Large trees as a barrier between solar radiation and sealed surfaces: their capacity to ameliorate urban heat if planted strategically to shade pavement

The urban heat island is in part caused by sun-exposed pavement absorbing radiation and releasing it overnight. This keeps the neighbourhoods warmer than if the surface was vegetated or shaded. Mature large-growing road-side trees overlap streets to a degree where a large proportion of the pavement is shaded, intercepting radiation before it reaches sealed surfaces. Existing literature recognizes the cooling effect of trees in urban areas. No reports exist on the specific influence of road-side trees.

Land cover in cities is generally highly fragmented with small patches of vegetation interspersed with buildings, roads, parking lots, and other features. Coarse scale satellite information does not pick up these details. In this study high resolution satellite images were combined with coarser thermal infrared data to examine the influence of vegetation on temperatures at a local scale in Huntington, West Virginia.

Vegetation was mapped using high resolution data. The difference in spatial distribution of high fractional vegetation cover between spring images (green lawns but bare canopies) and summer images (canopy fully developed) indicates the location of tree canopies over sealed surfaces. This method of determining tree cover over roads simplifies canopy mapping, and large areas in cities can be processed. A preliminary study of pavement temperatures during a hot spell in July 2005 revealed that before sunrise on mornings after hot summer days the surface temperatures in exposed locations were still two degrees warmer than in shaded sites. However, these were only point measurements. In order to examine larger areas across the city, thermal information was extracted from Landsat Thematic Mapper images during that same time period, and converted to degrees Celsius. Vegetation proportions were aggregated to the same pixel size as the temperature data and regressed against the latter. Not surprisingly a highly significant inverse relationship exists between vegetation proportion and surface temperature.

Trees have the capacity to extend vegetation cover over areas where there would otherwise be pavement. Even a single block of high vegetation cover in the middle of an otherwise highly sealed downtown area, such as the City Hall block in Huntington, caused a clear depression in surface temperatures at the available satellite resolutions. Large buildings and parking lots in otherwise vegetated residential neighbourhoods on the other hand form clearly detectable hotspots.

Mostly due to budget problems, the City of Huntington is currently in the process of removing all large growing trees along the city's right of way. Results of this study provide strong arguments against this practice. The technique of combining high resolution satellite data with relatively coarse thermal information to identify cool and hot spots in cities could be adapted by city planners in order to ameliorate the environmental quality in cities, and to identify gaps in urban forests.