Flaming prairies: when burning out is a good thing

prescribed burn photo courtesy of Vanderbilt UFire is a doozy of a maintenance regime; burning resets the sere (accumulation of habitat and other ecological factors that move a site from young to mature) with a starkness that can only be rivaled by hedgerows and coppice lots.  What had been land covered with grasses and/or trees and shrubs is reduced to ash, but this happens along a gradient.

Burns on generally cool days that are fed by very little debris and low winds, and mildly thwarted by relatively high soil moisture levels, tend to burn cool (for a fire.)  Burns on hot days that happen in forests that are filled with fallen trees and branches, and that are amped up by high wind velocities and low moisture levels can burn exceedingly hot.

These factors add up to temperature, and this makes a tremendous difference: a fire of 500° F (260° C) will impress the heck out of you, me, and your newspaper, but very few (or no) tree canopies will bother to burn at that temperature- they are too moist.

A fire of over 1300° F (705°C), on the other hand, is hot enough to not only consume the canopy layer of a forest (making it a “topfire”), a significant amount of the organic content of the top layers of soil gets involved, too.  The Great Hinckley Fire of 1894 in Michigan burned so hot (estimates put it at +2000° F, or 1100° C) that large patches of ground were burnt to the bedrock and fireballs of gas escaping from the burning forest re-ignited in midair. 

image possibly from the Telegraph newspapers? not sureThe Hinkley fire was fueled by an excess of timber brash left behind by lumbermen following standard timbering procedures of the time: trees were cleared of all their branches at the site and only the trunks removed.  Large piles of branches built up as vast acreages were cleared.  This was the era of the railroad expansion and a lot of railroad ties were coming out of the Great Lakes Region, much to the chagrin of the Native Americans who had been living there, and this region had experienced heavy logging since the 1830s.

Forest management policies began to shift after this and other massive fires (the Great Chicago Fire and the far larger Pashtigo Fire, and the series collectively known as the Great Michigan Fire were all in 1871).  At first, forest fire management stragety focused on suppression of fire.  For several decades, this worked.  But, as ground fuel levels built up again, suppression became a harder and harder goal to achieve: fires that got out of control started raging again.

At the same time, grassland ecologists were studying prairire management, tackling such topics as How to Not Have Another Dust Bowl, sustainable grazing patterns, and the thwarting of invasive species that were taking over prairie restoration projects. 

Nichols Arboretum burn managed by studentsBurning grass on purpose is always less intimidating that burning trees on purpose, so fire as a management tool made a lot of strides with the grassland folks.  My favorite of research projects has long been that at the Nichols Arboretum in Ann Arbor, Michigan.  Here, Professor Robert Grese has been burning the same patches on a variety of schedules for years; the impact of repetition is showing intriguing results.

Where Grese’s team has burned every fall, a near monoculture of big bluestem has arisen.  Where they have burned every spring, a bountiful collection of flowers has accumulated.  Spring burns on a 3-5 year schedule seem to produce the most biodiverse responses, with some flowers and grasses able to use the extra time to rebuild their populations a bit better than the annual burn sites.

The burns I’ve been tracking here are all on these 3-5 year schedules.

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Images here courtesy of Nichols Arboretum, the Telegraph newspapers (?), and Vanderbilt University.

 

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