We had an unusual visitor in our woods the other day. Chandler Johnson is a forester working on contract with the Minnesota Department of Natural Resources (DNR) to ground-truth the information and images coming to us from aerial LiDAR (LIght Detection and Ranging) imagery. The data available to forest planners from airplanes is rich and useful but combining it with detailed measurements on individual plots gives a more accurate picture of the Minnesota woods.
Chandler’s task on that bright sunny day was to count and measure the trees in a tenth-of-an-acre plot chosen randomly by a computer algorithm for our woodland. Luckily this plot wasn’t far from one of our snowshoe trails, so we didn’t have far to bushwhack.
Chandler set up a tripod and attached a GPS unit to it. It sat there for an hour, talking to the satellites that together pinpointed an extremely accurate location, within a centimeter or two. Then Chandler counted and measured all the trees in the plot at least five inches in diameter, including dead trees.
He used a special measuring tape to check diameters: one side of the tape includes a pi calculation, allowing him to simply wrap it around the tree and see the diameter.
Then he estimated the height of each of these trees, walking backward and using a very long tape measure attached to his hip. From a distance of 66 feet he pointed a clinometer to measure the height.
He measured both the total height and the merchantable height, the part of the tree a logger will be interested in.
Then he chose a “site index tree,” an aspen to represent most of the surrounding trees, and the tallest and healthiest in the area. From this tree he took a core sample to determine its age. It was 46 years old. These trees are a lot younger than I am!
Next, he entered information about the smaller trees at the site: the sizes and densities of each species regenerating in this area. He entered all this data into his sturdy laptop, gathered his equipment, and drove to the next site, about a mile away. These sites are scattered about a mile apart from each other and each of them represents 1,500 acres.
DNR foresters will double-check about ten percent of these sites each year. This new “plot-based inventory” (PBI) is specifically designed to complement remote sensing maps. The data from each small plot is combined with the aerial LiDAR images, and the DNR says it’s accurate enough to replace earlier methods and sharply reduces the cost of an inventory. It does not, however, make any attempt to plot or measure old growth trees.
Here’s an introduction to LiDAR and how it is used in Minnesota. The pictures are fascinating.
Brian Huberty, a retired DNR forester and now President of the Minnesota Forestry Association, told me the new system can make a big difference both for forest planners and woodland owners.
Brian recalls—40 years ago—using aerial photos to map state forests and talking about how the agency should be mapping private woodlands at the same time. There wasn’t enough support for the idea then. “But with recent advances of LiDAR, which is a laser pointer on steroids, the idea took hold,” he told me
LiDAR uses laser beams sent from aircraft to scan the earth’s surface. It provides an accurate picture of what it sees, and “specifically maps tree heights and densities very well, which gives us a better idea of forest structure,” said Brian.
As a former woodland firefighter, Brian knows fires go where the fuels are. Lidar gives a clear picture of the trees and other vegetation on the land, helping managers predict where wildfires might occur. And there’s another reason the DNR and some woodland owners want to know quite precisely what is on the land.
Trees store carbon, and carbon is of great interest in this era of climate change. In the U.S., trees and forests annually store approximately 11% of all the greenhouse gas emissions we emit. About half of a tree’s dry weight is carbon. Depending on size and age, a single tree can sequester as much as 10 pounds of carbon dioxide each year. Woodlands store carbon in live trees, roots, dead wood, litter (leaves and small woody material), and soil. In Minnesota, on average, woodlands store about 75 tons of carbon per acre, mostly in soil and live trees.
Markets are beginning to operate to monetize this phenomenon. In carbon offset markets, companies pay landowners for the carbon storage their trees provide.
These new markets are challenged by various factors; as the saying goes, “the devil is in the details.”
Brian Huberty is bullish on the future of these markets, and although only a small number of landowners are participating now, he says that number will probably grow, and these inventories of private land will help. “Landowners want to understand what they have in their backyard and whether they have the potential to get compensated for it,” he explained.
Also, some industries are hoping that biomass in the form of trees and other organic material can contribute to a less carbon-dense jet fuel than we use now. If that industry matures, harvesters will want to know how much carbon is stored in a given stand.
I was astonished to learn that Minnesota is home to two or three times as many woodland owners as farmers. “Of the forested land in Minnesota, almost half is owned by private owners—that’s more than the DNR owns, and more than the U.S. Forest Service owns,” Brian told me. “Private woodland owners are a big part of our forests, but they get little support compared to farmers.”
Personally, I don’t have plans to try to enter a carbon market, and we mainly use our woods for snowshoeing and privacy. But it’s fascinating to learn more about what’s out there.
