Photo by Marco Simola/CIFOR

For really accurate calculations of tree cover, or above-ground biomass, there’s no substitute for getting out into the woods with a measuring tape. But that’s time-consuming and expensive, and those measurements cover a limited area. Satellites provide a broader view, but miss the nuance that’s visible on the ground.

“You get much closer to reality if you look at both,” says CIFOR-ICRAF senior associate Martin Herold, an author of a study published in August in Science of the Total Environment that examines the challenges and possibilities of combining the two types of data.

Can they be combined to provide more precise estimates of carbon stocks, at both the country level and globally, for monitoring Paris Agreement commitments and programs for Reducing Emissions from Deforestation and Forest Degradation (REDD+)?

Herold and his colleagues say yes — though some adjustments would be needed to integrate them fully.

Each data-gathering method has advantages and limitations. National forest inventories (NFIs), which include biomass estimates and are conducted in most of the world’s countries, are based on field plots. But monitoring those field plots is costly, so the number is limited, and results must be averaged out over the country, resulting in less-precise calculations.

Satellites gather data over large areas, but although remote biomass estimates have become more accurate in recent years, they still rely on models rather than direct measurement. And for technical reasons, remote sensing tends to underestimate the biomass of dense forest and overestimate where forest cover is low.

The good news is that most countries now conduct NFIs. They are still fairly recent in tropical countries, where the surge in interest stems from the need to monitor carbon stocks under international commitments, Herold said.

Using NFI data poses challenges, however, because not all inventories are conducted using the same methods or with the same frequency, said Karimon Nesha, the first author of the study and PhD student in the Laboratory of Geo-Information Science and Remote Sensing at Wageningen University & Research in The Netherlands.

Those variations made a difference when she compared data from a remote-sensing system against biomass estimates for 2018 using the UN Food and Agriculture Organization’s Forest Resources Assessments, which draw on NFIs.

Nesha found that 150 of the world’s countries had NFIs, and all but 30 were complete. Most of the partial NFIs were in tropical countries.

Many tropical countries did have full data, however, and their inventories tended to be most recent, dating to 2018–2019, while temperate or boreal countries had gathered data between 2011 and 2017. The greatest difference between NFI biomass estimates and remote-sensing calculations were in countries that had not updated their NFIs since before 2010.

Some countries had multiple clusters of field plots, while others had only one plot. The distance between plots varied from country to country, ranging from 5 kilometres to 20 kilometres for main plots and 10 meters to 500 meters for sub-plots in clusters. Sizes also varied, from 0.01 hectare to 1 hectare, although most were one-quarter of a hectare or smaller.

Those differences made correlation and comparison more difficult, because different NFI designs require different statistical methods for analysis, Nesha said.

“For scientists working with specific countries, this study provides comprehensive support for understanding the methods of the NFI and better interpreting the data,” CIFOR-ICRAF senior scientist and study co-author Kristell Hergoualc’h said. “It also offers the opportunity for discussing improvement of the NFI together with the country.”

The study also points to steps that could make it easier to integrate NFI and remote-sensing data.

Small plots, especially circular ones, were more difficult to correlate with satellite data, because the footprint of remote-sensing systems is square and the resolution may be too low to capture a small area, Nesha said. Countries could combine their ground-based information with satellite data more accurately if their NFIs include “super-sites” of about one hectare, especially if they measure biomass there using LiDAR, which creates a more precise 3D image.

Many countries manage NFIs on five-year cycles, but updating only once in that time creates a data gap. Updating data from 20% of the field plots every year for five years would increase accuracy, although that must be weighed against the cost of the fieldwork, she said.

“Both the space-based community and the NFI community need to understand these as complementary sources,” Herold said. “For example, NFI designers can work on some additional measurements that could make integration with space-based data easier, while the space-based community should think about what kind of information and integration can be best for different countries’ circumstances.”

That way, he added, “We can make biomass information at both the country and the global level more accurate, up-to-date and efficient.”