Some Results
A few results from the Kyloe Wood Experiment
Wind profiles
The sectorwise mean wind profiles for the two masts are shown in Figure below. Due to the poor resolution of the cup anemometers in low wind speed the analysis was restricted to time periods in which the hourly mean wind speed at 30.8m exceeded 3 ms-1. The shelter the site receives from easterly winds means that the data pool for these wind directions is very small.
The wind speeds in the trunk space at the two locations are very low. In direct comparison, the wind speeds at Mast-I are higher than they are at Mast-II. The second wind speed maximum in the trunk space is also more pronounced at Mast-I. For the Mast-II a second wind speed maximum is only recognised for the sectors W to NNW.
The inflection point of the wind profile - defined as the height of maximum shear - is higher at Mast-I. This indicates that the wind in-forces more drag on the canopy in higher parts. Since the turning moment is calculated as the integral of force over height this should result in higher turning moments.
Momentum absorption
For this experiment two sonic anemometers were available. Both sonics were mounted onto Mast-II at 29.8m and 16.0m. The surrounding trees were about 23.5m tall. Hence the upper sonic was located several meters above the canopy (1.27 z/hC). The lower sonic was located within the canopy (0.68 z/hC), a few meters above the average crown base at 13.2m (0.56 z/hC) and also some meters above the understorey.
The difference in momentum flux at the two sonic heights is a measure of the amount of momentum which has been absorbed over the height between the two sonics and transferred into tree motion. For comparison the difference is scaled by the momentum flux at the upper sonic height, which represents the atmospheric stress above the forest.The relative momentum absorption (RMA) is calculated as:
where u' and w' are the instantaneous deviations of the horizontal and vertical wind component and the overline indicates an average over a time period (here:~8192/10~seconds).
If the measured momentum fluxes at the two heights were equal no momentum was absorbed between the two heights and the value for RMA would approach zero. In contrast, if the momentum flux at the lower sonic height was to be zero, all momentum would have been absorbed by the canopy above the lower sonic height and the RMA becomes unity.
In the Figure below the relative momentum absorption is plotted versus wind direction. The pattern of the data points suggest that less momentum is absorbed when the wind comes from southerly to westerly direction, which coincides with the area where there is an understorey. Values greater than unity indicate an upward momentum flux at the lower sonic height. Since the vertical momentum flux is caused by the shear stress of air layers, an upward momentum flux indicates an inversion of the normal wind profile in the trunk space of the forest. Hence a second wind speed maximum has to be present in the trunk space to cause an upward momentum flux. The occurrence of a second wind speed maximum is also recognisable in some of the wind profiles.
The drag of the plant canopy is a function of the frontal crown area. Before bud burst the crown area of the canopy is reduced. Hence the data points for this time period (red crosses) do line up at the lower margin of the data pool.
Turning Moments
All of the nine experimental trees were planted in the same year. The height of the trees 102, 104, 106, 108 and 109 are very similar (upper value in the table), so that it make sense to compare those trees to see if there is a difference in the turning moment.
The scatterplots below suggest that the tree without an understorey (102) experiences a higher turning moment than the trees 104, 106, and 108. The scatter plot for the values for tree 102 and 109 is very close to the 1:1 line (slope: 1.10 and 1.02). The red lines in the plots represent a best linear fit.
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