ECR Feature: Paulina Meller on factors shaping diversity in geoxyles

Paulina Meller is finishing her PhD at the University of Hamburg, Germany. She studies the evolutionary forces that generate diversity in plants. Paulina shares her recent work on the environmental factors that have given rise to diversity in geoxyles, plants with disproportionately high below-ground woody biomass.

Paulina Meller taking a break from digging in the Afromontane grasslands of Tundavala, Angola. Photo credit: Manfred Finckh (Twitter: @ManfredFinckh)

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Institute. Institute of Plant Science and Microbiology, University of Hamburg, Germany.

Academic life stage. PhD (but finishing soon!).

Major research themes. Patterns and drivers of diversity, with a research focus on tropical vegetation (and a personal interest in birds).

Current study system. I study geoxylic plants (geoxyles) in Afrotropical grasslands. Geoxyles have a low growth form and disproportionally high belowground woody biomass. Many geoxyles evolved from tree species in response to environmental changes that proceeded the Miocene. Emerging fire occurrences have been hypothesised as the evolutionary driver of geoxyles. Hundreds of geoxyle species exist in Africa, and there are more in other tropical grasslands. They can be so diverse and abundant that they form “underground forests”. The belowground storage organs and bud banks enable geoxyles to resprout again after environmental disturbances. Their abundance, diversity and resilience make geoxyles a key element of tropical grassy ecosystems.

Recent paper in JBI. Meller, P., Stellmes, M., Fidelis, A., & Finckh, M. (2022). Correlates of geoxyle diversity in Afrotropical grasslands. Journal of Biogeography, 49, 339–352.

Motivation behind this paper. Geoxyles have been mostly overlooked in studies on tropical grassy ecosystems, probably due to the fact they are hidden underground. Despite some very good studies on geoxyles in Southern Africa and the Cerrado in South America, the main narrative remains that they all evolved from trees in response to fire. However, this narrow focus neglects other environmental factors that might have shaped the evolution of geoxyles and their taxonomic diversity. As a result, the knowledge on geoxyles has been rather fragmentary and one-sided so far.

As part of my PhD, I was working on a geoxyle species list for Angola when a nice paper by Pausas et al. (2018)* came out, providing a framework to classify belowground plant structures. It was therefore an exciting opportunity to combine the species list with analyses on their ecology, belowground functionality, and taxonomic and biogeographical origin, linking the fragmented knowledge together to better characterise geoxyle evolution.


Caloncoba suffruticosa (Achariaceae) resprouting and flowering shortly after fire. The belowground woody storage organs and bud banks are partly excavated, typically only the green parts and flowers are visible aboveground.

Key methodologies. Our recent paper is quite complex because we measured so many facets of geoxyle biology, which was necessary to understand how heterogeneous and diverse this group of plants is. Each facet of geoxyle biology required a different analysis. Our study is the first to integrate different kinds of data – from field-derived functional traits, over literature research on origins, to large scale spatial and environmental modelling – to show how the facets of geoxyle biology interlink with one another. We took the vivid discussion on the main drivers of geoxyle evolution – fire versus frost – as an incentive to test for correlations between belowground traits, spatial distribution, and environmental pressures. Strikingly, some species seem to be more fire dependent than others, and most geoxyles had a strong link to frost-prone sites. So both frost and fire are important evolutionary drivers.

Major results. It was important to show that the geoxylic life form is much more diverse, complex and heterogeneous than previously assumed. The focus so far has been on the species with close tree relatives, which is totally reasonable since they are the most striking geoxyles. But they make up less than 50% of the geoxyle pool studied so far. In order to better understand tropical grassy biomes, and manage these often-threatened ecosystems sensibly and sustainably, it is crucial to characterise the diversity in geoxyles. Another important point to me was to reconcile whether fire or frost is the one driver of geoxyle evolution. We showed that there is no single driver, as implicit in previous discussion on the overarching importance of fire. Although some geoxyles clearly evolved in response to fire, there are species that have evolved in response to dry season frost events, and likely some also evolved in response to edaphic conditions. By recognizing that geoxyles are diverse and heterogeneous, determining an ultimate evolutionary driver of their diversity becomes less important.

Unexpected results and challenges. A particularly surprising finding was that many species in our study sites exhibit considerable belowground woody biomass. We had to include almost every species we looked at, and the more species we excavated, the longer grew our list of geoxyles. We therefore show that it is not sufficient to describe and understand an ecosystem by its visible, aboveground parts alone. Moreover, it was hard work to excavate all these over 100 different species, as some formed tubers big as a football in 1.2 m depth, others grew in particularly rocky ground. Our field assistant Segunda was a great help in this regard! Neither have I had so many blisters on my hands before, nor dirty finger nails, than during this campaign. But afterwards I lost some 5 kg and had hands as tough as a sailor’s.

Paulina in the field. This is a drone image of a geoxyle grassland near Chitembo, Bié province, in Central Angola. Geoxyles resprout and colour the landscape brightly at the end of the dry season, long before grasses or trees do.

Next steps. There are still so many open questions, but I would like to compare the geoxyle diversity hotspots of Africa (Miombo) and Brazil (Cerrado) more closely. Our Brazilian colleagues have done some amazing work on geoxyles, and often on different aspects to what we have done with African geoxyles. A combined, intercontinental analysis would fill many knowledge gaps and yield important new insights. For instance, whether similar families or genera produce geoxyles, thereby hinting at a phylogenetic predisposition to evolve this life form, or whether species and functional diversity are dependent on similar environmental factors on both continents.

If you could study any organism on Earth, what would it be? I don’t know if I am competent enough, but I would like to study big birds like the Southern Ground Hornbill (Bucorvus leadbeateri), or the Great Blue Turaco (Corythaeola cristata). I have seen both during my trips to Angola, and I love to see and hear them, they make me happy. There is something magic in waking up in the early morning to the bass sound of a foraging group of Ground Hornbills. They always look like the Blues Brothers on a stroll.

Anything else to add? Doing research in Angola is exciting and exhausting, beautiful and bizarre, endearing and endangering – but you come home the wiser and more experienced, and you will always be able to tell an interesting story. So even after 7 years of countless blisters, sunburns, malaria, and a barely survived crocodile attack I still enjoy digging in Angolan grasslands, and collaborating with our Angolan colleagues and friends.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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