Autumnal Hail and Early Browndown in the Upper Midwest
It can be challenging to detect disturbances during seasonal periods of transition such as Fall or Spring in the eastern deciduous forest. The baseline conditions we use to compare with current conditions harbor a lot of normal variation that results from the onset and progression of cold temperatures. In the upper Midwest, Fall is typically reflected by a gradual drop in NDVI during September, then a more rapid decline in October. By the beginning of November during most years, this decline in NDVI is largely complete. When Fall progresses normally, ForWarn’s phenologically-derived baselines capture no change (i.e., conditions remain light blue), but as long as the summer and winter tail ends of this transition are normal, an early Fall stands out as a temporary flash of yellow, orange and red, as NDVI is below seasonal conditions for those periods. Given ForWarn’s legend, an unusually late Fall browndown shows up as bluer than baseline conditions and an early Fall is yellow to red.
The Fall browndown of 2014 was somewhat earlier than the Upper Midwest experienced in 2013, the latest Fall of the prior 3 years, and average conditions since 2000. This region-wide decline greenness was driven by a sharp cold weather event in mid-September as shown by the accompanying graphs. One week earlier, a severe storm front moved south bringing 1-2 inch diameter hail to northern Wisconsin. ForWarn change maps show that the forest areas hit hardest constituted two linear storm tracks, 30 km apart that extended 200 km.
This wind and hail storm caused substantial damage in portions of the Chequamegon-Nicolet National Forest, perhaps in part because many of the deciduous trees still had their leaves making branches more susceptible to breakage. Much of the observed change may have been caused by either high wind or hail stripping trees of their leaves prematurely, yet in similar vegetation types, downed branches and limbs can increase forest fuels and fire hazards.
Note the persistence of the NDVI decline in northeastern-most Minnesota caused by the 2011 Pagami Creek Fire. Unlike the deciduous forests to the south, conifers dominate northeastern Minnesota so there is limited change in NDVI from one Fall to the next. Like other evergreen forests, this makes the NDVI of these landscapes relatively insensitive to year-to-year variation in weather, so they are more likely to remain blue (normal) during September and October while other vegetation types are in flux.