How root morphology and functional traits enable geoxyles to thrive in nutrient poor and disturbance prone Angolan miombo ecosystems

Abstract

The miombo ecoregion encompasses a vast African dry woodland ecosystem that dominates about 47% of Angola's territory, especially in the central areas of the country. The ecoregion, which extends from the Angolan escarpment to the Indian Ocean in Mozambique and Tanzania, covers more than 3.8 million km². In general, miombo ecosystems form a mosaic of forest and woodland patches with grasslands co-dominated by geoxyles. Geoxyles (also known as geoxylic suffrutices) are low shrubs or semi-shrubs with mostly short-lived above-ground shoots but strong development of underground woody shoots, roots and tubers. Geoxyles have their global diversity centre in the Zambezian phytochorion, where geoxyles from more than 30 different plant families occur. As grasslands dominated by geoxyles are an important and little studied part of the miombo ecoregion, I have conducted studies on diversity, root morphology, functional traits (PFTs) and ecological functions of geoxyles on the southern slope of the Bié Plateau (Chitembo municipality, south central Angola) as part of the Future Okavango (TFO) project. My objectives were a) to provide an updated overview of the diversity and distribution of geoxyles and geoxyle grasslands in Angola, b) to describe the vegetation dynamics, root architecture, wood biomass and soil properties of geoxyle grasslands, c) to study the role of functional traits (PFTs) of geoxyles in pairwise comparisons with closely related tree species, and d) to see if the functional traits (PFTs) of certain phylogenetic groups differ and can help us explain the dominance of detarioid Fabaceae in miombo ecosystems. Chapter 1 provides a general introduction to miombo ecosystems and to geoxyle grasslands. An overview of the ecosystems dominated by geoxyles in Angola, information on their early scientific exploration, their distribution, biodiversity and endemisms is given in the first essay of my dissertation (Chapter 2). Based on our observations and data as well as the existing literature, I provide the reader with detailed information on the importance of geoxyle grasslands and emphasize the need to protect these special ecosystems, which have been scientifically largely unexplored so far. Together with my co-authors, I have identified the human activities that threaten these ecosystems, in particular intensive agricultural development in these areas. A striking feature of geoxyle species is the weak development of their above-ground shoots and the comparatively very strong development of woody rhizomes, roots and tubers in the soil. This woody network of subterranean woody shoot axes and storage organs forms the main pool of biomass and carbon stocks of the geoxyle grassland ecosystems, which so far have not yet been considered in carbon balances of miombo ecosystems. To get an idea of its carbon stocks, I measured the belowground biomass pool in my second paper (Chapter 3) and described the structure of the subterranean woody organs of six dominant geoxyle species (three for each of the two soil types occurring in my study area). I selected Brachystegia russelliae, Cryptosepalum exfoliatum ssp. suffruticans and Syzygium guineense ssp. huillense from ferralitic areas and Parinari capensis, Pygmaeothamnus zeyheri and Ochna arenaria from sandy sites. I calculated and discussed the subterranean biomass and the corresponding carbon stock, whose magnitude is comparable to dry tropical forests. Both, biomass and the disturbance regime by fire differ greatly between the two geoxyle vegetation types. Another important feature of geoxyle grasslands is the existence of phylogeneticaly related tree or shrub species for each geoxyle species. My third paper describes and discusses the differences in PFTs of six of these congeneric pairs (geoxyles versus trees) in the Chitembo study area (Chapter 4). Together with my co-authors, I measured and compared specific leaf area (SLA), leaf thickness (LT), wood density (WD), leaf contents of nitrogen (LN), carbon (LC), phosphorus (LP), potassium (LK), calcium (LCa) and magnesium (LMg) as well as leaf dry matter content (LDMC). Additionally, we measured the leaf water potential (WP) of the compared pairs during the dry and rainy season. These traits are known to show responses to a wide range of environmental factors and a close association with ecophysiological mechanisms. The results of this study suggest that differences in functional characteristics of geoxyles and trees are adaptations to the specific environmental stresses of their respective habitats. In my fourth paper (Chapter 5) I measure and compare the functional traits (PFTs) of an extended set of geoxyle and tree species which occur frequently in my study area. For this purpose the database was extended to twelve geoxyles and fourteen trees. To investigate the different adaptations with which these species cope with the environmental conditions of their habitat, I analysed and discussed differences in PFTs between phylogenetic groups and symbiotic strategy types (Fabaceae versus non-Fabaceae; Ectomycorrhiza associated versus non-mycorrhizal species). The results clearly show ecophysiological and symbiotic adaptations that developed in certain phylogenetic lines during evolution, enabling them to successfully cope with extreme habitat conditions. The general dominance of detarioid Fabaceae on the nutrient-poor soils of the miombo belt seems to be strongly influenced by their ectomycorrhiza association and thus superior phosphorus uptake capacity. Studies from comparable ecosystems of the miombo ecoregion would be required to support this hypothesis. Finally, I summarise and discuss the most important results of my four papers in a synthesis chapter (Chapter 6).