Maths replaces chainsaws to measure tree carbon

Research in Western Kenya will help in the development of a quick and reliable method of predicting the amount of carbon stored in the landscape; essential if smallholder farmers are to benefit from carbon markets by growing trees on their farms.

PhD student, Shem Kuyah, has developed an equation which is proving 90% effective in determining the biomass of trees, both above the ground as well as in the roots. Once biomass is known, the amount of carbon stored by each tree or per hectare can be calculated.

“Measuring the amount of carbon stored in a tree usually involves cutting the tree down, taking samples of different parts of the tree, drying them, calculating the biomass and then converting this into a figure for carbon,” Kuyah explains.

“I am confident that by measuring the diameter of each tree and plugging this into the equation, it is possible to get a very good prediction for biomass and therefore carbon.”

For small farms, such is the case in Western Kenya, all the trees can be measured relatively quickly. For larger farms or whole project sites, several plots can be randomly selected across the site.

Kuyah’s research is part of the Carbon Benefits Project and will be launched this week during the scientific meeting of the United Nations Framework Convention on Climate Change in Bonn, Germany along with a range of other tools that have come out of the project. All of these are aimed at a consistent methodology for accurately measuring, monitoring, reporting, and projecting just how much carbon each kind of land use system is storing.

Without consistent methods, it is impossible to calculate incentives for payments that farmers, conservationists, communities and land owners could receive for the global environmental benefit they provide.

While farmers in developing countries are one of the world’s largest, most efficient producers of sequestered carbon, at present there is no reliable way to calculate or verify how much they are removing from the atmosphere.

Kuyah’s is certainly not the first equation to be developed for measuring biomass. A number exist for individual tree species but, as Kuyah points out, these are difficult for farmers to use as they rely on the farmer knowing the scientific names for the all the trees on their farm. There are other generic equations too, but their accuracy is low.

Kuyah has tested his equation on 879 randomly sampled trees in his study site in the districts of lower, mid and upper Yala northwest of Kisumu in Western Kenya. He has also harvested 72 trees to cross-check this against the measured biomass. And he is quick to add that for every tree harvested, two are planted.

“Not only is the biomass of the tree above the ground measured, but also what is below the ground. “ To do this, Kuyah and his team of helpers had to dig up the roots of the harvested trees. He admits that predicting below-ground ground biomass is more difficult and there is far greater variation but tree diameter still gives the best estimate.

“Many people use a shoot to root ratio to estimate below-ground biomass, mostly because it is extremely hard work and time consuming to do what we’ve done,” Kuyah adds. “But these ratios are not as reliable because the roots of every tree grow differently and are influenced by soil, drought or perhaps the way the tree has been managed by the farmer.”

Kuyah explains how the performance of his equation varies depending on the diameter class of trees. “For tree diameter less than 10cm, the error is greater than for trees with a diameter larger than 40cm,” he outlines. “But in the landscapes I’ve been surveying just five percent of the largest trees contain 60% of the biomass.”

Kuyah’s equation is working well in agroforestry-dominated landscapes in Kenya and Uganda, and spanning a substantial rainfall and temperature gradient as well as 1,000m in altitudinal difference. The next step is to test it at a national level in Kenya and across a broader area in East Africa.

“In theory it should work wherever similar climatic and management conditions exist,” Kuyah concludes.

Shem Kuyah is a lecturer at Jomo Kenyatta University of Agriculture and Technology in Kenya and undertaking his PhD research attached to the World Agroforestry Centre.