The Limiting Effect of Deep Soil Water on Evapotranspiration of a Subtropical Coniferous Plantation Subjected to Seasonal Drought

Seasonal drought is a common occurrence in humid climates. The year 2003 was the driest year during the period 1985-2011 in southeastern China. The objective of this study was to elucidate the impact of the exceptional drought in 2003, compared with eddy flux measurements during 2004-11, on the dynamics of evapotranspiration (ET) and related factors, as well as their underlying mechanisms, in a subtropical coniferous plantation in southeastern China. It was found that daily ET decreased from 5.34 to 1.84 mm during the intensive drought period and recovered to 4.80 mm during the subsquent recovering drought period. Path analysis indicated that ET was mainly determined by canopy conductance and deep soil water content (50 cm) during the intensive drought and recovering drought periods, respectively. The canopy conductance offset the positive effect of air vapor pressure deficit on ET when suffering drought stress, while the canopy conductance enhanced the positive effect of air temperature on ET during the late growing season. Because the fine roots of this plantation are mainly distributed in shallow soil, and the soil water in the upper 40 cm did not satisfy the demand for ET, stomatal closure and defoliation were evident as physiological responses to drought stress.