Tree rings link climate and carbon in Africa

Writer: 
Kristi Foster

“Trees are history books,” says Aster Gebrekirstos, a scientist with the World Agroforestry Centre (ICRAF). “Just ask a tree what conditions were like in the past and it can tell you about rainfall, temperature, river flow, the frequency of wildfires, and about how much and how fast it grew.”

Using dendrochronology—the science of analyzing and dating tree growth rings—Gebrekirstos and colleagues at ICRAF and partner institutions are capitalizing on the knowledge stored in trees to both learn about the past and plan for the future.

A brand-new laboratory facility at ICRAF’s Nairobi headquarters, set up by Aster Gebrekirstos under the auspices of the Climate Change research program and funded by the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), will allow other ICRAF scientists and partners in Africa to apply dendrochronology to carry out exciting new research, such as:

  • Reconstructing past climatic history, variations in atmospheric circulation patterns, river flow and frequency of extreme events;
  • Determining how climate change affects tree growth
  • Examining how trees allocate carbon over time and in different places; and
  • Investigating past and present changes in the frequency of wildfires.

“By understanding what climate was like in the past, we’re better positioned to predict and plan for the future,” says Gebrekirstos. “Dendrochronology can help us predict which agroforestry tree species will do well into future climates so that we choose the right trees for the right place.”

Tropical dendrochronology has long been viewed as having limited potential. In temperate and boreal regions, where there is an annual halt in growth during the winter, trees produce distinct annual rings. But in the tropics, tree ring formation is controlled by changes in moisture brought on by dry and wet seasons. Periodic bushfires and climate fluctuations within seasons can cause false rings, unclear ring boundaries, incomplete rings and missing rings, greatly complicating analysis.  

Yet there are several ways to prove the formation of annual rings in the tropics and there are even new techniques to date trees without rings, says Gebrekirstos. Research in Ethiopia’s highlands has shown that there is potential to reconstruct several centuries of growth history for Podocarpus falcatus, a conifer known locally as zigba, using dendrochronology. The authors discovered that zigbas – unlike broadleaved trees – could re-activate growth at any time of the year given sufficient rain.

The study, a partnership between ICRAF and the University of Erlangen-Nuremberg in Germany, used specialized instruments called dendrometers which measure variations in tree diameter. These measurements were then combined with analyses of tree core samples to distinguish between swelling due to water uptake and actual formation of new wood cells. The dendrometers were also used to monitor tree growth rates and examine the formation of tree rings in a particular year.

Based on annual rings counted from stem disks, the scientists estimated the age of a particular zigba at 500 years, and confirmed it by carbon dating – a method that uses changes in the amount of radioactive carbon to estimate the age at which a tree died.

“This species is one of the oldest dated trees known from East Africa. If we can date it confidently, we should be able to reconstruct its entire growth history, along with associated environmental changes on Ethiopia’s highlands over the past several centuries,” explains Gebrekirstos.

Other ICRAF scientists have already started applying dendrochronology in a number of ways in Africa. A recent study, for instance, was able to determine the amount of biomass produced by West Africa’s savanna trees in years past.

“This finding holds important implications for climate change mitigation interventions that require us to monitor the carbon sequestered by trees,” says Cheikh Mbow, senior climate scientist with ICRAF’s climate change unit and lead author of the study.

“If we understand the relationship between climatic conditions and tree growth, we can develop models that estimate carbon sequestration over entire landscapes based just on climate data.” Mbow is enthusiastic about the potential to use dendrochronology on the continent.

In southern Senegal, Mbow and other scientists have managed to reconstruct the complete growth histories of nine tree species using dendrochronology. They did so by developing equations that allowed them to relate tree ring size to annual biomass production.

According to Mbow, these models could help overcome the high effort and high cost of carbon accounting, which are major obstacles for developing countries. More frequent reporting of carbon stocks could potentially allow projects to receive carbon income before year 5 – the usual certification period, he adds.

African ecosystems have a pivotal role to play in climate change mitigation, yet they are also home to some of the world’s most vulnerable rural populations. “Dendrochronology can help us unpack the information held in trees,” says Gebrekirstos. “It can give us the hard evidence we need to make decisions on how best to mitigate and adapt to future climates.”

----------------------------------------------

Read the full journal articles:

Potential of dendrochronology to assess annual rates of biomass productivity in savanna trees of West Africa – Cheikh Mbow, Sophan Chhin, Bienvenu Sambou, David Skole in Dendrochronologia 2013

Growth dynamics and potential for cross-dating and multi-century climate reconstruction of Podocarpus falcatus in Ethiopia – Julia Krepkowski, Achim Bräuning, Aster Gebrekirstos in Dendrochronologia 2012

Visit the Dendrochronology Lab page

Related Reading:

Uncovering the past with trees

The carbon in blue gums

Climate–growth relationships of the dominant tree species from semi-arid savanna woodland in Ethiopia – A Gebrekirstos, R Mitlöhner, D Teketay, M Worbes, Trees 22 (5), 631-641

Cambial growth dynamics and climatic control of different tree life forms in tropical mountain forest in Ethiopia – J Krepkowski, A Bräuning, A Gebrekirstos, S Strobl, Trees 25 (1), 59-70

Links:

The CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS)

University of Erlangen-Nuremberg