How the Nation’s Forest Census Improves Satellite-Based Mapping

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It’s possible to map vegetation type with Landsat, but getting the maps right requires more than satellites alone.

The teams at the USGS Earth Resources Observation and Science (EROS) Center behind the LANDFIRE program’s mapping products use an extensive network of ground control points to check their work, thereby bolstering the reliability, accuracy and utility of the data.

Color photo of Karen Schleeweis with the logo for the USGS EROS podcast Eyes on Earth

Karen Schleewies, FIA Techniques Research Band Lead/LANDFIRE Liaison, USDA Forest Service, pictured with the graphic for the USGS EROS podcast Eyes on Earth.

The largest single source of “ground truth” for LANDFIRE comes from a USDA Forest Service program called Forest Inventory and Analysis (FIA). Karen Schleeweis of the Forest Service, who serves as the FIA’s Techniques Research Lead and its LANDFIRE liaison, recently sat down to talk about the program and the partnership for an episode of the EROS podcast Eyes on Earth.

Below you’ll find excerpts from the show, edited for clarity. Follow this link to hear the full episode. Follow this link to hear every episode. You can also find Eyes on Earth at Apple podcasts or Google podcasts.

Eyes on Earth (EOE): First off, Karen, tell us about FIA. How long has this been around, and where did it come from?

Karen Schleeweis (KS): In the FIA logo, it says we’re the Nation’s forest census. But really, we’re a survey. And to take that further, we are a collection of surveys. We have the forest monitoring survey, which is the kind of ground plots that LANDFIRE interacts with, but we also have a National Woodland Owner Survey, timber products, Tropical Island Survey. Our Forest Monitoring Survey is a probability based, statistical sample of field plots.

EOE: What does that mean in practice?

KS: The analogy would be the Nation’s people census. The idea is they go out, and they measure every person. That’s what makes it a census—the numbers are actually from counts. In our case, we are going out and measuring a sample of the forest, and then we make estimates about the forest area and where it is at. People use it to answer questions like how much forest exists and where? Who owns it? How is it changing in terms of growth, mortality and removals?” FIA is kind of the program that collects, analyze and reports on all those data.

But how long has it been around? The short answer is way longer than me. It’s been in continuous operation since 1930, and it’s really had a similar mission that whole time. As legislators tried to adapt to the Nation’s need for reporting on forests and rangelands, they adapt (FIA) somewhat. An example would be the 1999 Farm Bill: in direct response to user needs, we enhanced FIA from a periodic to an annual survey.

EOE: Give us an idea of the scope of FIA. How much data are we actually talking about? And can you tell us a little bit more about how it’s collected? Is this the kind of thing where it’s just constantly in motion, the plans for the next survey?

KS: Definitely constantly in motion. We measure forest land on public and private forests, all the way out in Guam and the Pacific Islands, to Puerto Rico and the U.S. Virgin Islands, and all of the 50 states. So that’s the spatial extent. The total number of plots in the survey? Roughly, we have one permanent field plot every 6,000 acres. If you imagine draping the U.S. in a hexagon grid, almost like a soccer ball, with hexagons with an area of roughly 2,400 acres, we randomly locate a spot within that hexagon. That’s kind of the spatial grain. Then, we visit those plots roughly every five to ten years, depending on the location. Then sometimes our spatial intensity is much higher. Some states pay for 2- to 3 times the intensity. And then how’s it collected? Those plots … there is kind of an intense design of the plot itself and how we go out on it. But we do actually measure the diameter breast height—the DBH—of the trees, the species types, we collect invasive data. There’s quite a lot of variables that we collect on the ground. We don’t measure every tree (in the Nation), but we do measure every tree on our plot.

EOE: So how large is the plot itself?

KS: Each field plot consists of a roughly 24-foot radius circle subplot. That’s a little hard to describe: four sub-plots within a macro plot, then we also have micro plots. But I think to get the rough idea, you could say the plots encompass or describe the area of an acre.

EOE: How are these inventories used? Who uses them?

