Drought Influence on a Missouri Oak-Hickory Forest Ecosystem

A severe drought was observed in the mid-summer 2005 at the Missouri MOFLUX oak-hickory forest site. It largely covered the entire month of July and the early August. The monthly mean temperature in July measured at the site was 26.2 °C, 1 °C higher than the climatic mean between 1970 and 2000 and almost 3 °C higher than 2004. The cumulative precipitation in July was only about 19 mm, 77 mm less than the climatic mean (1971-2000) and 118 mm less than 2004. Mean daytime vapor pressure deficit measured at each 30-minute period was 1.97 KPa in July 2005, about 0.44 KPa higher than July 2004, and its daily peak often exceeded 2.5 to 3.0 KPa during the drought. At the site, fluxes of CO2, latent heat and sensible heat are routinely measured by an eddy covariance system above the canopy. As volumetric soil water content between 0.6 and 1.2 m declined from 0.4 at the beginning of the drought to 0.32 at the end of drought, daily maximum CO2 uptake dropped from 30 μmol m-2 s-1 to less than 10 μmol m-2 s-1. Concurrently, daily maximum latent heat flux dropped from 500 W m-2 or more to 200 W m-2 or less and daily maximum sensible heat flux rose up from about 200 W m-2 to 500 W m-2. Mean predawn leaf water potential of trees decreased from -0.3 MPa just prior to the drought to -2.5 MPa at peak drought. Soil respiration measured by 8 automated chambers also decreased dramatically, and the 30-minute mean was reduced from more than 5 μmol m-2 s-1 to about 2 μmol m-2 s-1. As a result of stomatal closure in the afternoon under water stress and high vapor pressure deficit, the daily peak of CO2 uptake was shifted from noontime to the morning hours. Light saturation occurred at about 500 μmol m-2 s-1 in the drought season, which was only half of that in the pre-drought period of 2005 and in the growing season 2004. It is clear that water stress was the primary driving force for the drought. However, we also found evidence indicating that high vapor pressure deficit added to the drought effect, as CO2 uptake showed a transient recovery under relatively low water vapor deficit in cloudy days.