Relationship between treefall direction, slope-aspect, and wind in eight old-growth oak stands in the Central Hardwood Forest, USA

This study examined the relationship between direction of treefall, slope-aspect and prevailing wind in five old-growth stands where single-tree canopy gaps characterize the dominant disturbance regime. All live and downed trees were inventoried in 0.45-ha sample plots, and crown sizes of live trees were measured along two perpendicular axes. Directions of fall and slope-aspect of downed trees also were recorded. Regional prevailing winds and wind gusts were obtained from four nearby airports. I used circular statistics to determine if directions of treefall for each study stand had a mean direction or if the directions of fall were uniformly distributed. If directions of treefall had a true mean, they were then compared to mean slope aspect and prevailing wind directions. At two of eight plots, treefall directions were uniformly distributed (i.e., no mean direction). Only one plot showed a statistically significant similarity between mean slope-aspect and mean direction of fall, and only three of the eight study plots showed statistically significant similarities between mean treefall and wind directions. Mean crown asymmetry (ratio of long and short diameters) was 1.26, and there were no significant differences in asymmetry values when current gap border trees and non border trees were compared. While trees may fall downhill and downwind, the high variation in treefall and wind directions precluded establishing a consistent statistical relationship between these data sets. I suggest that crown asymmetry, resulting from differential crown growth of trees on sloped-hillsides and within canopy gaps, exerts a strong additional influence on direction of fall.