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

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.