Friday, April 24, 2015

Andy Asell's maps: Savanna, prairie and woodland

I promised earlier in the week to pull from PowerPoint four of the maps used by Andy Asell for his "Little County on the Tallgrass Prairie" presentation Monday evening at the Lucas County Historical Society annual meeting. So here they are.

Keep in mind that three of the maps belong to Andy, not to me (or you). The map from the 1875 A.T. Andreas Illustrated Historical Atlas of the State of Iowa is in the public domain. Andy used the maps to illustrate historic vegetation patterns in the county.

The first map consists of two parts --- a base indicating vegetation patterns noted by the field surveyors who first mapped the county ca. 1847. Green represents woodland; tan, open land; brown, transitional areas. Over the top, Andy has dropped a grid of historic man-made features --- cultivated areas noted by the surveyors (tiny splotches of bright red), historic but no longer extant towns (orange squares), schools (blue dots) and cemeteries (blue crosses) in addition to roadways and the towns that still are with us.

The surveyor notes do not adequately represent the extensive oak savannas the marked the transition between woodland and prairie, so those areas are under represented on the base map. It might be noted, too, that not all of Lucas County's rural schools are located on the overlay --- although most are. There were, for example, two school buildings in the Benton Township district I know best --- Myers. What was known as "Myers School" is indicated, but the other --- Brush College --- is not.

The second map, taken from the Andreas atlas, illustrates patterns of vegetation noted by 1875 mapmakers, relying no doubt on earlier sources.

In the third map, the 1847 vegetation pattern has been dropped onto the 1875 grid to show how they related as the century progressed.

The final map results from science rather than obscrvation and is based upon soil surveys. In other words, soil samples have been used to map under what conditions Lucas County soils developed --- prairie (tan), savanna (brown) and woodland (green).

I was amazed by the extent of the savannas --- although this is not necessarily the exact pattern EuroAmerican pioneers found when they entered Lucas County since it is based upon millennia of development. Note that the big forests of Lucas County were located along White Breast and Cedar creeks; the Chariton River --- rather puny here in Lucas County close to its headwaters --- was bordered primarily by savanna.

There seems to be a little confusion about what exactly a savanna is. I (unscientifically) think of a savanna as "park-like." In other words, mature trees are scattered across the landscape in such a manner that their canopies are not closed, sunlight reaches the soil and grasses and other vegetation flourish under and around them. Prairie fires swept through savannas, but the trees --- mostly oaks --- were hardy enough to withstand the fire and flourish while lesser varieties of trees and brushy vegetation were kept in check.

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