Lasers reveal forest structure in HD
Not long ago if you wanted to measure the height of a tree, you had to do trigonometry on the ground — or gear up for a climb. But these days you have a more sophisticated option: beaming lasers from the sky.
A revolutionary airborne technology called LiDAR (“light detection and ranging”) is making it possible to measure and map entire forests in a sliver of the time—and for a fraction of the cost—of earlier methods. By bombarding forests with hundreds of thousands of light pulses from laser equipment mounted on airplanes, scientists in the LTER program at Oregon State University are getting never-before-seen 3-D images of dense old-growth stands such as McDonald Forest in Oregon’s Willamette Valley and H.J. Andrews Experimental Forest in the McKenzie River Basin. And they’re doing it for the bargain-basement price of $2 an acre (not counting computer processing, which will add at least another dollar per acre to the cost). In contrast, the cost of putting two technicians on the forest floor with notebooks and measuring tapes is about $30 an hour. At a pace of about one hour per tree, mapping a forest the size of the Andrews on foot, with its 15,000 rugged acres in the Cascades foothills, would take years, if it could be done at all. With LiDAR you can start after breakfast and have the raw data in hand before lunch.
In fields as diverse as geology, oceanography and forest ecology, LiDAR is in fierce demand. “LiDAR is everywhere,” says Tom Spies, a research ecologist at the USDA Forest Service, Pacific Northwest Research Station, who has a courtesy appointment at OSU. “It’s the hot new technology, the hot stuff.”
BOUND FOR THE CROWN
Boots on the ground, however, still have a role. That’s why researchers have been out in the field manually double-checking the height of the Andrews’ tallest 10 or 12 trees the old-fashioned way: with a tape measure.
One cool autumn afternoon, Spies and Mark Schulze, director of the Andrews Forest, one of the original LTER network sites, stand at the foot of an ancient Douglas fir as they strap on the harnesses and snap on the carabiners they will use to
leverage their body weight. With gloves and helmets secured, the College of Forestry researchers clip their ascenders onto two of the colorful nylon ropes rigged in advance by professional climbers Rob Miron and Jason Seppa of the Pacific Tree Climbing Institute. Craning their necks, they can barely see where the orange and red lines disappear into the deep-green canopy. Crowning at 280 feet, the tree towers are as tall as a 25-story building.
The scientists are soon dwarfed as they hoist themselves skyward, dangling beside pitch-stippled bark as gray and craggy as a weathered mountainside. This silent colossus was a seedling about the time Shakespeare was writing his plays.
Spies and Schulze are “ground truthing” the LiDAR readings — that is, they’re comparing the laser readings against manual measurements in order to verify the LiDAR’s accuracy. “We use a 300-foot tape measure,” says Schulze. “We stake one end to the ground at the base of the tree and attach the other to our climbing harness and take it up in a straight line along the trunk. Eventually, we reach a point above which we’re not comfortable climbing, and use a telescoping height pole to measure the remaining distance to the tip of the crown.”
So far, accuracy has been within a whisker. “LiDAR can measure heights to the nearest centimeter,” reports Spies.
LiDAR’s beauty, aside from being fast and cheap, is its 3-D capability. It can characterize a forest’s structure at every layer: from streambed to treetop, from open clearing to tangled undergrowth, from massive coniferous branches to twiggy deciduous boughs. Sitting at their computers, scientists can rotate the colorful LiDAR images to view the forest from an infinite number of angles.
This remote sensing tool is similar to the radar that air traffic controllers and meteorologists use to monitor jets and hurricanes, except one uses electromagnetic waves while the other uses pulses of light. Radar (originally dubbed RADAR, for “radio detection and ranging”) works by bouncing radio waves off a target to gauge its distance and position. LiDAR does the same thing with lasers, targeting anything from woodlands to coastlines to rainclouds.
For the H.J. Andrews Forest project, 10 laser points per square meter are beamed to Earth from a sensor mounted beneath a small twin-engine plane owned and operated by Watershed Sciences, a Corvallis-based firm. After hitting an object — a fallen log, a rocky outcropping, a thick mesh of branches, a logging road — light from each pulse scatters backward to the sensor. This bounce-back is called an “echo.” The period of time each beam takes to return to the sensor indicates the object’s elevation. So if the beam comes back fast, that means it bounced against something tall. If it comes back later, it bounced against something lower in the forest layers, maybe even bare earth where foliage is thin. The digital images that emerge provide a comprehensive picture of forest structure unlike anything possible pre-LiDAR.
“Forest structure is key to its ecology,” says Spies. “Knowing the details of forest structure not only allows us to better predict and manage habitat for wildlife but also to understand microclimates, measure carbon and biomass, manage wildfires and design restoration efforts.”
OSU ecologist and wildlife biologist Matt Betts explains that “vertical structure” — how vegetation is layered throughout the forest — determines habitat selection and even survival for forest species.
“Many experts increasingly believe vertical structure is the primary driver of biodiversity,” asserts Betts, an assistant professor of forest ecosystems and society. “Researchers can often predict with considerable accuracy the diversity of birds, mammals, even insects and butterflies that will live in areas, based on what you can tell of the vertical structure of the forest.”
Forest ecologists like Spies and Betts comprise only one LiDAR user group. The current and future uses for this new tool are as vast as Oregon’s storied woods. Already, other OSU geoscientists have used LiDAR to study post-tsunami landscapes in Samoa and detect hidden earthquake faults in Puget Sound. NASA is using it to estimate global carbon stocks and detect atmospheric changes across the planet.
The National Oceanic and Atmospheric Administration is tracking topographic changes along coastlines. The list is long and varied.
Spies goes so far as to liken LiDAR to such transformational technologies as the telescope and the microscope. “Anytime there’s a new tool in science and research, it opens up a whole new avenue of investigation, one that you couldn’t necessarily anticipate,” he notes. “You end up discovering that it can give you answers to questions you never thought you could ask before.”
Read more about this story in “LiDAR Use Expands into Monitoring Biodiversity, Ecosystem Health,” OSU news release, September 14, 2010