Trees have long been regarded as vital indicators of environmental health, and now scientists are turning to them as "Witness Trees" to gather crucial data on climate change and air quality. These trees, some of which can live for over a thousand years, provide an unparalleled historical record of environmental conditions. They are also highly responsive to changes in their surroundings, making them excellent natural sensors for monitoring ecological shifts.

A team of researchers, including Midori Yajima, a PhD candidate at Trinity College Dublin, and Michelle Murray, Outreach Manager at Trinity Botanic Garden, is using these trees to measure changes in environmental factors such as atmospheric particulate pollution and carbon dioxide levels. Led by Professor Jennifer McElwain, Professor of Botany in Trinity’s School of Natural Sciences, the project aims to collect long-term data that can guide future environmental policies and conservation efforts.

“Trees are incredibly reliable witnesses to the effects of climate change,” said Professor McElwain. “They act as giant biological sensors, providing detailed, long-term datasets that are crucial for understanding the state of our environment. By monitoring how trees respond to pollution and atmospheric changes over the next 30 years, we can identify which species are best suited to clean our air and which are most likely to survive in changing conditions.”

The Witness Trees project has two primary missions:

  1. Evaluating Green Policies: The team will measure the amount of particulate matter, or “smog,” captured by the trees and correlate these findings with periods of varying green policy implementations. This will help assess the effectiveness of different environmental strategies.

  2. Assessing Tree Health: By tracking the health of the trees over time, the researchers will provide insights into which species should be protected and planted for future environmental resilience. This data will inform green policies aimed at preserving and enhancing urban and rural forests.

To gather this information, the team will monitor critical tree functions every summer. They will measure the amount of inhalable particulate matter (PM10) and fine particles (PM2.5) each tree captures, providing a gauge of the tree's ability to filter air pollution. Additionally, the scientists will assess the trees' “stomatal conductance”—the rate at which water and CO2 are exchanged between the plant and the atmosphere. This measure is a key indicator of the tree’s overall health and its ability to adapt to environmental changes.

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

The project has expanded significantly, with the addition of 21 new Witness Trees from Ireland’s National Botanic Gardens to the existing 21 species at Trinity’s Botanic Garden. Among the species under study are the Alder, Strawberry Tree, Hazel, Gingko, Common Oak, and the rare Wollemi Pine. These trees are not only diverse but also offer varied responses to environmental stressors, providing a rich dataset for the researchers.

Professor McElwain expressed her enthusiasm for the project’s growth, stating, “We are delighted that the National Botanic Garden joined the Witness Tree project in 2024. More trees mean more data, which will help us to be more confident in the messages we pass on to researchers and policymakers who can influence future environmental decisions.”

The Witness Trees project is funded by Dr Beate Schuler and the European Research Council. To learn more about the project and follow the stories these remarkable trees are telling, visit the project’s website.

By enlisting these natural witnesses, the scientists hope to deepen our understanding of how trees interact with their environment and to guide the development of green policies that will ensure a healthier, more resilient future for our planet.