A new study conducted by NASA revealed that previous measurements of the carbon content of millions of trees which are found in temperate United States forests were most likely to have been overestimated.
Trees can absorb and store carbon for a long time, but how much carbon is deposited in global forests is still unknown, researchers said.
Laura Duncanson of NASA's Goddard Space Flight Center explained that the estimations of living trees' carbon content rely on a method based on cutting down trees, but it would take a lot of effort to do so, especially cutting down the biggest ones. She said the method is not practical to apply in large quantities.
Most field studies aim to just strategically sample trees because of the limitation. With U.S. forests, an average of about 30 trees per species would be taken down and measured. Experts would use mathematical models to extend those measurements to many thousands and millions of trees. This would result to an estimation of the amount of carbon stored or biomass for an entire forest.
Duncanson and her colleagues discovered that the method tends to miscalculate the height of large trees, and this leads to biomass estimations that are much too high for temperate forests.
Researchers said that the overestimation happens because of a sampling bias. Younger and smaller trees get chosen for analysis than older and larger ones. Mathematical models that estimate biomass are mostly based on smaller trees, and in turn, inaccurate models of the largest trees are created.
The new NASA study used lidar-based technique instead of the widely-common method. The technique could enable scientists to analyze whole swaths of forests from high above. Data for the study was provided by G-LiHT or Goddard's Lidar, Hyperspectral and Thermal instrument.
"One of the innovations of this work is our use of lidar remote sensing to measure potentially millions of trees," said Professor Ralph Dubayah, one of Duncanson's co-authors. "This is a dramatically different approach when you consider how few trees are normally used to develop these relationships."
Scientists extracted digital measurements of the crown radius and height of the tree from the lidar data. They studied six forests in the U.S. and digitally measured 10,000 to more than a million trees at each site. They re-ran the mathematical models through a sample of measurements, beginning with tens of trees and then to a few thousand. More trees included in the measurement led to better calculations, especially because now, the larger trees were accounted for.
From the study, the team concluded that when the conventional method was used, the calculations overestimated the biomass by an average of 70 percent.
"Our findings underscore the importance of sampling more trees," explained Duncanson. "When you include more trees, and especially more big trees, you get a much better idea of how much carbon is being stored."
The team said that new lidar instruments, such as NASA's Global Ecosystem Dynamics Investigation or GEDI, are being built to help examine the biomass of forests. GEDI will be the first device ever to methodically characterize forests from space.
The team's findings are published in the journal Scientific Reports.
