184 Journal of the Mitchell Society [March 



micro-organisms which promote nitrification. Type replacement may 

 be effected 



(1) By direct self -replacement under cover, as spruce beneath 

 spruce, or hemlock beneath hemlock, 



(2) By direct self -replacement without cover, as southern jack 

 pine or mountain pine in openings caused by the death of old trees 

 of the same species. 



(3) By alternation: Yellow poplar, for example, replacing chest- 

 nut when it windfalls or dies; and chestnut establishing itself be- 

 neath the cover of the poplar ; rosemary pine appearing in oak wind- 

 falls; and oak appearing beneath the cover of the pine. 



(4) By succession (often following fire or resulting from wind- 

 falls, etc.), as a nurse stand (temporary type) of aspen, birch, and 

 popple followed by a stand (permanent type) of spruce. 



PERMANENCE OF TYPE 



While the type is essentially permanent, oscillation in its composi- 

 tion from time to time or intermission in the continuity of its perma- 

 nent form may occur. In addition to such oscillations within the 

 type, the site may be subject at least to slight or temporary modifi- 

 cation by the type (edaphic changes). For example, there is evidence 

 for believing that the accumulation of humus beneath the oak-yellow 

 poplar mixture on certain situations due to retarded nitrification may 

 result in accumulation of raw humus and the development of a 

 subpeaty site and thus favor a change to a chestnut site and chestnut 

 type ; or a chestnut type quality 1 may with increase in peaty accumu- 

 lations and greater acidity become highly favorable for invasion by 

 laurel, and the site decline to chestnut quality 2 with laurel. On 

 swampy sites there may eventually be better drainage as a result of 

 accumulation of humus, resulting in a drying of the site (Appen- 

 dix 2). For instance, an alder and red maple site may become suit- 

 able for red maple and white oak. The type also is subject to change 

 by natural stresses within itself (Appendix 3). The killing of 

 chestnut by Endothia parasitica on large areas is resulting in pro- 

 found changes in the forest types on what originally were chestnut 

 sites; and undoubtedly this must be regarded as constituting an 

 organic change in the forest types in the region which is affected, 

 until or unless a resistant strain is developed from individual trees. 

 Such resistant individuals are often noted. 



