Hydrological response of an upland oak/pine forest on the Atlantic Coastal Plain to drought and disturbance

Background/Question/Methods

We investigated the hydrologic balance of an upland oak/pine forest on the Atlantic Coastal Plain. During 2005-2011, sap flux measurements were used to quantify transpiration and eddy covariance measurements were used to estimate evapotranspiration. The forest experienced a defoliation by Gypsy moth (Lymantria dispar L.) in 2007, a partial defoliation in 2008 and severe drought conditions in 2010.

Results/Conclusions

From 2005-2011, annual canopy tree transpiration (E_C) averaged 175 mm a^-1 ± 44 mm a^-1. In this forest, E_C was reduced by 10% in 2007 compared to 2005 but surprisingly similar to the months of July in 2006-2009. A severe drought in 2010, however, reduced stand transpiration by 25% in the month of July compared to the seven-year mean for July. Therefore, prolonged drought had a seemingly stronger effect on E_C than reduced foliage or episodic defoliation suggesting compensation of the remaining foliage following defoliation. Alternatively, this stand may also experience higher sensitivity to drought because of the history of prior defoliation. Evapotranspiration measured with the eddy covariance system was also reduced during defoliation (92 mm in 2007 vs. 193 mm in 2006 from June 1^st to July 15^th), and sensitive to drought conditions.

            Stream water discharge scaled to the watershed area showed greatest correlation with transpiration in a 24 day time lag and a 219 day time lag signifying hydrological connectivity on the watershed scale.  In other words, on a biannual time scale, thus seasonally the stream and transpiration undergo similar fluctuation coinciding with leaf on and leaf off condition accounting for the capacity of the system.

Under future climate change, frequency and intensity of precipitation may have a larger influence on local water balance components than disturbance effects in this upland oak/pine forest.