The lives of more than two billion people are affected by drought and could benefit from monitoring plant stress in real time. The extensive data collection and in-situ calibration that would be needed for continental scale estimates are difficult to implement in ways that could provide assessments of current conditions.
We used a simple relationship between the atmosphere and plants to extract real-time plant stress. Ordinarily, plant canopies are cooled by leaf water loss through stomata. These small pores in leaf surfaces lose water as they admit CO2 from the atmosphere. This evaporative cooling limits the canopy-atmosphere temperature differential, explaining why summertime mid-day temperatures within forest canopies are cooler than in cities. This cooling mechanism is driven by water uptake from the soil and remains effective until plants are moisture-limited, leading to stomatal closure, and surface heating. This simple relationship led us to speculate that the canopy-atmosphere differential, or “thermal stress”, could provide a real-time index of moisture limitation and of plant water consumption in warm seasons.
Combining the measurements from thousands of land-based weather stations and satellite-based ‘skin temperature’ of the canopy, Seyednasrollah et al. found thermal stress to be a reliable indicator of drought-induced water stress at the continental scale. Comparisons with land-based measurements showed that a 1 °C change in thermal stress is comparable to 1 to 1.2 mm of daily evapotranspiration, depending on site and climate conditions. Read more here.