Thirty-year 'Witness trees' project to see which are best at cleaning our air

The scientists from the School of Natural Sciences in Trinity College Dublin say the research promises to inform critical green policy and could end up changing which species are selected as street trees in towns and cities.

The scientists include Midori Yajima, PhD Candidate in Trinity, and Michelle Murray, outreach manager at Trinity Botanic Garden.

Principal investigator is Jennifer McElwain, Professor of Botany in Trinity’s School of Natural Sciences, She says this research can provide crucial information that can help us plan for the future and explained that 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.

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 of 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.”

Prof McElwain noted that trees have ‘thousands of surfaces’ due to their leaves. The particulates affecting our health are in the air and stick to surfaces, so trees stop the particulates harming people.

Why is this a 30-year investigation?

The reason this is a 30-year project is because climate is considered to be 30 years’ worth of weather.

“We find that long-term data sets get really important at year-five. We are at year-three already now. Every year after that it beomes more and more valuable. We don’t have to wait the full 30 years to get information, after year-five it will start reinforcing the information.” One-off or isolated polluting incidents such as ash from a volcano eruption or serious gorse fires etc will also show up in the research, confirmed Prof McElwain.

Likewise, during covid lockdowns when there was significantly less traffic on the streets, there was less pollution. “We would have instantly seen the covid lockdown effects on trees if this project had been running for 10 years before covid.

Is there early information on which trees perform better when it comes to filtering out harmful smog?

“Oak is amazing at taking particulates. We are also looking at a unique Irish whitebeam, a relative of rowan. And lime is in the dataset too. Interestingly, lime is thought to be one of the best street trees but it’s only in the middle of our tables so far. And one doing the best so far at taking particulates out of the atmosphere is black walnut — it’s doing so at 2-3 times the rate of hazel.”

But it's not a guarantee that we will see Irish streets lined with black walnut as other factors are taken into account also when deciding what species to use as street trees: “Black walnut sheds walnuts so councils may not want that on the streets, it’s a trade-off of sorts.”

Are there only native Irish trees featuring in the study?

“An open view on this is important so we are testing both native and non-native trees. With climate change our country could end up being too warm for a lot of our native species so we have to spread the choice.”

How do you actually test the trees?

Our method for this is we take the leaf at same time each summer. We wrap it in fibre-free paper and dry and press it. We then use a scanning electron microscope to view the surface and count all the particulate on the leaf to get a sense of how good the tree is,” explains Prof McElwain.

To assess the trees’ 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.

Cork City Council installed five high-tech CityTrees as part of a broader programme to tackle air pollution — are these being tested also?

They are not included in this study: “I’m biased as a botanist... nature has usually ‘invented’ if first. If you want a large surface area covered in plants or leaves then that’s a tree, you can’t beat a tree. If you think of the amount of surface area a mature tree provides then you can see how valuable they are.”

What and where are the Witness Trees?

Twenty-one Witness Trees 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 alder, strawberry tree, hazel, gingko, common oak and wollemi pine.

Professor 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.”