Assessing the biogeophysical cooling potential of constructed prairie with eddy covariance & thermal UAS remote sensing

Anthropogenic activity has altered the Earth system in numerous and complex ways, including through climate warming, extensive land-use change, biodiversity loss, and alternation of nitrogen and phosphorous cycles. Warming caused by human activity is also contributing to record-breaking heatwaves and droughts in the Midwestern United States. To mitigate these effects, prairie restoration can potentially sequester carbon, increase biodiversity, and prevent excess fertilizers from running off to streams and rivers. However, small-scale prairies as urban green spaces have not been explored for their cooling potential during the Midwestern summer. To quantify the cooling potential of constructed prairies and the mechanisms underlying it, we combined half-hourly eddy covariance flux measurements, which track the exchange of water and energy between the surface and atmosphere, with monthly unpiloted aerial vehicle (UAS) surface temperature and height imagery over a constructed prairie and an adjacent non-prairie in Iowa City, Iowa, USA.

The UAS temperature imagery captured consistently lower surface temperatures in the constructed prairie than in the non-prairie area, with peak differences occurring near midday. Providing an explanation for this temperature difference, the sensible heat fluxes, or transfer of heat which results in temperature change, were generally higher in the non-prairie area, while latent heat fluxes, or transfer of heat which causes a phase change of water such as evaporation, and evaporative fraction (ratio of evaporation to total energy exchange) were higher in the constructed prairie. Monthly height imagery captured the prairie’s consistently rougher canopy surface and increasing seasonal rugosity (roughness). The biodiversity survey shows that greater species richness in the prairie produced this greater canopy surface rugosity through a more structurally complex plant community. The increase in canopy structural complexity resulting from greater species richness led to a more turbulent transfer of energy, as indicated by evaporative fraction and latent heat fluxes, resulting in evaporative cooling and lower surface temperatures in the constructed prairie. Here, we show that a biodiverse and structurally complex constructed prairie can reduce local air and surface temperatures at a greater magnitude than a less complex and diverse vegetation present in a non-prairie area. In urban and suburban contexts, small-scale prairie restoration may be an effective, nature-based solution to mitigating warming caused by human activity within the Midwestern United States.