Trees can benefit when their neighbour recruits ants

Research conducted by Staab and colleagues, published recently in Ecology, highlights an ingenious yet natural way to enhance tropical reforestation and carbon capture. By promoting tree species with Extrafloral Nectaries (EFNs), small secretory structures on plant surfaces that secrete nectar, they found it possible to increase the diversity and productivity of forest ecosystems.

Extrafloral nectaries, attracting ants to the plants, set up what’s known as a defence mutualism where ants protect the plants from herbivores in return for the sugary treat. The surprising finding from Staab’s team is that this protective effect spills over to neighbouring trees without extrafloral nectaries. This indirect defence results in these trees having fewer caterpillars, one of their primary pests, leading to improved growth. Thus, a high presence of extrafloral nectary-bearing trees can boost overall forest productivity.

This mechanism, known as facilitation, is a well-studied principle in ecology. It functions through “mutualist-mediated facilitation,” directly linking plant diversity and ecosystem productivity. This research is critical because it provides a cost-effective and environmentally friendly strategy to support reforestation projects, improve agricultural productivity, and enhance carbon capture efforts, thereby combating climate change.

Moreover, the researchers found evidence that trees neighbouring extrafloral nectary-bearing trees adjusted their composition of defence traits, reducing resources allocated to defences and increasing growth. This is a striking example of how biodiversity and interspecies interactions can lead to unexpected yet beneficial outcomes, suggesting that maintaining biodiversity is not just about conservation but also about harnessing these valuable ecological services.

This innovative approach can have substantial implications for reforestation and agricultural practices. Traditionally, nectar-providing plants have been planted next to crops to attract natural pest enemies, enhancing crop yields. These findings suggest that interspersing extrafloral nectary-bearing trees in forests or crop fields could similarly increase productivity. Staab and colleagues conclude:

Facilitation between plant species can be driven by mutualistic predatory arthropods that enhance conditions for plants without defense mutualisms. This understudied role of EFN-bearing trees might be important for understanding BEF [Biodiversity-ecosystem functioning] mechanisms in early successional tropical and subtropical forests. In reforestation and forest restoration, adding EFN trees to species pools (sensu Jones et al., 2017) could, at least in the early establishment phase, foster tree growth, carbon capture, and myriad other forest ecosystem functions.

Staab et al. 2023

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Staab, M., Pietsch, S., Yan, H., Blüthgen, N., Cheng, A., Li, Y., Zhang, N., Ma, K. and Liu, X. (2023) “Dear neighbor: Trees with extrafloral nectaries facilitate defense and growth of adjacent undefended trees,” Ecology, p. e4057. Available at: https://doi.org/10.1002/ecy.4057.