Introduction

Our understanding of savanna dynamics typically focus on trees and grasses. However, there is much still to be learned about the diversity, environmental determinants and functional ecology of other life forms. Across the miombo savanna ecoregion of south and central Africa, forbs (herbaceous flowering plants) comprise a large proportion of old-growth savanna ground-layer diversity but have typically received less attention in research and land management. In addition, geoxyles or ‘underground trees’ (subshrubs with extensive woody underground storage organs) have generally been overlooked despite comprising Africa’s ancient, widespread and charismatic ‘underground forests’. During a collaborative field campaign in February 2024, members of the Future Ecosystems for Africa (FEFA) team visited Bicuar National Park in southwest Angola, a ‘darkspot’ of plant species records, to sample across a gradient of different vegetation types to capture these multiple facets of ground-layer diversity.

Figure 1. Flowers in the miombo.

Overview

Typical of the miombo ecoregion, Bicuar is characterised by mosaics of savannas and grasslands in regular arrangements contouring shallow valleys called catenas. Up on the catena crest are miombo savannas with open canopies and a continuous grassy understorey. On the slopes are the grassy ‘underground forests’, which transition into seasonally waterlogged grasslands called ‘mulolas’ in the valley bottom. The recurrent and predictable distribution of each vegetation type across Bicuar’s catena is known to result from an interplay between various environmental drivers such as fire, frost and especially topography. However, we wanted to ask exactly how these patterns manifest in terms of the diversity and distribution of different ground-layer species.

Figure 2. From left to right, the miombo, the ‘underground forest’ and the mulola grassland vegetation types across Bicuar National Park.

Main Content

We sampled herbaceous plants using the Global Grassy Group (GGG) protocol across plots that span the catena gradients of the different grassy vegetation in Bicuar. The GGG methodology involves collecting:

1. a) ecological data about each plot including evidence of disturbances such as fire and grazing, proportions of the ground that is covered by plants, tree canopy cover and soil properties;
2. b) species composition across sub-plots by noting the presence/absence of species and collecting pressed specimens that are used to confirm identifications of the plants in a herbarium.

Figure 3. Collecting ground-layer composition data and plant specimens using the Global Grassy Group (GGG) protocol.

Our data can be used to compare metrics such as species richness, diversity and similarity in order to quantify the distinctiveness of each vegetation type. Although they are all grassy ecosystems, we noted many unique species in each and an especially high diversity of forbs and geoxyles relative to grasses. This fits into global patterns of very high non-grass biodiversity in grasslands.

In addition, we set up three transects along the catena and measured environmental conditions such as soil moisture and temperature at a fine scale to calibrate microclimate models, with the aim to predict seasonal frost occurrence as a response to vegetation shifts.

Our work was complementary to the SEOSAW network, who have established permanent plots across Bicuar to monitor dynamics in the miombo overstorey such as growth and mortality rates of trees and smaller woody stems.

Conclusion

Angola’s flora remains unevenly documented, with frequent observations of new species and new country/provincial records. It is also highlighted as a priority for plant collecting globally. Our data will contribute to furthering knowledge about local and regional plant diversity, endemism, and conservation prioritisation. We hope that greater consideration will be given to the different life forms that comprise savannas beyond trees and grasses, and to the nuance in ground-layer diversity that arises at smaller scales across mosaiced ecosystems.

We thank colleagues from Instituto Superior Politécnico Tundavala (Angola), the University of the Witwatersrand (South Africa), and the University of Edinburgh (UK) for supporting the fieldwork, which was funded through NERC.

Figure 4. The team!

Author Bio

Anya Courtenay is a postgraduate research assistant at the University of Edinburgh and Royal Botanic Garden Edinburgh. Through managing the Global Grassy Group network with Future Ecosystems For Africa and the Global Centre on Biodiversity for Climate, Anya’s work collates data on herbaceous species composition across savannas and grasslands especially across southern Africa and Madagascar. Her research interests lie in how the distributions of grassy plant communities shift along gradients of disturbances such as fire, frost, herbivory and waterlogging.

Related Posts
For more field observations from Bicuar: https://globalgrassygroup.github.io/2024/02/28/bicuar.html
For information on the Global Grassy Group: https://globalgrassygroup.github.io/
For some science on underground trees: https://academic.oup.com/aob/article/133/5-6/757/7252959

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