Factors influencing windthrow in balsam fir forests: from landscape studies to individual tree studies

Abstract

Windthrow hazard depends upon the interaction of numerous factors whose relative importance has yet to be assessed in the specific context of Eastern Canadian boreal forests. This paper presents results from three studies looking at their relative importance at different scales in the context of balsam fir stands. First, factors involved in a catastrophic windthrow are examined in order to assess their importance at a regional and local scale. Wind speeds were estimated from a numerical model and overlayed with windthrow, soil and stand maps. The two regions that were damaged presented many predisposing factors: high occurrence of shallow soils, of vulnerable species and of overmature stands. Moreover, both regions have been identified as high wind areas. No consistent effect of wind speed on windthrow could be demonstrated, but the increasing vulnerability of stands with increasing proportion of balsam fir in one region and increasing age in balsam fir stands in both regions was significant. In one region, shallow tills experienced more damage in comparison with deep tills but not in the other. The second study looks at windthrow in riparian buffer strips at a local level. It includes a wind tunnel study and a field monitoring in stands of similar species composition and age. Results have shown that windthrow after 7 years was very variable and did not differ with strip width or thinning in the strip. In fact, it was more closely related to locations exposed to stronger winds stressing the importance of wind exposure estimation. It also confirms the greater vulnerability of balsam fir. The last study addresses the identification of external indicators of mechanical weaknesses on individual trees. It involves dissections and winching of trees with and without defects. Results tend to indicate a reduction in resistance when cracks are present on balsam fir. These three studies provide some information to begin considering windthrow hazard in the management of balsam fir forests.

Introduction

The disturbance regime in Eastern Canadian boreal forests is largely dominated by fire and insect outbreaks. In this region, clearcutting has been the preferred method for harvesting mature even-aged stands. In such a context, windthrow is mostly associated with forested strips left alongside roads or waterbodies in order to mitigate the impacts of clearcutting. This explains why very little research on windthrow has been conducted in Eastern Canada.

In recent years, the need to better integrate multiple uses of the forest brought a reduction in the size of clear-cut areas, increasing the amount of exposed edges. Concurrently, recommendations have been made to increase the amount of mature trees or stands left after logging to provide a better habitat for wildlife. The need to increase the use of partial cutting has also been stressed (Franklin, 1990, Seymour and Hunter, 1992, Coates and Steventon, 1994). With these trends, wind-related damage is likely to become a primary preoccupation (Laurance and Yensen, 1991, Esseen, 1994).

Windthrow hazard depends upon the interaction of numerous factors related to climate, topography, soil and stand characteristics. Even though the factors involved are relatively well known, their relative importance has yet to be assessed in the specific context of Eastern Canadian boreal forests (Ruel, 1995).

Vulnerability to windthrow can also be modified by silvicultural operations. Trees left at the edges of clearcuttings are exposed to stronger winds making them more susceptible to windthrow (Moore, 1977, De Walle, 1983, Oke, 1987). Since this increase is related to the amount of exposed perimeter, the amount of windthrow has been related to the ratio of exposed perimeter/total area in strip clearcutting (Elling and Verry, 1978, Fleming and Crossfield, 1983). In Québec, forested riparian strips are used to mitigate the impact of clearcutting on water quality and wildlife. The current practice is to leave 20 m forested strips alongside all permanent waterbodies and harvest a third of trees over 10 cm DBH in those strips. Even though riparian buffer strips have been used since the beginning of the century in Eastern Canada (Loucks, 1957), there is still limited information available on their stability so that strip width recommendations differ greatly between regions (Van Groenewoud, 1977, Quebec, 1986).

Thinning also increases wind penetration into the stand and could increase the amount of windthrow for some years after treatment (Somerville, 1980, Cremer et al., 1982, Bouchon, 1987, Gardiner et al., 1997). The increased use of partial cutting requires the selection of trees that are likely to live through the next cutting cycle. Also, the retention of trees to create structure after clearcutting requires the identification of trees resistant to windthrow. It then becomes necessary to look at factors that make individual trees within a stand more resistant.

Since factors can act at different scales, studies must also be conducted at different scales. First, broad scale studies must be conducted to identify general patterns that can lead to a specific regional vulnerability. Secondly, other studies must be conducted at the local level to better weigh the different factors in similar climatic conditions. Lastly, some attention should be paid to the identification of vulnerable trees within a stand. Hence, this paper integrates results from three studies providing information at different scales for windthrow hazard management in balsam fir stands. These studies have been conducted in an attempt to start building tools to incorporate windthrow hazard into forest management activities.