Local decision-makers looking for ways to reduce the impact of heat waves on their communities have a valuable new capability at their disposal: a new study on vegetation resilience. 

Scientists at the Department of Energy’s Oak Ridge National Laboratory completed a study of how well vegetation survived extreme heat events in both urban and rural communities across the country in recent years. The analysis informs pathways for climate mitigation, including ways to reduce the effect of urban heat islands.

Vegetation such as trees provide a valuable cooling effect, shading surfaces and deflecting solar radiation while releasing moisture into the atmosphere through evapotranspiration — the process in which plants absorb water through their roots and release it as water vapor through their leaves. 

The study, published in the journal PNAS Nexus, is the first nationwide accounting of vegetation resilience that takes into account the influence of human-built infrastructure. Using machine learning methods, ORNL researchers examined about two decades’ worth of satellite and other data covering 85 large cities and surrounding rural areas. The team found that impervious surfaces such as roads and other infrastructure, moisture conditions and type of land cover affect vegetation resilience. They also evaluated how vegetation is impacted by the intensity, duration and timing of heat waves.

The data provide crucial insights into how ecosystems can be guarded against climate change, including pathways to counteract the influence of urban heat islands and to improve the stewardship of natural resource areas, said Jiafu Mao, ORNL Earth system modeling scientist and the project’s lead.

“The empirical evidence we provide from this research can help urban planners better understand which plants are more vulnerable to heat waves and stressors such as water availability in the local environment, guiding decisions about plant selection and location and urban design improvements,” Mao said. “The study suggests that preserving and enhancing vegetation could significantly contribute to urban sustainability, air quality improvements and the well-being of residents.”

The work extends ORNL’s research on climate impacts in urban and rural ecosystems. In a previous study, Mao and colleagues found that while all regions of the country can expect an earlier start to the growing season as temperatures rise, the trend is likely to become more variable year over year in hotter regions. The research found a trend of accelerated spring budding and blooming of plants in rural areas as temperatures rise, for instance, but suggested the trend will slow as warming continues.

“Our analysis is the first large-scale quantification of urban and rural differences in vegetation and its resilience to extreme events across the contiguous U.S., capturing these very broad patterns on environmental change,” Wang said. Future investigations that capture more high-quality data would benefit both urban planners and ecosystem modelers, she added.