A Method to Study Response of Large Trees to Different Amounts of Available Soil Water 1

Introduction 3TRV \0 A method was developed to manipulate available soil water on large trees by intercepting thrufall with gutters placed under tree canopies and irrigating the intercepted thrufall onto other trees. With this design, trees were exposed for 2 years to either 25 percent less thrufall, normal thrufall, or 25 percent additional thrufall. Undercanopy construction in these plots moderately affected patterns of thrufall entry onto the forest floor when compared to a plot without construction. In this pilot study, a variety of aboveground and belowground measurements were recorded from trees over the 2-year period. Additional amounts of thrufall increased circumference growth, litter fall, fine-root biomass, ectomycorrhizal development, and number of fruiting bodies of ectomycorrhizal fungi. Intercepted tlirufall decreased amounts of ergosterol, a surrogate for soil and root-origin fungal biomass. Recommendations are made to improve and simplify the design, including arranging gutters to intercept more or less thrufall at any time of year.