Microburst, July 2006

In July 2006, we had an amazing microburst 

windstorm come through our region. I don't know its exact trajectory, but I think it blew SW-NE, hitting Somerville,

 Palermo and Liberty especially hard. I heard that some southern Kennebec County towns also got hit, and there is impressive windthrow from about the same time on some islands in Casco Bay. 

When this storm occurred we had not had a serious large-scale disturbance in Somerville since the Ice Storm of 1998. The microburst created gaps in the canopy from single-tree size to 5+ acres. The arrangement of snags, downed logs and residual trees after the event was fascinating: a textbook example of how natural disturbance impacts stand development. Certain species were hit harder than others. Tall, emergent white pines were "combed out" of mixed forests. Pure, old-field pine stands (including some that I helped my friend Shaun to thin immaculately) were totalled, often through snapping.  On mixed hardwood-hemlock ridges, aspen tended to be the hardest hit, while red oak and red maple were thrown selectively. A lot of hardwoods sustained heavy crown damage but didn't fall. I have read that yellow birch and red maple are adapted to slough off large branches in wind storms to reduce their crown "sail".

Where winds were strong enough, the storm did amazing damage to pure stands of oak sawtimber, which I had thought of as a wind-resistant species. 

Hemlock was pretty susceptible, especially if it was already decadent. In Casco Bay (Whaleboat Island) the storms knocked down swathes of red spruce about 200 feet wide, while leaving adjacent areas untouched. But even within areas of susceptible species like spruce, there were still lots of "canopy legacies"- snags, heavily-damaged trees and surprisingly intact survivors.

One thing that really fascinated me was how well this storm replicated, on a much smaller scale, what I have read about the Hurricane of 1938. Just as in 1937, in 2005 there was a large supply of dense old-field pine stands in stem exclusion. These were growing on abandoned farmland, on sites where such a structure and composition was probably unnatural. The storm knocked them down almost without exception. After the blowdown and resultant salvage, the advance regeneration of oak, maple and ash is dominating the site. I suspect that many of the mixed hardwood stands in Somerville had a similar origin.

Of course, salvage has really changed the development trajectory for these areas. In Somerville and Palermo, almost the entire affected are was salvaged. (Often quite well). Where there was a lot of wood on the ground, salvage tended to destroy most of the advance regeneration. I think that the best place to see how storm-affected stands develop without salvage will be Whaleboat Island, which is owned by TNC as a coastal reserve.