Subterranean Wonders: The Fascinating Resilience of African Geoxyles 

Geoxyles, a distinctive feature of Afrotropical savannas and grasslands, are a type of tree that survives recurrent disturbances by resprouting from large belowground woody structures. A new study by Anya Courtenay and colleagues, published in Annals of Botany, found that underground trees inhabit significantly distinct and extreme environments relative to their taller, woody tree/shrub congeners.

Underground trees have independently evolved within the woody genera of over 40 plant families in Africa. But what factors dictate the presence and distribution of these geoxyles? Is it the chilling touch of frost, or the fiery embrace of frequent fires that shape their world? Researchers used occurrence records of four Afrotropical genera (Parinari, Ozoroa, Syzygium and Lannea), and environmental data of nine climate and disturbance variables to compare the biogeography and niche of underground trees with their open and closed ecosystem congeners. 

The authors found that across a myriad of environmental gradients, and within the multidimensional space of their habitat, underground trees have staked out territories in strikingly distinct and extreme environments compared to their open and closed ecosystem relatives.  

The researchers also found that the niche overlap among underground trees and their congeners is remarkably low. In essence, underground trees carve out their own unique niches, distinct even among their own kind. Each of the four studied genera reveals a different facet of the underground tree’s story. 

Parinari underground trees, for instance, brave the harshest of conditions, inhabiting hotter, drier, and more seasonal environments where herbivory pressure is at its peak. Ozoroa underground trees, on the other hand, dare to thrive in fire-prone landscapes, displaying their resilience in the face of frequent blazes. 

Syzygium underground trees, masters of adaptation, have made their homes in areas prone to frost, flourishing in relatively wetter conditions with seasonal waterlogging. Meanwhile, Lannea underground trees venture to the extreme, enduring the lowest temperatures, basking in the highest precipitation levels, and experiencing a wide spectrum of disturbance conditions. 

As scientists continue to delve deeper into the ecology and biogeography of these geoxyles, one thing becomes clear: the distinctiveness of their environments should be recognized and considered in the management of African grassy ecosystems. Courtenay and colleagues write:

Across Afrotropical environments, underground trees occupy more extreme environments and stressed niches than their tall tree/shrub congeners. When underground trees are considered as a single group incorporating the four genera, the environmental niche occupied is broader than that of OE [open ecosystem]and CE [closed ecosystem] congeners. Hence, grouping all underground tree taxa masks how their niche stretches into diverse extreme environments and demonstrates the necessity of understanding the variety of geographical contexts in which underground trees are found.

Courtenay et al. 2023

These enigmatic trees, hidden beneath the surface, hold the key to a deeper understanding of the intricate dance between nature and adaptation in the heart of the African savannas.

READ THE ARTICLE
Courtenay A., Moonlight P., Pennington R. and Lehmann C. (2023) “Underground trees inhabit varied environmental extremes across the Afrotropics” Annals of Botany. Available at: https://doi.org/10.1093/aob/mcad124

Updated 14 Sept 2023 to change the image for a better example of an underground tree.