Scientists enlist “Witness Trees” to track climate change and pinpoint which species can best clean our air

Posted on: 23 September 2024

Trees make wonderful natural witnesses to environmental change for two key reasons: first, they are generally very long-lived (some flowering trees live beyond 1,000) so typically witness change in a way that shorter-lived species do not; and second, they are highly responsive to their environment, moulding the parts they grow every year such as new leaves and twigs – and even their physiological behaviour – to the prevailing conditions of the time. 

As a result, it is relatively easy for the scientists – who include Midori Yajima, PhD Candidate in Trinity, and Michelle Murray, Outreach Manager at Trinity Botanic Garden – to measure various elements of their response to assess what changes are occurring, and how the trees are responding. According to PI Jennifer McElwain, Professor of Botany in Trinity’s School of Natural Sciences, this can provide crucial information that can help us plan for the future.

Witness Trees project researchers, Sate Ahmad, Christos Chondrogiansis, Jennifer McElwain, Tina Monterio and Morgane Weissenburger, in the National Botanic Garden.

Prof. McElwain said: “Trees are incredibly reliable witnesses to the effects of climate change. They are, in effect, giant biological sensors with the potential to provide scientists with detailed, highly accurate, long-term datasets on the state on the environment.

“In this project we are teaming up with our Witness Trees to study how they respond to atmospheric particulate pollution and to changes in levels of carbon dioxide. By recording data from the Witness Trees every year for the next 30, we will build up a valuable long-term data set and be able to make highly informed decisions as to which species can best clean our air, and which species are most likely to survive and thrive in our changing environment. In this way, we hope to shape ongoing and future green policy and make a major societal impact.”

Two key missions underpin the project:

1. Evaluating green policies, by measuring the quantity of “smog” (or particulate matter) captured by the tree and relating these data to periods in which different green policies were active, to assess how effective they are.
2. Evaluating the “health” of the trees over time as the environment changes, to advise which species should be proactively protected and planted with the future in mind, and to shape green policy in this regard. 

To understand what the Witness Trees are trying to tell them, the scientists have to ask the right questions. Every summer they will observe critical functions in the trees, which will outline their health and ability to capture particulate matter pollution.

To assess their ability to capture smog from the atmosphere the scientists will assess the quantity of inhalabale (PM10 – relatively large particles) and fine inhalable particles (PM2.5 – relatively small particles) captured by each tree.

And to assess the trees’ general health (and track how this changes with the environment), they will measure their “stomatal conductance”, which is the rate of exchange of water and CO₂ between the plant and the atmosphere. As this is central to keeping the tree alive, it is a good indicator of general health.

Tina Monterio, a researcher on the Witness Trees project, collects data from a tree in the National Botanic Garden.

The Witness Trees

Twenty-one Witness Trees that reside in Trinity’s Botanic Garden are the stars of the show, while an additional 21 have just been added to the roster from Ireland’s National Botanic Gardens, which recently jumped aboard the project..

The 21 species in Trinity’s Botanic Garden include the likes of the Alder, Strawberry Tree, Hazel, Gingko, Common Oak and Wollemi Pine. You can meet them all on the Trinity Botanic Garden website. 

Prof. McElwain added: “We are delighted that the National Botanic Garden joined the Witness Tree project in 2024 to provide an additional set of witness trees. More trees means more data, which will help us to be more confident in any key messages we pass on from the Witness Trees to the researchers and policymakers who can make a difference in the future.”

You can read more about the project and track the tales the Witness Trees are telling on the project website. The project has been funded by Dr Beate Schuler and the European Research Council.

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