Proof Positive: Fewer Trees Mean Higher Temps

In the back of our rock garden, shaded on three sides by arborvitae and neighboring trees, stands a chest-high, white, capped PVC tube with holes drilled along its sides. Beneath the tube is a wooden stake; attached to the stake are two small climate sensors, one at the top and one at the bottom.

Every day for several months, now, the sensors have been silently recording temperature changes and related data from our garden. Occasionally, a group of graduate and undergraduate students from Clark University’s Geography Department, which owns the equipment, come by to download the data onto a laptop. We chat, they pet our aging golden retriever, who is most curious about any visitors, and then go on their way.

That data, as well as similar information collected from a handful of other sensors placed around Worcester in a variety of spaces with varying tree canopy, have formed the basis of a study this summer about how tree cover affects land surface temperature.

A Ten-degree Drop in Temp Where Trees are Present

The findings, presented on August 1 by undergraduates participating in Clark’s Human-Environment Regional Observatory (HERO) project, are significant: Trees reduce temperature near the ground by between 4 and 10 degrees Fahrenheit.

No surprise for anyone who seeks shelter under a tree on a hot summer’s day. But well worth quantifying and analyzing, given the significant number of trees lost to the Asian Longhorned Beetle infestation here—nearly 32,000 since the eradication effort began in 2008.

About 22,000 trees have been replanted, since, but more trees, still, have been lost to other, human-driven factors. According to a May 21, 2013 op-ed piece by Clark Geography faculty Deborah Martin and John Rogan, between 2008 and 2010, the city lost another 30,000-plus trees to urban development.

Without Tree Cover, Hotter Temps and Higher Energy Bills

To better understand the temperature impact of deforestation and replanting, the HERO student team analyzed satellite images of land surface temperature in Worcester. They compared two periods: 2007-2010, when most of the ALB-infested and at-risk trees were removed, and 2010-2012, when replanting efforts intensified.

The results are striking. In the Burncoat-Greendale neighborhoods that were hardest hit by the beetle, land surface temperatures increased by nearly 2 degrees to a whopping 16 degrees Fahrenheit after trees were removed (1-9 degrees Celcius).

By contrast, during the replanting phase, land surface temperatures began to slip slightly, by nearly a degree Fahrenheit. Given that new plantings are still saplings, it will be decades before neighbors can once again enjoy the cooling comfort of trees on a hot summer’s day.

Hotter temperatures translate into higher energy bills. Researchers from the University of Massachusetts-Amherst found that the presence of trees saved $85 in cooling costs for the average Burncoat resident.

Shared Losses and a Sense of Sacrifice

For now, until the new trees grow taller, higher energy costs will be the norm for neighborhoods stripped of their old, shady tree canopy—just one impact profoundly felt by residents of the affected areas.

Those impacts were the focus of a second team of HERO students, who conducted and analyzed dozens of interviews with residents, government officials and other policy makers to define how tree loss has affected the community’s sense of place.

Even as neighbors grieved the changed landscape, higher energy bills, lowered property values, lack of comforting shade and less attractive views, many of the stakeholders interviewed agreed that the crisis had some unexpected benefits: improved communications between government officials and citizens, increased funding to replant an aging urban forest, and a stronger sense of neighborhood commitment and environmental awareness.

Admirably, residents in the most affected neighborhoods also recognized that their sacrifice of infested trees most likely saved the city, region, and—we hope—the great northern maple forests from ALB devastation.

The HERO students’ presentation concluded a summer of research and the second year of a three-year National Science Foundation grant, headed by Associate Professors Martin and Rogan. The undergraduate teams included students from Clark and around the country.

The students are heading home, now, for a few weeks of well-deserved rest until the fall semester begins. I weeded the back of my rock garden, to make it easier for the next group to download data. Until they return, the sensors continue to silently record temperature changes in my little, tree-shaded corner of the world.

Evelyn Herwitz is the author of  Trees at Risk: Reclaiming an Urban Forest and blogs about ALB prevention and tree stewardship at She predicted the 2008 Asian Longhorned Beetle infestation of Worcester, Mass., in her book, published by Chandler House Press in 2001.