46 THE STUDY OF PLANT COMMUNITIES * Chapter 111 



a ruler placed at the right until its lower edge describes a tangent 

 to the curve. The point of the tangent is the center of the region 

 where the 10 percent relationship holds. If greater accuracy is de- 

 sired, the minimal area could be placed at the point of 5 percent 

 rise for a 10 percent increase in sampling. 



By another procedure, 49 the ratio of the x-y axes can be ignored, 

 a sample size or number can be selected, and a value set upon the 

 sampling. If the total number of species obtained in the sampling 

 is divided by the total number of sample units, the average incre- 

 ment of new species per additional sample unit is obtained. The 

 point on the curve is located (point A), in the region of which ad- 

 dition of a unit sample produces an increment of species equal to 



Oak- Hickory Forest 

 piedmont of n.c. 



MINIMUM NUMBER OF 10 X I0M. 

 FOR SAMPLING TREE STRATA 



8 10 12 14 16 18 20 22 



NUMBER OF 10 X 10 M. QUADRATS 



24 26 28 30 



C 



Oak-Hickory Forest 

 piedmont of n.c. 



MINIMUM NUMBER 0F4X4M. SHRUB 

 AND TRANSGRESSIVE STRATA 



8 10 12 14 16 18 20 22 



NUMBER OF 4X4M. QUADRATS 



24 26 



28 



30 



FlG. 17. Species : area curves for an oak-hickory forest, (A) indicating a 

 minimum of six 10 by 10 m. quadrats for sampling the arborescent strata, and 

 (B) a minimum of ten 4 by 4 m. quadrats for sampling the transgressive and 

 shrub strata. (C) A dune grassland community required a quadrat of hot less 



