A strained relationship: the effects of pruning treatments on wind-induced bending moments of Colorado blue spruce (Picea pungens)

Abstract

Wind and trees often have a strained relationship; and severe wind events can lead to tree loss, personal injury, and costly litigation. Pruning is a common practice used by arborists to mitigate the risk of tree failure by selectively removing parts of a tree crown exposed to the wind, but there are few existing studies examining changes in wind loads after pruning, especially on large, open-grown, evergreen trees. The objective of this research was to identify the effects of pruning treatments on wind-induced bending moments on Colorado blue spruce (P. pungens). Ten, open-grown spruce at the Colorado State Forest Service Nursery (CSFS) in Fort Collins, Colorado were monitored before and after a series of pruning treatments. Trees were pruned to raise or thin crowns over a range of severities between 0% and 40%. Axial trunk deformation was measured using two displacement probes installed orthogonally on each tree, and each displacement probe was calibrated using a static pull test to convert trunk deformations to bending moments. Wind conditions and trunk bending moments were simultaneously monitored and continuously recorded from July 2023 through January 2024. At a given wind speed, mean wind loads did not differ between pruning treatments, but pruning severity significantly affected wind loads. Mean separation analyses were conducted at wind speeds of 10 m∙s-1, 12.5 m∙s-1, and 15 m∙s-1, representing the upper range of 30-minute maximum winds during all severity periods. Compared to the other severities, the average 30-minute maximum wind-induced bending moments decreased significantly after deadwood removal at the higher wind speeds (12.5 and 15 m∙s-1). At 10 m∙s-1, bending moments were significantly lower after deadwood removal compared to the 20% severity period, but no differences were observed among other severity levels. These findings suggest that arborists should carefully consider the amount of material removed from large, open-grown evergreens to reduce wind loads and mitigate tree failure risks. Further research is needed to assess the long-term impact of pruning on tree health and whether the mechanical benefits align with aesthetic expectations in urban greenspaces.