36 ALFRED DACHNOWSKI 



V. THE MESOPHYTIC FOREST SUCCESSION 



As the soil processes migrate toward the center of a filled-in 

 basin, and the bog conditions become eliminated, the advance 

 of the bog associations continues toward the center also. The 

 direction and rate of the movement varies but little from the 

 slow progressive changes in the substratum. In some deposits 

 the depth to the mineral subsoil is greater than 30 feet (9m.), but 

 the deficiency in mineral substances, such as lime, potash, phos- 

 phoric acid, etc., does not render it difficult or impossible for trees 

 to grow luxuriantly. The deciduous forest association arises 

 in part by the varying relation between trees, light and evapora- 

 tion. The reaction of the deciduous invaders consists mainly 

 in the addition of leaf humus and assimilable organic compounds. 

 It would seem on account of the great difficulty experienced in 

 extracting the water held in peat by any means except high tem- 

 perature, that the lower water-table would determine the succeed- 

 ing association to be the most xerophytic, and that any tendency 

 to excessive transpiration accompanied by slow absorption 

 from the soil would lead to more highly developed protective 

 features against the loss of water. However, with the descent 

 of the ground water follows a more thorough decomposition 

 and oxidation of injurious organic matter. The layer of humus 

 is now kept wet by the upward movement of water. Facility 

 of percolation, power of raising water, all of which are greatest 

 in peat, assume an importance increasingly favorable to the incom- 

 ing vegetation, and to the action of fungi and mycorrhiza. The 

 gradual invasion of deciduous trees and the replacement of coni- 

 fers eventually leads to the complete extinction of bog conditions, 

 to stability and the permanent occupation of the habitat by an 

 association here characterized as the climatic unit of Ohio for- 

 ests — the predominance of maples, ash, oak, elm, walnut and 

 others. 



1. Acer-Fraxinus-Ulmus association. Several of the tama- 

 rack bogs of northwestern Ohio have a peat soil, the conditions 

 of which favor a number of deciduous trees. Among the species 

 that succeed best are in addition to Acer rubrum, Ulmus fulva. 



