Comparing forest and grassland drought responses inferred from eddy covariance and Earth observation

Temperate forests and grasslands have different drought response strategies. Trees often control their stomatal opening to reduce water loss to prevent hydraulic failure and ensure the sustainable above-ground biomass production. In contrast, grasses generally have a less strong stomatal control and maintain high photosynthesis and transpiration until the soil moisture gets depleted. That is when their leaves wilt and the grasslands reduce their aboveground green biomass. Both the increased stomatal control and the reduction in aboveground biomass decrease the canopy-surface conductance and decrease the exchange of water and carbon between the leaves and the atmosphere. Here, we study to which extent remote sensing data reflect the drought-induced reduction in canopy-surface conductance for forests and grasslands. We use eddy covariance observations over 63 sites across the northern hemisphere to infer the conductance. We identify severe droughts from low soil moisture content and reduced canopy-surface conductance. We further analysed how the drought response is reflected in thermal and optical data derived from MODIS satellite data. The results show that the land surface temperature increases with drought-induced reductions in canopy-surface conductance for both forests and grasslands. By contrast, the optical indices (e.g., the normalized difference vegetation index) show a much stronger response for grasslands as compared to the forests. We conclude that the different canopy-level drought response strategies of trees and grasses are widespread and that these different responses are reflected in remote sensing data. Hence, a combination of thermal and optical satellite data should be used to monitor and study vegetation drought responses of forests and grasslands to ensure accurate inference on the implications on water, energy, and carbon fluxes.