238 ILLINOIS ACADEMY OP SCIENCE 



CORRELATION BETWEEN FOREST AND SOIL TYPES. 



The field study has been inestimably aided by the State 

 Soil survey map of Cumberland county, of which an ad- 

 vance copy was kindly furnished by Professor J. G. 

 Mosier. One problem has been to find what agreement 

 there may be in local distribution of soil types and forest 

 associations. It was soon determined that the boundaries 

 between areas marked as upland timber soils and those of 

 upland prairie soils, serve equally well as boundaries be- 

 tween prairie and forest vegetation. The light gray silt- 

 loam on tight clay, with xerophytic oak vegetation, differs 

 from the gray silt-loam with prairie vegetation, only in 

 having a slightly lower organic content. This at once sug- 

 gests that the slight difference in soil may be the result of 

 the difference in vegetation, a hypothesis which is sup- 

 ported by the facts of vegetational history. The writer is 

 rather confident that the original distribution of xero- 

 phytic oak forest, now mostly destroyed, corresponds very 

 nearly with that of the light gray silt-loam, as shown con- 

 spicuously on the soil map in yellow. Likewise prairie 

 vegetation boundaries have been found to coincide with 

 prairie soil boundaries. The soil map is therefore of very 

 great value to students of vegetation and to foresters.^ 



Correspondence between forests and soils on the hill 

 slopes and bottomlands is less detailed and exact than on 

 the uplands, and without causal connection. The soils are 

 in horizontal strata and are successively uncovered by 

 erosion, so that the soil boundaries coincide more or less 

 closely with the contour lines. Thus soil distribution is 

 controlled by topography. Vegetation distribution is also 

 controlled by topography but in somewhat different ways. 

 Thus, in ravines, a south-facing slope is commonly covered 

 with oak-hickory forest, while the north-facing slope has a 

 mesophytic forest, with red oak, maple, etc. But the soil 

 types of the two slopes are the same and, on both slopes, 

 the same forest type extends over two or three soil types. 

 A re-examination of Table I will show the kinds of soils 

 associated with particular kinds of forests and may per- 

 haps also show that vegetation is more closely dependent 

 upon topography and aspect than upon soil. To sum- 



' It might be expected that the vegetation served somewhat as a guide in the 

 field study for the soil survey, and this is suggested by their classification of soils 

 into upland timber soils and prairie soils. 



