Windthrow in riparian buffer strips: effect of wind exposure, thinning and strip width

This paper discusses the effects of topography, riparian buffer strip width and thinning on the amount of windthrow over 9 years in balsam fir (Abies balsamea) stands. Monitoring of windthrow was conducted in 25 riparian areas representing five treatments: uncut control, thinned 20 m strip, unthinned 20, 40 and 60 m strips. Wind tunnel measurements were made on a topographical model of the study area to provide an estimate of local wind behaviour. A cluster analysis performed on the wind speed data led to the identification of four major topographic units of similar wind behaviour. Wind speed in valleys varied greatly depending upon the direction of the approaching winds and the presence of small topographic features. When the wind blows perpendicularly to the valley, wind speed tends to be lower but more variable than when it blows parallel to the valley. Windthrow was not related to strip width or thinning. The field study showed that windthrow 5 years after cutting was found to be correlated with the speed of winds blowing roughly perpendicular to the strips. Windthrow after 7 and 9 years, following an unusual wind event that occurred between years 5 and 7, was no longer correlated with this wind direction. High levels of damage were observed where the valley widened, offering less shelter to the buffer strips. (C) 2001 Elsevier Science B.V. All rights reserved.