Tara Strand

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

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.

Mass-Balance of Pheromone Surrogate Plumes in the Canopy Trunk Space

A large, multi-year study has been completed to evaluate the dispersion of insect pheromone in forest canopy trunk spaces. The study was undertaken to guide operational pest managers in the placement of pheromone sources in forest canopies. A very dense network of tracer sampling devices was deployed and over 200 average (half-hourly) plumes consisting of over 13000 chemical samples are available for analysis. This spatially dense near-field array provides an important opportunity to conduct a mass balance exercise. The subset of tests analyzed here indicates that mass recovery in this situation is possible within a factor of two and often close to 100%, though even in this dense sampler field there is substantial uncertainty and a tendency to over recover (>100%).

Diffusion of Insect Pheromones in a Forest Canopy: Co-Located Tracer/Electroantennogram Experiments

Sulfur hexafluoride (SF6) is a tracer gas that has been widely used to simulate behavior of air pollutants in the atmosphere. In this study, however, tracer experiments were conducted to study diffusion of insect pheromone through a forest canopy. Experiments were performed during morning and afternoon hours in August of 2003 at a forested field site near Pellston, Michigan. An SF6 source was collocated with a gypsy moth pheromone source, and concentration data for both gases were collected downwind with near-instantaneous equipment at a source-to-receptor distance of 5 m. In addition, a sonic anemometer was used to measure turbulent changes in horizontal and vertical winds throughout the study. The results illustrate the use of SF6 as a surrogate for insect pheromone, and to evaluate the performance of a new, portable, electroantennogram (EAG) device.

Near-Field Trunk Space Dispersion

A near-field tracer study was conducted to study the movement and dispersion of gas in the lower forest canopy in an attempt to improve guidance for forest managers deploying anti-aggregation pheromone sources to protect high value forest stands. Data are shown from three forest canopies and include over 13000 chemical tracer samples compiled into half hour dispersion fields around a point source. A high frequency sampler was also deployed to ascertain the structure of the gas plumes at 1 Hz. The plumes showed strong Gaussian tendencies in many cases and very high peak-to-mean ratios. Average maximum ÷/Q values were relatively consistent over the canopies studied though high variance in the maximum ÷/Q values was observed.