A New Measuring Stick for Forests

"The amount of carbon that is being held in the land surface by trees, and how it has changed over time through disturbance and subsequent regrowth, is the least understood aspect of the global carbon cycle," said Ralph Dubayah, a remote sensing scientist at the University of Maryland. "Until we know how much carbon there currently is in Earth's forests, and how that has changed over the last 20 to 30 years, we will have a difficult time predicting how much more carbon forests will absorb in the future, and what role they'll play in helping to mitigate or accelerate atmospheric carbon dioxide concentrations."

Dubayah and colleagues are working hard to measure that carbon, and they have a great new tool to do so. The Global Ecosystem Dynamics Investigation (GEDI) mission has been flying for nearly a year on the International Space Station, and researchers have just released to the public the first GEDI measurements of forests around the world.

GEDI surveys Earth's forests through light detection and ranging (lidar). The instrument sends out rapid pulses from three lasers and records how the light bounces off of surfaces and objects to detect their location and shape. From these lidar signals, the GEDI team can construct detailed, three-dimensional maps of forest canopy height and the distribution of branches and leaves.

"Existing pan-tropical biomass maps use laser data acquired nearly 15 years ago and were based on less than 5 million laser observations in total," said Dubayah, the GEDI principal investigator. "GEDI collects six million laser observations every day. So over the tropics, we've already collected about two orders of magnitude more data than what was 'state-of-the-art' before."

"Measurements over vegetation are challenging," said Bryan Blair, GEDI's deputy principal investigator. "To make these accurate height measurements, we use a technique called waveform lidar that maps the 3-D structure of the forests. To do this accurately, we need to penetrate the canopy and see a sometimes very weak ground signal."

GEDI views a forest as a collection of waveforms that show the treetops, the ground, and the branches, leaves, and open space in between. Put together, the collections of waveforms begin to show the forest's structure—not only vertically, but horizontally. This allows scientists to calculate the amount of biomass and carbon contained in a given area of forest.

With a global map of the mass contained in forest tree trunks, branches, and leaves, researchers can estimate how much carbon different forest areas contain and how that changes over time. Researchers will also be able to model biodiversity around the world.

What this means for a Legal Reserve

GEDI's carbon measurements from space show why forest area alone does not tell the whole story: two hectares that look the same on a map can store very different amounts of carbon, depending on tree height and density.

At Neutraza, each FRU is one hectare of Legal Reserve with an address — the polygon registered in Brazil's rural environmental registry (CAR). What we track on that hectare is whether the forest cover is still standing, using Sentinel-2 imagery at every pass and a vegetation index (NDVI) computed only over the reserve's own pixels. Structural measurements like GEDI's are the next step in the science: they help estimate how much carbon that forest stores, not just whether it exists.

For someone supporting a reserve, the practical question is simple: is the forest I protect still there? That is the answer each FRU's public page shows, with the date of every image.

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Source: A New Measuring Stick for Forests. Public domain text and image from a U.S. government agency.