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

Ametsitsi, George Kwaku Duku


Forest-savanna transitions are often discussed as systems that operate next to each other with distinct woody species and separated into pyrogenic and non-pyrogenic systems with sharp transition between the two systems. It is often hypothesized that fires cannot enter closed canopy vegetation. Potential changes in vegetation cover, soil heterogeneity and impending climate change have important consequences on tree species selection for reforestation to adapt to the derived environment and there is the need to specifically study the interplay between fire and tree cover and tree species at the forest-savanna ecotone. There is also the need to determine if there are soft transitions and the mechanism that controls them. In this thesis, I analysed empirical evidence to test four paradigms commonly associated with the forest-savanna transitions with field evidence supported by remote sensing. I explain the mechanism and interplay by addressing the following questions: (1) How stable are the vegetation patterns in West-African forest-savanna boundaries? (2) Do tree functional types mix in the landscape? (3) How does fire penetrate the forest landscape in KSNR if forest burns? (4) How do tree functional type and stand structure affects the phenology of leaf shedding?  (5)  How does canopy variation selects for tree species recruitment?

The results show that forest and savanna were dominated by associated species but instead of two groups of vegetative types, a third group has emerged as non-selective that bridges the gap between forest and savanna. The open woodland and closed forest were separated by a vegetation leaf area index (LW) threshold of 1.5 to 2.0 and two fuel load sources with inverse relationship, resulting in two distinct types of fires (high intensity and benign). There was evidence that the strong phenological pattern of species and tree functional type observed seem to support the possibility of litter fires across a closing cover of trees and help to explain the existence of litter fires under these conditions. I hypothesise a mechanism for the maintenance of non-specific species with their early deciduous nature of a critical importance. This observed duality of fire types as observed in current study leads to what we term “soft fire transitions”. This in contrast to a simple savanna versus forest, fire versus no-fire dichotomy that results in a “hard fire transition”, such as current prevails in the literature. I suggest that additional groundwater supply such as in gallery forests plays a role in the “sharpening” of transitions. Vegetation structural parameters were associated with higher soil exchangeable potassium and silt contents supporting recent suggestions of interplays between potassium and soil water storage potential as a significant influence on tropical vegetation structure. The results also reveal that tree cover variation has species-specific effects on tree seedling recruitment which is related to root storage functions.

This thesis challenges the traditional view of a simple forest versus savanna dichotomy controlled by fire, and with the newly identified third non-specialized species grouping capable of bridging the pyrogenic gap which will contribute to understanding ecotonal responses to climate change and the evidence that fire penetrates the forest in the landscape in KSNR, that forest-savanna transitions burn regularly. Most broadly, this study contributes to the ongoing debate about the relative importance of fire and soil in determining vegetation function and structure and concludes that soil, climate, and fire all interact to create boundaries and transitions in the forest savanna landscape. This finding also helps explain the mechanisms underlying the apparent softness of forest-savanna transitions in West-Africa.