Nation’s Urban Forests Losing Ground
National results indicate that tree cover in urban areas of the U.S. is declining at a rate of about 4 million trees per year, according to a U.S. Forest Service study published recently in Urban Forestry & Urban Greening.
Tree cover in 17 of the 20 cities analyzed in the study declined while 16 cities saw increases in impervious cover, which includes pavement and rooftops. Land that lost trees was for the most part converted to either grass or ground cover, impervious cover or bare soil.
Of the 20 cities analyzed, the greatest percentage of annual loss in tree cover occurred in New Orleans, Houston and Albuquerque. Researchers expected to find a dramatic loss of trees in New Orleans and said that it is most likely due to the devastation of Hurricane Katrina in 2005. Tree cover ranged from a high of 53.9 percent in Atlanta to a low of 9.6 percent in Denver while total impervious cover varied from 61.1 percent in New York City to 17.7 percent in Nashville. Cities with the greatest annual increase in impervious cover were Los Angeles, Houston and Albuquerque.
“Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces,” said U.S. Forest Service Chief Tom Tidwell. “Community organizations and municipal planners can use i-Tree to analyze their own tree cover, and determine the best species and planting spots in their neighborhoods. It’s not too late to restore our urban forests—the time is now to turn this around.”
The benefits derived from urban trees provide a return three times greater than tree care costs, as much as $2,500 in environmental services such as reduced heating and cooling costs during a tree’s lifetime.
Forest researchers David Nowak and Eric Greenfield of the U.S. Forest Service’s Northern Research Station used satellite imagery to find that tree cover is decreasing at a rate of about 0.27 percent of land area per year in U.S. cities, which is equivalent to about 0.9 percent of existing urban tree cover being lost annually.
Photo-interpretation of paired digital images offers a relatively easy, quick and low-cost means to statistically assess changes among various cover types. To help in quantifying the cover types within an area, a free tool, i-Tree Canopy, allows users to photo-interpret a city using Google images.
“Trees are an important part of the urban landscape,” according to Michael T. Rains, director of the Northern Research Station. “They play a role in improving air and water quality and provide so many environmental and social benefits. As our Forest Service Chief says, ‘…urban trees are the hardest working trees in America.’ This research is a tremendous resource for cities of all sizes throughout the nation.”
Nowak and Greenfield completed two analyses, one for 20 selected cities and another for national urban areas, by evaluating differences between the most recent digital aerial photographs possible and imagery dating as close as possible to five years prior to that date. Methods were consistent but imagery dates and types differed between the two analyses.
“Tree cover loss would be higher if not for the tree planting efforts cities have undertaken in the past several years,” according to Nowak. “Tree planting campaigns are helping to increase, or at least reduce the loss of, urban tree cover, but reversing the trend may demand more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy.”
Only one city, Syracuse, N.Y., exhibited an overall increase in tree cover. However, the increase was dominated by European buckthorn (Rhamnus cathartica), an invasive small tree/shrub from Europe, and the most of the city’s cover increase is likely due to natural regeneration.
Cities included in the study were: Albuquerque, N.M., Atlanta, Ga., Baltimore, Md., Boston, Mass., Chicago, Ill., Denver, Colo., Detroit, Mich., Houston, Texas, Kansas City, Mo., Los Angeles, Calif., Miami, Fla., Minneapolis, Minn., Nashville, Tenn., New Orleans, La., New York City, Pittsburgh, Pa., Portland, Ore., Spokane, Wash., Syracuse, N.Y., and Tacoma, Wash.
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The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation's forests and grasslands to meet the needs of present and future generations. Recreational activities on our lands contribute $14.5 billion annually to the U.S. economy. The agency manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world.
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By Katherine Kornei
Clear-cutting a forest is relatively easy—just pick a tree and start chopping. But there are benefits to more sophisticated forest management. One technique—which involves repeatedly harvesting smaller trees every 30 or so years but leaving an upper story of larger trees for longer periods (60, 90, or 120 years)—ensures a steady supply of both firewood and construction timber.
A Pattern in the Rings<p>The <a href="https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/coppice-standards-0" target="_blank">coppice-with-standards</a> management practice produces a two-story forest, said <a href="https://www.researchgate.net/profile/Bernhard_Muigg" target="_blank">Bernhard Muigg</a>, a dendrochronologist at the University of Freiburg in Germany. "You have an upper story of single trees that are allowed to grow for several understory generations."</p><p>That arrangement imprints a characteristic tree ring pattern in a forest's upper story trees (the "standards"): thick rings indicative of heavy growth, which show up at regular intervals as the surrounding smaller trees are cut down. "The trees are growing faster," said Muigg. "You can really see it with your naked eye."</p><p>Muigg and his collaborators characterized that <a href="https://ltrr.arizona.edu/about/treerings" target="_blank">dendrochronological pattern</a> in 161 oak trees growing in central Germany, one of the few remaining sites in Europe with actively managed coppice-with-standards forests. They found up to nine cycles of heavy growth in the trees, the oldest of which was planted in 1761. The researchers then turned to a historical data set — more than 2,000 oak <a href="https://eos.org/articles/podcast-discovering-europes-history-through-its-timbers" target="_blank" rel="noopener noreferrer">timbers from buildings and archaeological sites</a> in Germany and France dating from between 300 and 2015 — to look for a similar pattern.</p>
A Gap of 500 Years<p>The team found wood with the characteristic coppice-with-standards tree ring pattern dating to as early as the 6th century. That was a surprise, Muigg and his colleagues concluded, because the first mention of this forest management practice in historical documents occurred only roughly 500 years later, in the 13th century.</p><p>It's probable that forest management practices were not well documented prior to the High Middle Ages (1000–1250), the researchers suggested. "Forests are mainly mentioned in the context of royal hunting interests or donations," said Muigg. Dendrochronological studies are particularly important because they can reveal information not captured by a sparse historical record, he added.</p><p>These results were <a href="https://www.nature.com/articles/s41598-020-78933-8" target="_blank">published in December in <em>Scientific Reports</em></a>.</p><p>"It's nice to see the longevity and the history of coppice-with-standards," said <a href="https://www.teagasc.ie/contact/staff-directory/s/ian-short/" target="_blank">Ian Short</a>, a forestry researcher at Teagasc, the Agriculture and Food Development Authority in Ireland, not involved in the research. This technique is valuable because it promotes conservation and habitat biodiversity, Short said. "In the next 10 or 20 years, I think we'll see more coppice-with-standards coming back into production."</p><p>In the future, Muigg and his collaborators hope to analyze a larger sample of historic timbers to trace how the coppice-with-standards practice spread throughout Europe. It will be interesting to understand where this technique originated and how it propagated, said Muigg, and there are plenty of old pieces of wood waiting to be analyzed. "There [are] tons of dendrochronological data."</p><p><em><a href="mailto:email@example.com" target="_blank" rel="noopener noreferrer">Katherine Kornei</a> is a freelance science journalist covering Earth and space science. Her bylines frequently appear in Eos, Science, and The New York Times. Katherine holds a Ph.D. in astronomy from the University of California, Los Angeles.</em></p><p><em>This story originally appeared in <a href="https://eos.org/articles/tree-rings-reveal-how-ancient-forests-were-managed" target="_blank">Eos</a></em> <em>and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.</em></p>
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