How Trees Tell Time: Dendrochronology Explained.

by Ken Muli - Eco Journalist

As the world races to plant trees for a greener future, the choice of which trees to plant and where is more crucial than ever. Picking the right trees for today and tomorrow’s climate can mean the difference between growing forests or wasted efforts, impacting both our environment and communities deeply.

To determine which tree species will thrive in specific climates, scientists typically create test plantations, but this process takes many year, time we don't have. Thankfully, there's a quicker, more cost-effective way to understand how species respond to climate challenges: we just need to learn how to listen to them.

Dendrochronology, the science of dating and studying tree rings, provides a way to understand tree species' responses to climate. While it's been used for decades in ecology and climate change studies, it's mostly applied in temperate regions where tree rings are clearer and funding is more abundant. For instance, Germany has over 15 dendrochronology labs, while the entire African continent has just about six.

Africa, being one of the regions hardest hit by climate change, urgently needs restoration efforts, but there are significant gaps in climate information. To address this, a group of scientists is launching the African Tree Ring Network for Resilience (ATRN). This initiative aims to enhance the use of tree-based data in the tropics, especially in Africa. Aster Gebrekirstos, a global scientist at the Center for International Forestry Research and World Agroforestry (CIFOR-ICRAF). She leads the dendrochronology laboratory in Nairobi and spearheads this new initiative under the Past Global Changes (PAGES) project.

"Trees tell us a lot about climate and ecological history because they are the oldest living things on the planet, living up to 4000 years," says Gebrekirstos. "They also reveal how their species coped with past climatic challenges. Some species are highly sensitive to drought, showing little to no growth or even dying during dry periods, while others manage to grow even in harsh conditions."

"This means trees hold critical stories that can guide restoration efforts," Gebrekirstos explains. "For instance, we can assess a species' resilience index by observing how quickly they recover from drought and their adaptability to climate variability. This information is invaluable for choosing the right species for restoration projects."

In the dendrochronology laboratory, scientists essentially ask the trees about their experiences: How was life for you? How was the climate? How did we treat our planet? What did we do? And what shall we do for future generations?

I feel that I speak the language of trees. When we can read and understand their language, trees become history books. The lives of trees and ecosystems, from minutes to centuries, from the cell to the landscape, are recorded in their physical and chemical form. Dendroecology offers a fast and cost-effective way to understand, quantify, and provide insights on various aspects: identifying the most productive and resilient species, their contributions in terms of water and carbon, their interactions with crops, and the optimal time to harvest timber. This knowledge helps us plant the right trees in the right places, considering future climate projections. In doing so, we gain a comprehensive understanding of trees, forests, their ecosystem services, and much more.

Currently, many indigenous trees in the tropics are overlooked in restoration efforts, with a preference for planting exotic species like eucalyptus, grevillea, and cypress. This is largely due to a lack of knowledge about the ecology of most indigenous trees, such as their growth and mortality dynamics. The tropics lack many permanent plots and long-term data. Dendroecology provides an effective way to gather the necessary data in a timely manner. As ecosystems shift, this research can also support the ongoing survival of indigenous tree species by understanding their behavior and assisting them in migration and adaptation.

In temperate regions, clear climate seasonality marks the growing period distinctly, with everything dormant in winter. However, in the tropics, there isn't such clear seasonality; the main limiting factors are moisture and sometimes heat. There has been a misconception that, because the sun always shines in Africa, growth rings don't form in tropical trees. This outdated notion still appears in textbooks, but it's not true. Tropical trees have different structures, making them more challenging to read, but not impossible. With more effort and patience, the data obtained can be incredibly valuable. It's time to incorporate this science into the curricula of African universities.

We use the data to glean insights from climate and environmental history, and to inform policymakers accordingly. For example, if historical data shows that droughts occurred every ten years in a specific area, but now they occur every two to three years, what implications does this trend have for policy decisions?

We aim to expedite the breeding and planting of indigenous species. Instead of the traditional method of planting seeds from various provenances and waiting to determine the best performer, we advocate for a more efficient approach. By collecting and analyzing samples from different provenances upfront, we can quickly assess how each behaves and make informed decisions, saving valuable resources and time in the process.

The establishment of this working group marks just the beginning. It serves as a platform to identify effective methods, sampling techniques, and species that provide reliable chronology across different locations and over time. Our focus is on integrating long-term data to address crucial gaps in climate, ecological, and environmental knowledge and capacity. This effort is conducted in collaboration with our international partners to ensure comprehensive and impactful outcomes.

This initiative will also expedite the establishment of an African Tree Ring Society, fostering connectivity and facilitating the generation of extensive data and knowledge about trees and forests across the continent, akin to similar societies seen elsewhere in the world. At CIFOR-ICRAF, our focus on trees, forests, and agroforestry in the tropics positions us well to make significant contributions to this mission.

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