Windstorms can cause serious damage and destruction in Minnesota forests. The storms in early July 2012 caused extensive damage in both urban forests and rural forests and woodlots. We’ve pulled together some resources that may be of assistance to homeowners and landowners as they clean up after the storm and plan for what’s next in their woods and yards.
Research by University of Minnesota Department of Forest Resources researchers after the massive July 4, 1999 blowdown in Minnesota’s Boundary Waters Canoe Area Wilderness identified several factors that affect a tree’s vulnerability to blowdown. (Rich, R.L., L.E. Frelich, and P.B. Reich. 2007. Wind-throw mortality in the southern boreal forest: effects of species, diameter and stand age. Journal of Ecology 95:1261-1273.)
The following factors made forest trees more vulnerable to blowdown:
Species: Early successional, shade intolerant (often fire dependent) species tend to allocate more energy to fast height growth than to adaptations to improve windfirmness. Their greater height also leaves them more exposed to wind. In the BWCA research, species like jack pine, red pine, and trembling aspen were generally most vulnerable to blowdown.
Stand development stage: Mature (~90 yrs) stands were vulnerable than old and very old stands (126-200 yrs), likely because old stand have already lost early successional and transitioned to multi-aged stage of development. These stands include more gaps and wider spacing between dominant trees, so the dominant trees have been more exposed to wind for a long time and have adapted accordingly.
Size: Taller trees with larger diameter stems were more vulnerable.
Management history: The BWCA research occurred in an area of the wilderness that had never been harvested. However, other research has shown that dense stands in which individual trees have little room to grow are more vulnerable. Tall, spindly stems offer little resistance to strong wind.
Disease status: Armillaria root rot and other diseases tend to affect older trees, near end of natural life disproportionately. Decay at various points in the tree can make the tree more vulnerable to breakage.
The following factors made forest trees less vulnerable to windthrow:
Species: Later successional, more shade tolerant species like northern whitecedar and red maple were less vulnerable to blowdown. Fast growth is less of a priority, so these species invest more in strength. Also protected to some degree from wind by dominants. Spire-formed trees like balsam fir and black spruce also less vulnerable. Paper birch, while not commonly viewed as a late successional species, was also less vulnerable.
Long duration exposure to wind: Thinned stands or older stands in which trees have been exposed to high winds for decades have had time to develop wind resistance. While no stands are invulnerable, stands in which trees are more widely spaced tend to be less vulnerable because they’re better adapted to the effects of high winds.
What can landowners do to increase the resilience of their trees and woods to wind damage?
- Increase diversity. Be sure that your property includes some old and some young trees and stands, as well as some later successional species.
- Maintain young, healthy stands. Old stands that are beyond the natural life span of their dominant trees are naturally more vulnerable to disease that can destabilize them. Armillaria root rot and a whole host of diseases in aspen stands (most importantly hypoxylon and white rot) are particularly problematic.
- Thin stands (carefully) to increase wind firmness. While thinning can have important long-term benefits, in the short term it can actually make stands more vulnerable to wind (and ice) damage, as it increases the exposure of the residual trees. Until those trees develop more windfirmness, they will be vulnerable. Frequent light thinnings may be the best approach, particularly in old and very tightly spaced stands.
- Monitor your woods for health problems, and take action as necessary.
Other useful links and resources:
- Stem-girdling roots: The underground epidemic killing our trees. Stem-girdling roots often lead to stem breakage at the ground level.
- Storm Damage to Landscape Trees: Prediction, Prevention, Treatment.
- Pruning Trees and Shrubs. Among other benefits, pruning can help trees to produce strong, windfirm architecture.
- Storm damaged trees repair and replacement checklist (MN DNR)
- How to detect and assess hazardous defects in trees (see chapter 3 at the link)
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