Atmospheric Dispersion from a Point Source in Four Southern Pine Thinning Scenarios: Basic Relationships and Case Studies

Abstract

An atmospheric tracer experiment using SF6 was designed to assess changes in the dispersive environment in the trunk space of a southern pine forest through four thinning regimes. Over 6000 mean half-hourly tracer samples were collected and analyzed along with a high-frequency time series of tracer concentration sampling at 1 Hz over the course of the trials. The experimental plot was thinned in four stages, from a state of dense boles and thick understory, >32.1 m2 ha-1 (140 ft2 ac-1) basal area, to a final basal area of 16.1 m2 ha-1 (70 ft2 ac-1) with understory removed. In this study, plume dilution in the unthinned stand followed previously observed relationships; however, thinned stands deviated from expectations. The in-canopy atmosphere was largely coupled with the above-canopy atmosphere once a basal area of 23.0 m2 ha-1 (100 ft2 ac-1) or lower was attained, so additional tree removal had less effect in changing the in-canopy environment. Dispersion exhibited surprisingly strong, near-linear relationships with stand density.