Local tree removal in residential neighborhoods: temperature changes detected by satellite

Sealed surfaces in cities, such as pavement and roofs, absorb incoming solar radiation, converting the energy into sensible heat, hence raising cities' temperatures above those of their surroundings. This creates the urban heat island effect. Vegetation on the other hand converts incoming radiation into chemical energy through photosynthesis or latent heat by evaporating water, neither of which causes warming. Larger proportions of vegetation keep residential neighborhoods cooler than downtown areas. Even within residential neighborhoods the pattern of landscaping will determine the micro-climate: large-growing, mature road-side trees have the potential to extend across sealed surfaces, keeping even these cool. These trees therefore should have a much greater cooling effect on the local micro-climate than similar sized areas of lawn.

The City of Huntington started cutting down many large road-side trees in 2006. Approximately 800 trees were tagged for removal. In some cases half a city block was cleared at once. A pilot study was conducted, using Landsat Thematic Mapper thermal data to measure the effect of such clear-cuts on local temperatures. The images are too coarse to identify individual city blocks or trees. However, an earlier study showed clearly that even the coarse resolution allowed for the detection of hotspots that are caused by as little as a quarter of a city block of sealed surfaces (e.g. schools) in otherwise vegetated residential neighborhoods. A single block of high vegetation cover in the middle of an otherwise highly sealed area, such as the Court House block in downtown Huntington or Marshall University campus, caused a clear depression in surface temperatures. It was the goal of this study to test if the thermal infrared information can still be used to detect surface temperature changes caused by local removal of individual or small groups of large trees.

For this purpose, areas that were known to have been cleared of trees between 2005 and 2008 were identified and digitized. Randomly selected areas for which it was unknown if vegetation changes occurred, were used as controls. Surface temperature estimates for control and cleared areas were extracted from the Thematic Mapper thermal infrared channel of images that were acquired during heat waves in July 2005 (before tree cutting) and in July 2008 (after cutting). Paired t-tests were conducted to compare average temperature changes between cut and control areas. Initial results were promising: despite the coarse nature of the thermal data set, temperatures in cleared areas rose significantly (p < 0.05) compared to those in control areas.

Results of this study support a transition to a more park-like design of urban centers wherever possible. Trees promise a particular benefit if they are planted strategically to shade sealed surfaces. Another advantage of trees is that their footprint is small on the ground, where human activity occurs. They instead they spread wide above the zone that is typically used, intercepting radiation before it heats the ground.