Take two very different tree species from Ethiopia’s Munessa-Shasamene Forest: one an evergreen conifer called ‘zigba’ and one a deciduous broadleaved tree. Do they both use carbon in the same way? New research shows that these trees cycle and store carbon very differently and this could change our understanding of forest degradation’s impact on carbon balances.
Scientists from the World Agroforestry Centre (ICRAF), the University of Erlangen-Nuremberg and partners have performed an experiment called ‘pulse-labeling,’ the first of its kind in a tropical ecosystem. They sealed entire individual trees in a plastic cover into which they injected carbon-13 – heavy carbon atoms which act as tracers to follow the path of carbon all the way from photosynthesis through to incorporation by wood tissues.
By analyzing tree rings for this trackable ‘carbon signature,’ the scientists discovered a big difference between the two tree types: the deciduous tree, Croton macrostachyus, stored new carbon for only six months, while the zigba, Podocarpus falcatus, retained carbon for three full years in wood storage tissues.
Carbon that is not stored in wood tissues is used for energy production and released back into the atmosphere, either directly or via roots, fungi and bacteria and the decomposition of organic matter in soil.
“The fact that these two tree types have such different carbon storage strategies has implications for plant-soil-atmosphere carbon balances of degraded forests,” said Aster Gebrekirstos, a scientist with ICRAF’s Climate Change Unit and one of the paper’s lead authors.
When an old growth forest suffers human disturbance such as logging, it creates canopy gaps that favour pioneer species like Croton macrostachyus. These species increase their abundance at the cost of climax species such as zigbas. If disturbance is persistent, changes to species composition could mean a much faster turnover of carbon from plants to soil and back to the atmosphere, depleting carbon at the ecosystem scale.
“We’re seeing that degradation may have an additional carbon footprint we hadn’t considered before,” said Gebrekirstos. “It’s crucial that we understand how degradation from human pressure – like timber extraction or forest grazing – can affect carbon cycling in these ecosystems so we can better manage carbon in tropical landscapes worldwide.”
This study is just one example of the kind ofresearch to be carried out by ICRAF scientists and partners, aided by a newly-launched dendrochronology lab at ICRAF’s headquarters in Nairobi, the first of its kind in Africa. Set up by Gebrekirstos and funded by the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), the lab will help scientists extract information from tree rings that could significantly impact policy decisions on climate change and food security on the continent.
Dendrochronology, the science of analyzing and dating tree rings, can allow scientists to map a tree’s age, unearth the climatic conditions that a tree grew in and provide insight into the hydrology of an area in order to better predict future drought and other stress conditions related to food security. This work is cutting edge in Africa, where climatic data only dates back 30 to 40 years.
For example, Gebrekirstos and other dendrochronology scientists in Ethiopia have been able to determine a cyclical pattern in Ethiopia’s drought, have discovered that drought frequencies are increasing and that Ethiopia’s climate is influenced by the El Niño Southern Oscillation.
“For a long time, scientists have avoided tree ring studies in the tropics because it can be challenging to date tree rings with confidence,” said Gebrekirstos. “We want to change that because African decision-makers have a lot to gain from the knowledge stored in trees.”
Visit the Dendro Lab page or see photos on Flickr