Wind and Snow Loading of Balsam Fir during a Canadian Winter: A Pioneer Study

Marine Duperat,Barry Gardiner,Jean-Claude Ruel

doi:10.3390/f11101089

Abstract

Widely distributed across Quebec, balsam fir (Abies balsamea (L.) Mill) is highly vulnerable to wind damage. The harsh winter conditions, freezing temperatures, and snow pose an additional risk. It is important to find the mechanical loads experienced by trees during winter to adapt forest management and minimize the risk of damage to this species. Many studies have been carried out on wind and snow loading damage risks in Northern Europe, mostly based on post-storm damage inventories. However, no study has continuously monitored the applied turning moment during a period with snow loading, and no study has investigated wind and snow loading on balsam fir. Therefore, our main objective was to conduct a pioneering study to see how trees bend under wind loading during winter, and to see how snow cover on the canopy contributes to the loading. Two anemometers placed at canopy height and 2/3 canopy height, air and soil temperature sensors, a hunting camera, and strain gauges attached to the trunks of fifteen balsam fir trees, allowed us to measure the wind and snow induced bending moments experienced by the trees together with the meteorological conditions. Data were recorded at a frequency of 5 Hz for more than 2000 h during summer 2018 and winter 2019. Two mixed linear models were used to determine which tree and stand parameters influence the turning moment on the trees and evaluate the effect of winter. The selected model for measurements made during winter found that including the snow thickness on crowns was better than those models that did not consider the effect of snow (ΔAICc > 25), but the effect of snow depth on the bending moment appears to be minor. However, overall, the turning moment experienced by trees during winter was found to be higher than the turning moment experienced at the same wind speed in summer. This is probably a result of increases in the rigidity of the stem and root system during freezing temperatures and the change in wind flow through the forest due to snow on the canopy and on the ground during the winter season.

Highlights

In Québec province, Canada, windthrow events in balsam fir (Abies balsamea (L.) Mill) stands can be considered as a significant source of disturbance and economic losses [1], and are known to be linked to forest management [2]
To improve forest management in these stands and mitigate the wind damage risk, the stability and vulnerability of balsam fir trees has been studied as a consequence of different types of thinning [3,4] and an existing British wind risk model: ForestGALES [5,6] has been tested, adapted, and improved to provide a decision support tool for forest managers [3,4,7,8]
Our results show that the turning moment experienced by the trees was highly influenced by season, with a turning moment globally higher in winter, but apparently not strongly influenced by the snow thickness on the crowns

Summary

Introduction

In Québec province, Canada, windthrow events in balsam fir (Abies balsamea (L.) Mill) stands can be considered as a significant source of disturbance and economic losses [1], and are known to be linked to forest management [2]. In balsam fir forests, winter and snow cover can last for 6 to 7 months and snow can reach more than 200 cm depth This thick layer of snow and ice produces a change in the wind profile as the snow offers less aerodynamic resistance to wind than ground vegetation and, the rate of change in wind speed is probably larger at ground level during the winter season [9]. The weight of the snow pressing on the branches causes a change in the shape of the crowns and a decrease in the canopy porosity likely to influence the stand wind profile as well This snow loading of tree crowns combined with freezing temperatures can cause severe damages to trees, such as stem bending or stem breakage, but rarely uprooting, as frozen temperatures increases stem stiffness [10,11], and as the weight of the snow cover on the soil, and/or frozen soil, reinforce the root system [12]

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Results

Discussion

Conclusion