Regression 



(1) A statistical method for the study and expression of 

 the change in one variable associated with and dependent 

 upon changes in another related variable or group of vari- 

 ables. (2) See Retrogression. 



Regulatory Mechanism 



Any influence in the physical or biotic environment of a 

 population that tends to maintain the number of individuals 

 which the resources of the habitat can support, cf. Reaction, 

 Coaction, Predation, Parasitism. 



Relative Humidity 



See Humidity, relative. 



Relev6 



An analysis sample of a Stand of vegetation in which are 

 given the data on characteristics such as kinds of species, 

 Cover, Density, and sometimes others, cf. Sample area. 



Relic (Relict) 



(1) A remnant or fragment of a flora or fauna that re- 

 mains from a former period when it was more completely 

 developed. (2) A remnant of the population of a species that 

 was formerly more widespread. 



Relief 



The inequalities in the elevation of the land surface, cf . 

 Topography. 



Rendzina 



An Intrazonal group of soils usually with brown or 

 black, friable surface horizons and light gray or pale yellow 

 calcareous material below, formed under grassland or mixed 

 grassland-forest in humid or semiarid regions, cf. Prairie 

 soil. 



Replacement Control 



The substitution of one kind of plant by another kind, 

 chiefly as a result of competition, e.g., downy bromegrass 



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