2006;Tomlinson et al 2012;Pausas et al 2018;Wigley et al 2019;Le Stradic et al 2021). Last, large-scale models have been developed for woody plants assuming they are all trees, while most individuals composing a savanna stand can be developing as shrubs, with several small diameter stems (Zizka et al 2014).…”

The Demographic Response of Grass Species to Fire Treatments in a Guinean Savanna

“…Because fire intensity depends on the period of burning [19,20], this period should have contrasting effects on the demography of savanna trees [21,22] and perennial grasses [5,23,24]. Indeed, despite their adaptation to fire, fire regularly kills them, especially if it occurs at the beginning of their development cycle at the beginning of the wet season [24,25].…”

“…Thus, fighting tree encroachment may involve fire management, which implies choosing the best fire period to maintain the abundances and diversity of grasses and the desired tree density. Because tree encroachment is a worldwide issue in savannas [28], people could advise fire regimes such as late fires that are detrimental to trees [22,25]. However, are these regimes favorable to grasses?…”