We usually think of plants that strut above the ground: showy blooms, fragrant blooms, and unique shapes abound. But their underground development is just as magical.
“Over the past 400 million years, since plants first colonized the earth, roots have been the real engine of the earth’s nutrient cycle,” marvels SFI Omidyar Fellow Mingzhen Lu, lead author of a new study published in Proceedings of the National Academy of Sciences. “Roots are the foundation of biodiversity.”
In the study, Lu and his team of international collaborators, which included scientists William Bond (University of Cape Town) and Lars Hedin (Princeton University), dug deep to better understand one of the world’s most extraordinary root systems.
Researchers conducted a four-year manipulated experiment to explore the sharp divide between the Fynbos and Afrotemperate forest biomes in the Western Cape of South Africa. Fynbos, a shrub biome with enormous plant diversity, adjoins Afro-Temperate Forest, a forest dominated by a small number of tree species. The unusual boundary of the biome is so narrow that in a few steps one goes from a warm, open shrub to the cool, mossy shade of the forest.
The sharp delineation is made even more distinct because the two biomes share underlying geology and are subject to the same climate patterns – they exist as alternate steady states. In the face of extreme disturbances, biomes could potentially shift to mirror nearby plant communities.
“Certain systems can exist in different states – like water and ice,” Hedin explains. “This makes them particularly interesting as models of radical change, as they can transition from one state to another, which is particularly urgent in a world stressed by climate change.”
In this context, the study revealed two important findings. First, the Fynbos and Afrotemperate forests showed marked differences in their root traits. Second, these root differences allow the Fynbos plant community to deter trees by limiting below-ground nutrient availability. Specifically, Fynbos plants repel invasion with the finest roots ever identified.
“We found that in all the ecosystems of the world, these roots are the finest of all,” says Lu. “For every gram of carbon – the weight of a paperclip – these plants produce roots of 15 long.”
Stringy roots allow Fynbos species to outcompete thicker-rooted plants in nutrient-poor soils.
“The fine roots of Fynbos are the underground weapon creating miserable conditions for nutrient-demanding forest plants,” says Bond. “We now see that it is not the intrinsic properties of the soil, but the plant feedbacks on the soil, that create misery for young forest trees.”
Compounding the “nutritional misery,” as the authors describe it, the Fynbos biome is prone to frequent, hot fires that burn away nutrients stored in the soil. The subterranean strategy of nutrient storage combined with a collective adaptation to fire allows the Fynbos plant community to promote its own persistence by modifying its environment. On the other side of the biome divide, the forest does the same thing.
The results suggest that alternative stable states may be maintained by biotic mechanisms, such as root traits, in addition to commonly understood abiotic factors such as climate. This idea is essential to the conservation of threatened ecosystems around the world.
“It’s profound to see microscale plant characteristics, like root thickness, linked to macroscale emerging ecosystem patterns,” says Lu.
“Who would have thought that it is the roots that help to explain this bi-stability? Hedin asks. “It’s impressive.”