Earthshots: Satellite Images of Environmental Change

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» Logging in Oregon, USA

Every year, it takes one tree, 16 inches in trunk diameter and 100 feet tall, to meet every American’s need for paper, packaging, and lumber products. If that tree was harvested in Oregon, it was logged using strict environmental protections.

In Oregon, the goal is sustainable forestry. The logging industry there works to meet current societal needs for forestry products while ensuring that future generations will be able to enjoy the same benefits from abundant forest resources that we do today. Much of Oregon has been heavily logged in the past, but in western Oregon, where these Landsat images are focused, nearly 1 million acres of intact, old growth forests remain. And almost all land in Oregon that is logged is reforested.

One reason for the goal of sustainable forestry in Oregon is the Northwest Forest Plan, adopted in 1994, which resulted in a decline in forest harvesting on federal lands. After the plan was implemented, harvesting rates became longer to accommodate regrowth, and more partial cuts were used instead of clearcuts. The plan also works to protect old growth forest while maintaining sustainable logging.

A multi-image time-series from Landsat can create “regrowth trajectories.” The 30-m resolution from Landsat is enough detail to see the harvest rotation cycle. Douglas fir in western Oregon can regrow to a closed canopy stage in 15–20 years, so the time frame of the Landsat record can show just over one harvest rotation cycle.

Even from one year to the next, there is change. For example, compare the 2015, 2016, and 2017 images in this and the other sections. Look closely. Can you find new cleared areas in the green forest land?

Map of the featured area.

A logging strategy that’s easy to spot in the Landsat imagery is clearcutting. All the trees in an area are harvested, and then replanting begins immediately. Replanting must be completed within 2 years of harvest. In Oregon, about 40 million new trees are planted each year. According to the Oregon Forest Resources Institute, the reforestation success rate is 95% on private land.

Clearcutting is a major change to the forest. Clearcuts look unattractive, and they disrupt wildlife habitat. The clearcut area can also increase streamflow in that area, and the soil can erode more quickly until reforestation occurs. Therefore, in Oregon, the size of clearcuts is limited to 120 acres. They are also limited further when near highways. Forested buffers must be maintained along streams, lakes, and wetlands. This protects against possible increased streamflow erosion and maintains wildlife habitat.

There are actually some positive effects to clearcuts. They create open space in the forest that many plants and animals need to flourish. The full sun promotes rapid regrowth of Douglas fir and other species that need full sun. Also, clearcuts are generally temporary. Oregon requires prompt reforestation.

The close-up Landsat images that accompany this section show an obvious checkerboard pattern in the forest. These lands are known as “O&C” lands, which stands for Oregon and California Railroad. Land granted to O&C in 1866 was every other square mile, which formed the checkerboard pattern seen in the imagery. These lands now belong to the federal government, managed by the Bureau of Land Management (BLM).

In these images, the squares that remain dark green generally are lands that belong to the BLM. In some of those squares, there is clearcutting and regrowth taking place in this time frame. Adjacent squares, however, have a more noticeable change in tan or white spots, indicating more frequent clearcuts in the logging/reforestation cycle.

View an animation of this area to see the changes as they happen.

A more up-close look in the Landsat images helps you to see the progression of clearcutting and regrowth. The colors in most of these images are a patchwork of tan, white, and various shades of green. First, notice that the landscape changes a lot during this time. How can you ensure that this series of close-up images is the same exact area? Identify features that do not change throughout the series, such as a river, as you click through the time series.

Next, notice the difference between the shades of light green and dark green. How would these two areas look if you were standing on the ground? Dark green indicates taller trees and an older forest. Light green areas are only beginning to recover from past logging, so the young trees in these areas are not very tall yet.

The tan areas are fresh clearcuts. Using the infrared wavelengths of light that Landsat can detect helps to more clearly see those cleared areas. Compare the amount of tan seen in these images throughout the time series. Can you tell how long it takes for these cleared areas to look as they did before the clearing took place?

On Earthshots we typically show “multispectral composites”—single-date images that combine three different wavelengths into one image. The example used in this section shows a different combination of infrared wavelengths than was used on the 2016 image from the “Interpret the Images” section. In this image, the clearcuts are pink against the bright green of the forest. This combination of infrared wavelengths makes the clearcuts that have occurred from one year to the next stand out more clearly.

We don’t often examine change over time in one image. That’s what the colorful image in this section shows. It’s a “multitemporal composite”—one image that shows three different dates combined. This image shows a combination of Landsat images from 1994, 2009, and 2016. Red shows clearcuts that were evident in the 1994 image. Green shows clearcuts that took place by 2009. And blue shows clearcuts that occurred by 2016. Darker tones are areas of forest that did not change in this time frame. This way of examining the imagery shows that logging has affected nearly the entire landscape shown, something that may not be evident when looking at a single-date image.

People derive many ecological and economic benefits from forests, and Landsat can help forest managers monitor changes to ensure these resources are available for future generations.


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