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

George K.D. Ametsitsi, Frank Van Langevelde, Vincent Logah, Thomas Janssen, Jose A. Medina-Vega, Hamza Issifu, Laurianne Ollivier, Koos Den Hartogh, Thomas Adjei-Gyapong, Stephen Adu-Bredu, Jon Lloyd, Elmar M. Veenendaal*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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.

Original languageEnglish
Pages (from-to)133-149
JournalJournal of Tropical Ecology
Volume36
Issue number4
DOIs
Publication statusPublished - Jul 2020

Keywords

  • Alternate stable states
  • canopy cover
  • forest-savanna ecotone
  • soil properties
  • vegetation structure

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