total number of species may be low — Agropyron spp. and Festuca 

 ovina var. duriuscula, for example, in northeastern Washington. ^^ 



The interrelations between individuals of the same species may 

 be very close. Natural grafting of roots is common in many kinds 

 of forest-tree species; for example, in the eastern white pine 

 {Pinus strobus) from two to a number of trees may become united 

 in this manner.^'* This interaction has pronounced effects upon the 

 nature of the grouping, for the individuality of the single organ- 

 ism is replaced by a complex organism — an interacting group of 

 plants that have many connected physiological processes. Hence, 

 intraspecific competition becomes less important than interspecific 

 competition, and diseases such as Dutch elm disease (Ceratostomella 

 ulmi) and oak wilt {Endocomdiophorafagacearum) can be transmitted 

 through such grafts from tree to tree. Tree poisons (silvicides) 

 can also move from poisoned to nonpoisoned trees ("backflash"). 

 The disease or the poison transmitted in this way may hasten the 

 death of many desirable trees. 



A species with wide ecological amplitude and strong competi- 

 tive capacity may occur in several different groups. It may be 

 dominant in one group and subordinate in another; for example, 

 in certain clay soils in the northern Great Plains, western wheat- 

 grass {Agropyron smithii) is often the chief dominant, while in loam 

 soils it may be present but subordinate to needle-and-thread 

 {Stipa comata) and blue gramagrass {Bouteloua gracilis). Species 

 may vary in adaptability to different concentrations of mineral 

 salts. As shown in Table 1-1, Pinus silvestris has a wide range of 

 amplitude, but it is dominant only when the ground-water level 

 is 30 to 40 cm or more deep. As shown in the table, similarity in 

 amplitudes permits grouping of species into marsh, transition 

 bog, and sphagnum bog. 



Largely on the basis of tolerance to depth and duration of 

 snow cover, alpine species may often be differentially aggregated 

 into definite communities. For example, in the Rondane region 

 in Norway there are four communities: Cetraria nivalis- Alectoria 

 ochroleuca lichens on exposed ridges, Cladonia alpestris lichens on the 

 slope, Vaccinium myrtillus heath zone still lower on the slope, and 

 Deschampsia Jiexuosa-Carex bigelowii grassland with a ground layer 



4^2 • Species ai«d Popimlatioxms 