KS: There’s actually a cool paper that was published in 2018; the main author was Tinkham. He did analysis over the last three decades, finding roughly 300 and some research papers that used FIA data. Which really got to the nugget of who uses it, particularly for research, and why. And forest health and carbon cycle applications were the two dominant (topics), but forest products, climate and remote sensing applications were in there as well. Fuels and fire hazards kind of fall under that forest health, and that’s one of the links to LANDFIRE. But our data supports a lot of operational needs. For example, state foresters, industry consultants, different environmental organizations, even forest service officials. Journalists have used our data for different reasons. And then we get a lot of calls from private citizens. Like “what’s this tree in my back yard? And is it native? Can I do this with it? Can I find out about this? Or someone planted this species and how tall can I expect it to get?”

It’s funny, because when you ask why it’s important, you can lead with all the uses, but you can also say it’s important because we focus on high standards. People trust our data. We’re consistent, and have been through time. And I think more than ever, it feels like there is a greater need for information about the supply and the condition of the nation’s natural resources, including the forest land and rangeland.

Eyes on Earth Episode 48 - Satellites and the Forest Census

EOE: How does this tie into LANDFIRE? How does this tie into remote sensing? Tell us how LANDFIRE came to use FIA data, and LANDFIRE obviously is a satellite-derived data set. How did those things come together? What’s the history there?

SCHLEEWEIS:

They knew from the beginning that they needed comprehensive, field-based reference data to make plausible maps with remote sensing. As soon as you start to ask, “what field data do we have in the U.S.?” particularly for forests, FIA will come up quickly. There were a lot of FIA scientists on that original (LANDFIRE) prototype, helping develop the methods for vegetation modeling or for canopy cover modeling. For for the areas that are forested, it’s really that data that helps LANDFIRE improve. The FIA canopy and height measures are the gold standard for forest plot data. And they use that with a lot of lidar data to improve their canopy cover and canopy height maps across the US.

EOE: So that’s the value of LANDFIRE. Knowing how tall the trees are, how tall the canopy is?

KS: For one of their spatial products, yes. They have a tree height product, but they also have a lot of data that helps now support height for shrub, and even grass. And that comes from other partnerships they have. Knowing the height of vegetation helps when you’re understanding or modeling how fire spreads through the landscape, how high the canopy crown is, and how intense the fire is before it has to get up into a crown and be a crown fire. So we do provide height measurements, but then there are other things we measure even more directly. We measure duff and litter, so fine fuels as well as coarse fuels—dead, woody debris that’s on the ground. That really plays into fire modeling.

EOE: LANDFIRE released its ReMap for the conterminous United States last year, and that updated its base map and offered a host of other improvements for users. Tell us how FIA contributed to that process.

KS: There were couple different ways. First, more plots always helps. We now have an all condition inventory, where we take these really detailed measurements on land that doesn’t quite meet our forest definition. Additionally, since the last mapping effort, FIA has pushed into interior Alaska. LANDFIRE was able to leverage all of these new plots to help with the vegetation and forest mapping. And there are other ways that FIA helps contribute through the various remapping efforts, including this last one: we’ll go through and do updates to say, the algorithms or the kind of logic that they use in their models to map, say, forest height or forest canopy cover.

EOE: So is there a way for the public to access FIA data? Where can people learn more about this?

SCHLEEWEIS: There’s a national FIA website which is easy to find www.fia.fs.fed.us. There’s an “About Us” page that really has a lot of the background information, but the national public database is there, as well. You also can access our state report, or things like our strategic plan. For of our research publications, you can also access on TreeSearch. That’s an electronic data base that has free copies of all the FIA and all the forest service publications. And you know, I think really exciting new resource is that we are leveraging things like Tableau and fact sheets. So now we have more interactive state fact sheets. Or even the questions like my mom might ask me, “what do you mean by forest?” We’ll have an interactive fact sheet on that. We’re really starting to recognize the different types of audiences that are interested in the data that we have, beyond the classic timber industry or state forester audience.

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