Q&A: What can trees tell us about environmental history and climate adaptation?

Aster Gebrekirstos (centre) collects samples from a dead tree in Burkina Faso. Photo supplied

As people around the world rush to plant trees to restore degraded land, meet climate mitigation commitments, and build biodiversity, the question of which trees to plant in which places is more relevant – and challenging – than ever. If we don’t choose well for current and future climatic conditions, our work will be wasted and the costs for ecosystems and people will be high.

To work out which species are likely to be climate-resilient in specific contexts, scientists often set up test plantations. But this method can take many years to yield the required results – and we don’t currently have the luxury of time. Fortunately, there’s another – faster and cost effective – way to learn about how species respond to climatic challenges: we just have to know how to listen to them.

Dendrochronology – the science of dating and studying the annual growth increments (known commonly as tree rings) in woody trees and shrubs – offers a means to do so. It’s been used in ecology and climate change studies for decades , but mostly in temperate regions where the rings are easiest to read and where there is relatively high capacity and funds to devote to this: Germany, for instance, has probably over 15 dendrochronology labs, whilst the entire African continent has just about six.

Yet Africa is hardest-hit by climate change, and restoration there is arguably most urgent, while the gaps in climate information are huge. As such, a group of scientists are launching a new initiative, the African Tree Ring Network for Resilience (ATRN), which seeks to advance the use of tree-based data in the tropics – particularly in Africa. We spoke to Aster Gebrekirstos, a global scientist at the Center for International Forestry Research and World Agroforestry (CIFOR-ICRAF) who leads the dendrochronology laboratory in Nairobi and heads up this new initiative under the umbrella of the Past Global Changes (PAGES) project. 

Forests News: How can looking at trees’ growth rings help with climate adaptation?

Gebrekirstos: Trees tell us a lot about climate and ecological history, because they are the oldest living things on the planet and can live up to 4000 years. They can also tell us about how their particular species coped with past climatic challenges: some species are really sensitive to drought and don’t do anything – or even die – in very dry periods, and then in a good year they’ll grow a lot bigger, while others manage to grow during droughts. 

This means that trees contain a lot of really critical stories that are especially helpful for guiding restoration. For example, we can determine species’ resilience index based on how fast they recover from drought events and their plasticity to adapt to climate variability.

So basically, what we do in the dendrochronology laboratory is we ask the trees, ‘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. If we are able to read and speak their language, trees are 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 is a fast and cost-effective tool to understand, quantify, and advise (for example on which species is the most productive, which is the most resilient, what is its contribution in terms of water and carbon,  which competes with and which is beneficial for crops, when is the best time to harvest timber, etc.) so that we can plant the right trees in the right sites, taking future climate projections into account. In this way, we generate solid knowledge about trees, forests, their ecosystem services, environment and so much more. 

Forests News: How might this initiative also help with biodiversity conservation in Africa?

Gebrekirstos: At the moment, we have many, many indigenous trees in the tropics, but we are selecting very few for restoration – we’re mostly planting exotic trees like eucalyptus, grevillea, and cypress. And we’re doing it because we don’t know enough about the ecology  [of most indigenous trees] – the growth dynamics and mortality dynamics and so on. In the tropics, we don’t have many permanent plots; we don’t have long term data. So this is effective way that we can get the data that we need in time. As ecosystems shift, this work can also help with the ongoing survival of indigenous tree species – if we know how they are behaving, then we can assist them to migrate and survive.

Forests News: Why is the misconception that tropical trees don’t have growth rings still so prevalent?

Gebrekirstos: Because in temperate regions, you have clear climate seasonality – in winter, everything is dormant – so the growing period is very well-marked. But in the tropics, we don’t have that clear seasonality; instead, the limiting factors are moisture and, in some cases, heat. It seems like the scientists thought, ‘okay, the sun is always shining in Africa, so why should growth rings form?’ That misconception is still textbook knowledge, but it’s not true. Tropical trees have different structures, and it’s more complicated to ‘read’ them, but it’s not impossible: you just have to put in more effort and patience, and once you have the data you can use it for so many things. It is time to incorporate this science into the curricula of African universities.

Forests News: How are you hoping to influence policy through the network?

Gebrekirstos: We’re using the data to learn from climate and environmental history, and to share that information with policymakers. For instance, if we know in the past the drought was happening every ten years in a particular location, and now it’s happening every two-to-three years, what does that mean for policy? 

We’re also hoping to accelerate breeding and planting of indigenous species. For example, in terms of provenances, people usually collect seeds from different provenances, plant them, and wait to see which one is best. This is a real waste of resources and time, when we can just collect and analyze samples from different provenances to see how they have been behaving. 

Forests News: What are your hopes for the future of the ATRN?

Gebrekirstos: The formation of this working group is just the beginning. It’s a platform  for identifying the methods, sampling techniques, and species that give good reliable chronology in space and time, with a particular focus on the integration of long-term data to fill in critical climatic, ecological, and environmental knowledge and capacity gaps, in collaboration with our international partners. 

This initiative will also accelerate the formation of an African Tree Ring Society to keep people connected and generate data and knowledge about trees and forests at scale, like similar societies that we see in other parts of the world. At CIFOR-ICRAF, our work revolves around trees, forests, and agroforestry in the tropics, so I believe we have the capacity to continue contributing meaningfully to this mission.  

Follow and join ATRN here