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Lee E. Frelich Large infrequent disturbances are important events in the long-term development of forests. In North America alone, examples include the 1988 Yellowstone fires, The 1980 Mount St. Helen’s eruption, Hurricanes Hugo (1989) and Andrew (1992), and the Mississippi river flood of 1993. And now we can add another world-class disturbance event to this roster: the tremendous blowdown of July 4th, 1999, in and around the BWCAW. What causes such ‘superblowdowns’ and what effects will they have on the future of forested ecosystems? Three of these superblowdowns have occurred in the Upper Midwest in recent decades: (1) The Wisconsin superstorm (also occurred on July 4th!) in 1977 damaged a 17 by 160 mile swath across northern Wisconsin and removed greater than 50% of the forest canopy on 350,000 acres; (2) the 1995 blowdown just north of Itasca damaged a similar acreage; and (3) the BWCAW storm of July 4th, 1999. The current damage total for this storm includes 180,000 acres in the 10-33% damage class, 158,000 acres in the 34-66% damage class and 140,000 acres in the 67-100% damage class. These are totals on the U.S. side of the border. Large acreages were also seriously affected on the Canadian side. These superblowdowns all involved thunderstorm downbursts, which are parcels of air that interact with the jet stream in the top of the thunderhead, become very dense and sink at great speed, producing shafts of air up to several miles in diameter that descend from the base of a thunderhead, hit the ground going straight downwards and splatter out in all directions, creating oval-shaped damage zones elongated in the direction of movement of the storm (individual downbursts are on the order of 5x10 miles, roughly the size of Minneapolis). Although, the mapping has not been done in detail, the BWCAW storm probably produced many downbursts aggregated together in a downburst family. The Wisconsin superblowdown was very similar, with a family of 25 separate downbursts in close proximity so that one very large area of blowdown was created. The BWCAW thunderstorm was what meteorologists call a ‘ring of fire’ thunderstorm. These are storms that form at mid-summer during years when the Bermuda high happens to sit over the eastern U.S. The high circulates clockwise, and the thunderstorms migrate around the edge of the high, feeding off of the warm moist air pulled from the Gulf of Mexico by the high. Because Minnesota, Wisconsin and Michigan sit at the northern edge of this high, the circulation around it is going generally easterly in our area, explaining the easterly movement of these thunderstorms and the east-west orientation of the blowdowns. Wind speeds in downbursts are usually F1 on the Fujita scale (73-112 miles per hour), but sometimes they reach F2 velocities (113-157 mph). We know from previous studies of wind damage in forests that F2 winds are necessary to cause massive forest destruction. We estimate that Wind speeds in the BWCAW storm were in the 110-120 mph range. The forward momentum of the storm was approximately 60-65 mph, and the descending air from downbursts added to that to create the extreme wind speeds. When the area affected by downburst families and high wind speeds are combined, it is apparent that the Midwestern superblowdowns cause roughly the same severity and extent of damage as a category 3 or 4 hurricane making landfall in a forested region (for example, Hurricane Hugo in 1989 in South Carolina). The pattern of damage in the BWCAW followed expectations in that many trees survived along the lakeshores, while vast interior areas were flattened. Lakeshore trees are more likely to survive windstorms due to the fact that they are solidly rooted in rocks, and, being exposed on a daily basis, are adapted to wind. Trees regularly rocked by wind allocate more growth to the base of the trunk and upper root system. They are shorter and stouter than trees in the forest interior. They also retain branches low on the trunk, which lowers the center of gravity. A low center of gravity is one of the best predictors of resistance to wind. Interior forest trees--especially in near-boreal forests such as the BWCAW--have the most susceptible growth form, the so-called artists paintbrush form with a small crown on top of a long spindly trunk. The BWCAW blowdown, along with its two companions listed above, is an order of magnitude larger than any blowdown recorded by the General Land Office land surveyors (those who laid out the familiar grid system of townships and square miles just prior to European settlement in a given area) between 1850 and 1900 in the northern Lake States forests. This may suggest that a change in disturbance size has occurred during the last century. However, we do not yet know this for sure, and there are three theories that may explain the observed blowdowns: (1) The superblowdowns always occurred, but by chance none were present at time of land survey; (2) Storms that used to occur in the heart of tornado alley (Iowa, Kansas, Oklahoma, etc.), now occur further north--in other words tornado alley has expanded; and (3) Storms are getting more severe and more frequent due to global warming. Greater temperature difference between ground and stratosphere that accompanies warming could in theory enhance the severity of storms. Whether or not you agree that humans have impacted the earth’s climate, there is no doubt that the climate is warmer now than a century ago. A warmer climate may mean that there will be fewer fires (which sounds counterintuitive, but see below) and more severe windstorms along the southern edge of the boreal forest. Blowdowns were not traditionally thought to be a major disturbance force in the boreal forest. Now, however, we have the BWCAW storm as well as blowdowns of 71,000 acres and 98,000 acres that occurred in the boreal forest of northwestern Ontario in 1988 and 1973, respectively. Research in Quebec by Yves Bergeron and colleagues shows that fire is more frequent along the southern border of the boreal forest during times of cold climate--such as the little ice age from 1500-1850--than during times when the climate is warmer (prior to the little ice age and today). Presumably the mean position of the jet stream during summer is further north during times of warm climate, allowing humid tropical air masses from the Gulf of Mexico to penetrate further north, resulting in warm, wet summers with many thunderstorms that lack dry lightning. Conversely, during colder times, the supply of moisture to northern Minnesota from the Gulf of Mexico would be cut off by the more southern position of the jet stream, resulting in dry vegetation and dry lightning strikes that ignite fires. Perhaps at this point we are moving to a more wind dominated disturbance regime in the BWCAW than during the 1600s to the 1800s. Disturbance regime change inevitably leads to change in the composition of the forest. Wind without subsequent fire tends to advance succession, by removing competing overstory trees of pioneer species, releasing understory shade-tolerant saplings, such as white cedar, black spruce, balsam fir and to some extent, white pine. Crown fires tend to lead to dominance by jack pine on poor soils and aspen on good soils. Bergeron’s research shows that this tradeoff between cedar and jack pine species groups has occurred in the boreal forest over the last few thousand years. A third disturbance effect is the double whammy of windstorm followed by fire in the windfall slash. This is very likely to occur in at least part of the BWCAW blowdown. For example, 80% of the 98,000-acre 1973 blowdown in northwestern Ontario was burned by wildfires during 1974. The statistical likelihood of lightning ignitions within the next few years in an area the size of the BWCAW blowdown is nearly 100%. Fires in windfall slash, especially on shallow soil areas such as much of the BWCAW are very severe, and can consume live seeds in cones of jack pine, and the seed bank and seedlings that carpet the forest floor. Some rocky areas with an organic moss layer as soil could be burned to bare bedrock. Given all these different influences on the forest, what will become of the forest in the BWCAW? With the current climate, intact forests of aspen and jack pine will continue to regenerate to aspen and jack pine if burned, succeed to shade-tolerant species if blown down, and regenerate to mostly aspen and paper birch if blown down and burned. The scenario may be different, however, if, as many scientists predict, the pace of global warming accelerates in the next few years. With more warming, some sites within the blowdown that burn could become scrublands with stunted bur and pin oak. The seedlings of these species are already present in some places. During a 1992 disturbance ecology workshop held at Wilderness Canoe Base, Bud Heinselman was amazed to find oak seedlings on the forest floor in 200-300 year old white and red pine stands around Seagull Lake. At the time we weren’t sure (and we still aren’t sure) if this was a sign of warming climate. Although the pace at which oaks begin to play a significant role in the BWCAW is uncertain, it is more predictable that continued warming will move the system in that direction. In any case, we know that the forests of the BWCAW are now in a period of dramatic change, and the last vestiges of the presettlement forests that we always think of--the forests traversed by the fur traders during the 1700s and 1880s--are on the way out. We know what has happened can happen again. Don’t be surprised if another superblowdown occurs in northern Minnesota within the next decade, and pushes the forest further towards an unknown future.
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