How does drought change the way a forest divides energy between evaporation and heating the air?

Details

The study examined what controls variation in how surface energy is divided between latent and sensible heat fluxes at a temperate deciduous forest site in central Missouri, USA.

This observational field study used multiple micrometeorological and ecophysiological measurements, taking advantage of a prolonged drought in the middle of the 2005 growing season. Researchers examined the effects of soil moisture, atmospheric vapor pressure deficit (VPD), and net radiation on surface energy partitioning. They stratified these factors to minimize potential confounding from correlations among them.

All three factors had direct effects on energy partitioning and, more importantly, crucial indirect effects as well.

For soil moisture, the direct effect depended on whether the soil was dry or wet: the Bowen ratio fell rapidly as soil moisture increased in dry soil, but was insensitive to changes in soil moisture in wet soil. However, atmospheric conditions determined both how rapidly the Bowen ratio fell in dry soil and the soil moisture level above which it became insensitive to further moisture changes.

Increased net radiation directly increased the Bowen ratio. The direct effect of VPD was strongly nonlinear: increasing VPD decreased the Bowen ratio when VPD was low, but increased it when VPD was high.

The indirect effects were more complicated. Reduced soil moisture weakened VPD's influence on surface energy partitioning but strengthened the influence of net radiation. Soil moisture also controlled how net radiation influenced the relationship between energy partitioning and VPD, and how VPD influenced the relationship between energy partitioning and net radiation.

Both increased VPD and increased net radiation made the Bowen ratio more sensitive to changes in soil moisture and strengthened drought's effect on surface energy partitioning.

The identified direct and indirect effects of atmospheric conditions and soil moisture provide a target for testing how atmospheric general circulation models represent land–atmosphere coupling.