Not just a tree: Plant scientists call for rethink on what a tree is

They’re not just tall woody things, say plant scientists, who want the planet’s trees to be recast as far more expansive and complex organisms, amid the challenges of climate change.

Environmental scientists have long forecast that living organisms will be tested by global warming and the ecological shifts that result from it, and trees are especially threatened.

They are long-lived and, unlike animals, which can relocate to new territory, are fixed in position, so adaptation to changing climate largely needs to take place where they are.

Yet a paper published by New Zealand, Australian and American plant scientists in the journal Trends in Plant Science, suggests tree species may withstand climate pressures through their fast-changing microbiome. This tree microbiome consists of a diverse range of microorganisms living within and upon their tissues.

“Climate change has happened before, however the rate of change we are experiencing is unprecedented in recent history,” says Sarah Addison, a tree-root microbiome researcher at Western Sydney University and NZ-based forest research organisation Scion.

“As trees live for a long time and can’t easily migrate, the plants established today could be stranded in an unsuitable climate. We need new tools to support trees facing change in the future.”

Over time, the relationship between host trees and microbial life may shift in ways that aid adaptation to new conditions. Creating suitable microbial environments may also help.  

Addison and her co-authors suggest that trees should no longer be viewed as singular entities, but rather ‘holobionts’ – systems of both tree and its symbiotic culture of bacteria, viruses, archaea, fungi and other microbes. That’s because the holobiont provides a more robust model to study climate change and its influence on trees and the surrounding environment.

“We need to shift our perspective from ‘trees and their microbiomes’ to one of a single, co-evolved entity interacting dynamically within and as part of the environment,” says Addison.

This ‘neo-Darwinian’ approach to evolution requires what the authors describe as a reimagining of a tree “from a singular sessile organism with limited ability for environmental response, into a node within a holobiont network that is interacting with other nodes (trees) and collectively interacting with the environment.

“Understanding the coevolution of trees and microbiomes linked with abiotic factors could be an area that creates improved understanding of the assembly of microbiomes and their intricate relationships.

“Unexplored questions remain concerning what adaptations are provided by the plant–microbiome relationship to equip the tree for future growth; how the environment affects this mutualistic relationship for long-lived tree species; and how these microbiomes are transferred through generations.”

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