Fixed or mixed? Variation in tree functional types and vegetation structure in a forest-savanna ecotone in West Africa

Abstract

We analysed thirty-five 400-m2 plots encompassing forest, savanna and intermediate vegetation types in an ecotonal area in Ghana, West Africa. Across all plots, fire frequency was over a period of 15 years relatively uniform (once in 2–4 years). Although woodlands were dominated by species typically associated with savanna-type formations, and with forest formations dominated by species usually associated with closed canopies, these associations were non-obligatory and with a discrete non-specialized species grouping also identified. Across all plots, crown area index, stem basal area and above-ground biomass were positively associated with higher soil exchangeable potassium and silt contents: this supporting recent suggestions of interplays between potassium and soil water storage potential as a significant influence on tropical vegetation structure. We also found an average NDVI cover increase of ~0.15% year−1 (1984–2011) with plots dominated by non-specialized species increasing more than those dominated by either forest- or savanna-affiliated species. Our results challenge the traditional view of a simple forest vs. savanna dichotomy controlled by fire, and with our newly identified third non-specialized species grouping also potentially important in understanding ecotonal responses to climate change.

A study of drivers and dynamics of forest-savanna boundaries in West Africa

“…The most interesting is the intermediate MAP (1000-2000 mm p.a.) which comprise both forest and savanna patches in mosaics (Staver et al, 2011) depending on other environmental drivers (Veenendaal et al, 1996;Ametsitsi et al, 2020).…”

“…Thus, forest ecosystems are non-pyrogenic in contrast to open woodland which contains the bulk of grass biomass to sustain fire. Evidence on the ground, however, suggest that fire incursion in the closedcanopy forests which are present as mosaics in the forest-savanna transition is a regular phenomenon (Ametsitsi et al 2020;Goetze et al, 2006). Regular fire incursion in the forests selects Chapter 1 for species leading to specific floristically distinguished plant communities.…”

“…transforming from forest to savanna or vice versa when disturbed above a threshold; commonly a shift in fire regime Pellegrini et al, 2016). However, many forest-savanna ecotones have been reported as stable even at decadal timescales (Cuni-Sanchez et al, 2016;Goetze et al, 2006;Sobey, 1978;Ametsitsi et al, 2020).…”

“…Fire occurrences in the West African FS ecotone are often frequent (e.g. every two years) and dry forest formations are present as patches (Ametsitsi et al, 2020;Goetze et al, 2006). The distribution of these dry forest formations has been linked to topography in the landscape, soil water availability and other soil factors (Ametsitsi et al, 2020;Markham & Babeddge, 1979;Swaine et al, 1976).…”

“…every two years) and dry forest formations are present as patches (Ametsitsi et al, 2020;Goetze et al, 2006). The distribution of these dry forest formations has been linked to topography in the landscape, soil water availability and other soil factors (Ametsitsi et al, 2020;Markham & Babeddge, 1979;Swaine et al, 1976). Despite the apparent dynamic nature of the forest-savanna ecotone they are also often reported to be remarkably stable at decadal timescales (Cuni-Sanchez et al, 2016;Goetze et al, 2006;Sobey, 1978;Ametsitsi et al, 2020).…”