Description and field performance of the Walker Branch throughfall displacement experiment: 1993--1996

Abstract

The authors are conducting a large-scale manipulative field experiment in an upland oak forest on the Walker Branch Watershed in eastern Tennessee to identify important ecosystem responses that might result from future precipitation changes. The manipulation of soil water content is being implemented by a gravity-driven transfer of throughfall from one 6400-m{sup 2} treatment plot to another. Throughfall is intercepted in {approx}1850 subcanopy troughs suspended above the forest floor of the dry plot and transferred by gravity flow across an ambient plot for subsequent distribution onto the wet treatment plot. Soil water content is being monitored at two depths with time domain reflectometers at 310 sampling locations across the site. The experimental system is able to produce statistically significant differences in soil water content in years having both dry and wet conditions. Maximum soil water content differentials between wet and dry plots in the 0- to 0.35-m horizon were 8 to 10% during summers with abundant precipitation and 3 to 5% during drought periods. Treatment impacts on soil water potential were restricted to the surface soil layer. Comparisons of pre- and post-installation soil and litter temperature measurements showed the ability of the experimental design to produce changes in soil water content and water potential without creating large artifacts in the forest understory environment.