Use of ecosystem flux data and a simulation model to examine seasonal drought effects on a subtropical coniferous forest

The Qianyanzhou (QYZ) forest site is located in the mid-subtropical region under typical subtropical continental monsoon climate. Despite its plentiful water and heat resources, changes in their seasonal distributions result in frequent occurrences of seasonal drought, influencing both the structure and function of forest ecosystems. In order to examine the effects of drought stress on energy and water fluxes and the related physiological processes, we used tower flux measurements from 2003 and 2004 from the QYZ site and the ecosystemmodel EALCO (Ecological Assimilation of Land and Climate Observations). The model explained almost 90%of the variance in the daily LE for both years; whereas 71%and 55% of the variance in H for 2003 and 2004, respectively. Simulations suggested that, during the 2003 summer drought, about 80% of water extraction by roots occurred below 0.45 m, and canopy stomatal conductance decreased in response to soil dryness at a threshold Θr (relative water content) of 0.5 m. The response of canopy stomatal conductance appeared to be linked to the soil water exposed to themajority of the roots, even if substantialwater was being extracted fromgreater depth. The decrease of canopy stomatal conductance made the ratio of transpiration to ET relatively constant along the years with only a small difference. The different extent of the seasonal drought resulted in different water fluxes for the two years with more distinct influence on evaporation than on transpiration.