Effect of Tree Shade on State Highway 34 Winter Maintenance Operations

Project summary:

For many years, it has been recognized that temperature is an important parameter in determining the environmental impact of pavements. Compared to natural (vegetated) land cover, pavement warmed by solar radiation introduces additional heat to the lower atmosphere, increases rainfall runoff from the pavement, and reduces infiltration to shallow groundwater beneath the pavement. Additional heating of the lower atmosphere by pavement leads to the heat island effect, which reduces urban air quality and increases air conditioning loads. Heated rainfall runoff from pavement and heating of shallow groundwater leads to thermal pollution of receiving water bodies, especially cold-water trout streams.

In winter, pavement temperatures determine the need for application of deicing agents, mostly road salt in cold climate regions. In northern regions, road salt application is increasing chloride levels to unacceptable values in lakes, streams, and groundwater aquifers; therefore, the temperature characteristics of pavement in winter has to be considered in determining environmental impacts.

While heat island effect has received considerable attention, research on pavement temperatures in winter has been less studied, in particular the effect of tree shade during very cold winter months.

Minnesota Department of Transportation (MnDOT) District 4 engineers are interested in evaluating this effect to better understand the balance between optimal sun exposure and tree-cutting activities as part of a new road project. The project involves paving from Becker County Road 29 to just west of Osage on State Highway 34 in summer of 2023. This winter, MnDOT will cut trees on both sides of the road back to 65 feet. In winter 2023-24, they plan to do a 50 percent tree cutting on the easterly seven miles of the project (Snellman to Shell River), on the south side only, roughly from 65 feet to 100 feet from the edge of the roadway.

There has been public interest in limiting any tree cutting on the corridor as much as possible to preserve the trees. MnDOT was interested in cutting the shade trees along the corridor to allow maximum sun penetration on the roadway. This helps melt off snow and ice, which in turn reduces staff/equipment time and material application of sand and chlorides. However, as a result of public input, the planned tree cutting was reduced, and this project will provide MnDOT with supporting information and analyses to determine if more cutting will be needed after the planned work is completed.