A plant thought to aid Nothofagus may be out-competing it

The idea was simple, Proteaceae plants are experts at releasing phosphorus from soils, so planting them alongside Nothofagus should provide the beeches with natural fertilizer. The reality is more complex.
Image: Chilean Firebush Canva.

Roots draw up water and nutrients from the soil, but some are better than others. What happens if when some plants have better roots than others? If a plant is good at releasing nutrients from the soil, but isn’t very efficient at collecting them, are these released nutrients available for other plants? Plants in one familiy, the Proteaceae, found in the southern hemisphere are very good at releasing nutrients, thanks to their unusual roots.

Gevuina avellana with cluster roots, click to enlarge. Photo: Alex Fajardo.
Plants in the Proteaceae have evolved cluster roots. Dr Alex Fajardo has been examining them to see what they can do. “Cluster roots are specialized root formations that some plant species have. They appear in certain part of the roots as a concentration of hairy, fine roots that resemble brushes. The good thing about them is the fine roots increase the absorption area. So the plant can exude organic compounds through them that allow the plant to mobilize phosphorus from the soil, which many times, particularly in volcanic soils, is stuck on mineral particles.”

This led Alex Fajardo and Frida Piper to investigate how Proteaceae species, with their cluster roots, interact with other non-cluster root species. The plants they chose were Embothrium coccineum, ciruelillo, or Chilean firebush and Gevuina avellana, avellano chileno, or Chilean hazel. These are the Proteaceae species and are the ones thriving in southern Chile Fajardo and Piper work. Dr Fajardo said that  G. avellana was a particularly good choice as it is one of the Proteaceae species with the biggest cluster roots. The non-cluster root species were Nothofagus betuloides, guindo, or Magellan’s beech and N. pumilio or lenga (beech).

Fajardo and Piper planted pots with cluster-root and non cluster-root plants paired with each other. In some the pots, the pairings were of plants that would live by each other in the wild, conspecific, in other the pairs were swapped, heterospecific. They also planted some plants as single plants for comparison.

The expectation was that, by exuding chemicals from a greater surface area, the plants with cluster roots would release nutrients from the soil that both plants could use as Dr Fajardo explained: “The initial idea was that Proteaceae species, especially in poor nutrient soils, would facilitate (not compete) other species because they are not that efficient in absorbing all the phosphorus.” The Nothofagus species should then grow better when planted with one of the Proteaceae species. This did not happen.

“We were wrong in our initial idea of facilitation given that the evidence we provide indicates that Proteaceae species outcompete Nothofagus species, most probably being more efficient in the absorption of nutrients and even stealing some of the phosphorus absorbed by the Nothofagus mycorrhizae, although this needs to be proved.” Dr Fajardo said what the results show are that there is indeed a great deal of competition for resources below the surface, as thought. This will have an impact on plans for planting.

“We initially thought that Nothofagus species could be planted along with Proteaceae,” said Dr Fajardo. “This was one of the motivations of our experiment. However, our results indicate that this would not be a good idea. Both species can be used in restoration efforts, especially in volcanic-origin soils (most of southern Chile), but apparently not together.”

The results suggest that there is plenty of research to do in rethinking how plants use cluster roots. Dr Fajardo said that the process of exudation of organic compounds by the cluster roots remains a puzzle. He would like to know how efficient this process is. The roots also have an unexplored ecological facet he said. “In general, cluster roots appear as an advantageous trait for Proteaceae species ‘against’ other species, which certainly can modify how a plant community is structured.”

Further reading

Fajardo, A., & Piper, F. I. (2019). Cluster-root bearing Proteaceae species show a competitive advantage over non-cluster root species. Annals of Botany. https://doi.org/10.1093/aob/mcz128