Appendix D. Implications of wind speed and direction for white spruce seed dispersal.
Data on wind speed and direction (dataset number BNZD00048:41) were obtained from the Bonanza Creek LTER database (http://www.lter.uaf.edu/). Half-hourly data on windspeed and direction were available from 1998–2004. We only used the data from the anemometer at 10 m height (not at 3 m) because this was more representative for the height of seed abscission and data are analysed separately for the potential period of seed abscission (September through April1 ). Mean wind direction θ was calculated with circular statistics (Batschelet 1981)2 using the mean sine s‾ and cosine c‾ of the wind directions according to
with
The standard deviation was calculated according to the algorithm suggested by Yamartino (1984):
with
and all angles are expressed in radians. The prevailing wind direction during the abscission period from September to April was northeasterly (68.2° ± 79.5°) and the inter-annual variability in wind direction was very small with coefficient of variation of 7.3 % (Table D1). From May–August the mean wind direction was almost the opposite direction, i. e., westerly winds (285.5° ± 91.0°). The nearest white spruce seed sources in the direction of the northeastly winds that characterized the abscission period are about 2–5 km from our transects (Fig. 1). Because very few seeds travel such distances, seeds from sources other than the southern ‘peninsula’ where our transects begin is unlikely.
FIG. D1. Histogram of hourly wind direction data for 1998–2004 during the seed abscission period (September–April) and outside this period (May–August). |
FIG. D2. Mean monthly wind directions (1 = January, …, 12 = December) where the vertical represents north. The main abscission period is from September to May, where north-easterly wind directions prevail. |
TABLE D1. Mean wind directions for six consecutive abscission periods (September to May) from 1998 to 2004.
Abscission period (Sep–May) Mean wind direction (mean ± SD) 98/99 73.1 ± 83.8° 99/00 65.7 ± 85.1° 00/01 62.9 ± 79.7° 01/02 75.6 ± 82.5° 02/03 66.2 ± 83.7° 03/04 65.6 ± 82.6°
LITERATURE CITED
Dobbs, R. C. 1976. White spruce seed dispersal in Central British-Columbia. Forestry Chronicle 52:225–228.
Batschelet, E. 1981. Circular statistics in biology, Academic Press, London, UK.
Yamartino, R. J. 1984. A comparison of several ‘single pass’ estimators of the standard deviation of wind direction. Journal of Climate and Applied Meteorology 23:1362–1366.