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forming the pine association. Its present extent is much less than 

 formerly. This is due to cutting, burning, erosion by the lake, and to 

 natural successions. Of the three species of conifers that form the 

 greater part of the association, only one is native to the region. This 

 species, Pimis strohiis, was formerly relatively common, but is now 

 represented only by a few rather old trees in isolated situations. From 

 the taxonomic nature of the other three species, Pimis laricio and Pinus 

 sihestris and Pinus sp. ? it is exident that they have, at some past time, 

 been planted there by man. It has been difficult to secure accurate 

 evidence as to the date, but it was probably sixty or seventy years ago. 

 As long as the groves were taken care of the pines flourished ; but with 

 neglect and succession they are slowly disappearing. 



Physical and Ecological Characteristics. — ^The pine association 

 occurs on sandy soil and especially on tlie ridges of sand. Here, for 

 the first time, there is a definite differentiation between the soil and 

 the subsoil. Where the pines are densest there is a carpet of pine 

 needles, which are gradually being converted into humus. The trees 

 afford plenty of protection for ground plants, but at the same time 

 cut off so much light that ground plants can only occur in the inter- 

 stices between the trees and in places where a tree has been removed 

 or cut, thus permitting more light to reach the ground. As a result 

 of the ground-covering, water is easily retained and conditions in gen- 

 eral are less xerophytic than those on the heath. 



The Association. — This association is a representative of the bo- 

 real element which has remained as a relic of the postglacial conifer- 

 ous forests which at one time were dominant in this region. In places 

 where the pines are dense, the association is more typical of its ap- 

 pearance in the northern regions. There are usually few or no sec- 

 ondary species in such situations. The exceptions are false Solomon's 

 seal {Smilacina stellata), Anemone cylindrica, and Poa compressa. 

 The ground is carpeted with needles and pine cones. In places where 

 this association is more open, as along the ridges, there is an abund- 

 ance of secondary species, all of which represent succeeding associ- 

 ations. Which association does follow, is, of course, determined by 

 the number and nature of the secondary species. In the ridges to- 

 wards the southward, where the soil is more xerophytic, prairie 

 plants surround the pine trees and often occupy the ground clear up to 

 the trunk of the trees. (PI. LVI, Pig. 2.) In such places it is impossi- 

 ble for the pine to reproduce itself, as the seeds can not get down to the 

 ground on account of the tangle of prairie grass, debris, etc. As long 

 as the pine trees live, they give the character to the area ; when they 

 die, the prairie dominates entirely. Toward the northward, although 



