OBSERVATIONS OF TURBULENT AIR-FLOW IN 3 STANDS OF WIDELY SPACED SITKASPRUCE

We examined air flow and turbulence in three stands of Sitka spruce (P icea sitchensis (Bong.) Carr.) at Cloich forest, near Edinburgh. The t rees were about 8 m tall and spaced at intervals of 8, 6 and 4 m. Obse rvations were made using a vertical array of six three-component prope ller anemometers positioned at heights from about 0.25h to 1.25h, wher e h is mean tree height. The horizontal spatial variation in turbulenc e statistics was large, being somewhere between 20% and 90%, depending on tree density. Mean profiles of turbulence statistics reflected the influence of foliage density; turbulence intensity, skewness and kurt osis values each increased with depth into the canopy, and increased w ith increasing tree density. Tree spacing played a major role in modif ying canopy turbulence. As tree spacing was increased, ventilation rat es and turbulent exchange were enhanced and momentum penetrated deeper into the canopy. Mean wind speed in the trunk space as a percentage o f the mean wind speed measurement at the same height (2 m) over open p asture, was 46%, 29% and 16%, in the 8 m, 6 m and 4 m spacings, respec tively. The corresponding low-level (z = h) canopy drag coefficients, as defined by C(d) = (U/U)2, decreased with increasing tree spacing, from 0.12 in the 4 m spacing to 0.06 in the 8 m spacing. The mean leve l of momentum absorption also decreased with increasing tree spacing, from about 0.75h in the 4 m spacing to about 0.61h in the 8 m spacing. Canopy turbulence was dominated by large-scale, intermittent events. Conditional sampling of the shear stress revealed that turbulence in t hese sparse tree canopies was less dominated by extreme and occasional events than has been found in similar studies conducted in denser for est stands.