WATER-USE IN THINNED LOBLOLLY-PINE PLANTATIONS

Soil moisture and throughfall were monitored for two growing seasons under three thinning intensities: basal areas of 7.8 m2 ha-1, 12.6 m2 ha-1 and 26.6 m2 ha-1 (unthinned control) in an 11- to 12-year-old loblolly pine (Pinus taeda L.) plantation. The first year after thinning soil water decreased at a rate of 0.8 mm day-1, 1.0 mm day-1 and 1.4 mm day-1, respectively, in the 7.8 m2 ha-1, 12.6 m2 ha-1, and 26.6 m2 ha-1 basal area plots between May and late August. The more rapid development of soil moisture deficits in the unthinned stand was owing to both greater soil moisture use and reduced throughfall. For the same time period in the second year, with below-normal rainfall, soil moisture deficits developed at a rate of 1.5, 1.5 and 1.7 mm day-1 for the two heavily thinned and the unthinned treatments. Estimated growing season soil water use rates were 4.4 mm day-1, 4.2 mm day-1, and 4.0 mm day-1, respectively for the 7.8 m2 ha-1, 12.6 m2 ha-1 and 26.6 m2 ha-1 basal area levels in Year 1. The second year after thinning water use rates for the same period were 2.7 mm day-1, 2.6 mm day-1, and 2.5 mm day-1, respectively. The results of this study suggest that the capacity to manage available water in loblolly pine stands by thinning is more a function of reduced interception loss and increased throughfall than reduced water use and is also weather dependent. Growing seasons with low rainfall or only one or two large rainfall events will not permit much opportunity to manage soil moisture.