Trees ‘stressed’ by climate crisis work together to form resource-sharing root networks, research suggests

The growing recognition of how trees communicate with one another is a 21st century phenomenon which has revolutionised how we think about forests, woodlands and the individual trees which populate them.

They are not all separate plants indifferently competing with their neighbours for nutrients and sunlight, but, we now know, team up and interact with one another in various ways to help nurture and protect each other and the communities they live in.

But new research highlights how trees facing adverse conditions brought by the climate crisis appear to share resources more readily – thereby protecting themselves and each other.

Previous studies have revealed that underground networks of fungi and roots facilitate communication between trees and plants, but the new research examines how separate trees from the same species “graft” their roots onto one another in order to share resources.

This process allows, for example, trees in better locations to share resources with their neighbours in worse places.

The research team, from the University of Glasgow, examined root grafts among black mangrove trees in Mexico.

The species plants shallow roots in soft underwater sediment, which made for easier excavation than other environments.

But importantly, the La Mancha Lagoon the researchers focused on, in the Veracruz region, is separated from the salty waters of the Gulf of Mexico by a sandbar which closes off a narrow inlet between the two bodies of water.

However, every year during spring, fresh water runoff eventually overcomes the sandbar, bursting it open, allowing seawater from the Gulf floods into the lagoon.

The Gulf waters add salt to the freshwater lagoon, doubling or sometimes even tripling the salinity of the sediments during the dry season – lasting several months each year.

This shifting cycle of salinity makes it more difficult for the mangrove trees to consistently draw water and nutrients they need to survive from the earth.

But the researchers found this salt stress could spur the trees into action, forming root grafts across groups of trees. These root grafts could allow the stands of trees to spread their resources across the group to help keep each other alive, and to also physically hold onto one another, making the whole group more resilient to powerful tides and high winds.

The research team said the study provides new insights into how forests around the world might adapt to the increasing environmental stresses of the climate crisis.

Dr Alejandra Vovides, of the University of Glasgow’s school of geographical and earth sciences, said: “Mangrove trees make good candidates for mapping root grafts because their roots are shallow – you can reach into the sediment and follow them with your hands excavating locally.”

However, the team’s method was unexpectedly halted when the forest became flooded due to an early closure of the sand bar. As a result, “we ended up having to improvise a solution to keep our heads above water”.

This involved strapping a doppler ultrasound monitor, of the kind used to measure unborn babies’ heart rates, to the trees, and then using a length of metal pipe to poke through the sediment and hit the roots. If the monitor made a clear sound, then the team knew it was connected to that tree.

“It was low-tech, but it worked!” Dr Vovides said.

This unusual method allowed them to painstakingly trace the root graft networks of black mangrove tree stands across eight different 900-square-metre sites around the lagoon.

In addition to tracing the connections between trees, they also measured each tree’s height and the diameter of their stems (trunks), and the salinity levels of the water.

They found that more than three-quarters of the tallest trees had root grafts, and that grafted trees were generally taller than non-grafted trees.

In the higher-stress environments of salty water, the trees were more likely to form root grafts with other trees, however they did this in smaller groups than those trees in less stressed environments.

The researchers suggest that larger groups were less likely to form in higher-stress environments to ensure that the smaller group had sufficient resources to survive through their root graft network.

Dr Vovides said: “Around 200 species of trees around the world are known to form root grafts, but the processes and circumstances under which the grafts happen are not well understood.

“The evidence we’ve collected strongly suggests that the more dominant mangrove trees do indeed provide resources to help smaller trees prosper, which challenges some of the established theories of evolutionary biology. It’s less about ‘survival of the fittest’ and more about the mutual gains of existing in a network.”

She said the insights can also inform us about human interactions.

“One intriguing aspect of our findings is that they align with theoretical models of both natural system evolution and human societies on the cost benefits of cooperation. From those models, we’d expect to see small networks creating more stable gains than larger groups, and that’s just what we’ve seen here.”

The research is published in the journal Communications Biology.