Laser mapping the California forests

California is experiencing one of its worst fire seasons in history. Firefighters have responded to nearly 6,800 fires this year alone, and we're only half way through the season. The devastating Valley Fire engulfed over 585 homes and 73,7000 acres, according to Cal Fire. Much of this is a result of a historic, multi-year drought and decades of fire suppression, leaving the California forests in a vulnerable state.

Managers need more than just fieldwork because there’s too much ground to cover.  And satellites, while important, do not give a sense of the overall condition of the state’s forest. For that macro perspective, Greg Asner, a global ecologist and lead scientist for the Carnegie Airborne Observatory (CAO), has spent his summer flying over the California forests. The special plane he uses is outfitted with two key instruments – LiDAR and an image spectrometer – that help Asner assess the health of each individual tree and generate three-dimensional snapshots.

TechKnow joined Asner on a mission flight. It may look lush and green from the naked eye: 

But the images from CAO’s spectral sensors tell a different story: a field of red pixels, indicating drought stress. 

Here’s more detail on Asner’s view from above.

Editor’s Note: The following was adapted from an interview with Marita Davison & Phil Torres.  It has been edited for length and clarity

TechKnow: You've been mapping for a little while now, what have you seen so far?

Greg Asner: We've been all over the state.  It's about seventy million acres of forest we've seen. And we’ve seen everything from forests that are doing just fine to other forests that are in real trouble where we're seeing lots of mortality, extreme drought stress, scary stuff in different parts of the state.

How is the Carnegie Airborne Observatory an advance from more traditional survey methods?

What we tried to do was create instruments that give a measurement of a tree that's as close as possible to the experience you would have in going to measure the tree by hand…The difference being we're flying over about 8 million trees per hour. 

This is the sensor system in the airborne observatory, tell us about the instruments that you have here

Our aircraft has a unique set of sensors that among other things can see very clearly how well each tree in the forest is doing in terms of it's water content and other types of measurements that tell us whether those trees are growing well or they're really sickened or stressed out by this drought that we're under going.

This instrument (LiDAR) is a laser system that fires two lasers out of the bottom of the plane in a pattern that images the forest canopy or whatever it is that we fly over in 3D. Our other primary instrument is called the imaging spectrometer, it can measure the chemicals in each individual tree. 

The gold spectrometer plays a special role in this system

That’s right, this spectrometer has a detector, which detects the light that comes from the sun, bounces off the tree canopy, and comes up through the bottom of the aircraft into the telescope of the sensor and on to the detector. That detector needs to be cooled to a very low temperature, about minus 132 or so Celsius, very cold…Without it being that cold, the light hits the detector and all we get is noise. Making it that cold, the materials in the detector are ready to receive the light and produce a signal that we can interpret as chemical. 

Why is it important to know the chemical composition of the forest?

Knowing the chemical composition is a lot like going to the doctor and getting a blood test…We can do the same with trees by looking at their chemistry, we can understand something about their present condition, something about their recent and past condition and we can predict from their chemistry their immediate future.

Are there particular chemical signatures that you're looking for?

In the context of drought, and how drought affects forests, we're really interested in three chemical properties: the amount of water in the tree, the amount of sugar in the tree, especially in the canopy itself, and the nitrogen in the tree canopy….we want to know about the biomass, how much the tree weighs. 

What has been the most surprising thing that you've seen during this mapping mission?

I've seen shocking views of severe drought stress, I've seen places that on the map or on Google earth look green, but when you fly over them, they don't look like forests anymore. We're starting to see this in the southern forests in particular. We haven't seen that in the northern forests yet, but if the drought continues we expect to see this massive shocking wave of mortality and tree loss occur up here.

What is the value of forests to not just the state of California but on a national even global scale?

Forests are critical for storing carbon. If you don’t put carbon in forests, then it ends up in the atmosphere and that contributes to climate change. Another service that forests provide is in terms of watersheds. What I mean by that is mountain forests are critical catch-mans for rainfall in the future and those catch-mans become sources of water to reservoirs, rivers, farms, and to the drinking tap. Without those forests that provide that mediation of rainfall… we end up with lots of problems in terms of erosion and runoff.

Another big one is the value of biodiversity and habitat and the recreational experiences and all of that that Californians hold dear to them as well, as many places do. Forests need to be there for all of those animals, including ourselves.