284 CAIN, DE OLIVEIRA CASTRO, PIRES, AND DA SILVA 



Table 6. Density variability of trees on a 2-hectare sample plot of equatorial rain 



forest at Mucambo, Belem. The plots of larger size are recombinations of contiguous 



10 X 10 m. sub-plots, with only the largest size not utilizing the entire plot 



^ All standard deviations were computed on the basis of the square root of the 

 sum of deviations squared divided by the number of sub-plots except for the 10 

 largest plots where « — 1 was used in addition to give the following results : stand- 

 ard deviation, 12.42; coefficient of variation, 13.5 per cent. 



average density per plot is 5.94 ± 2.0, with a coefficient of variation of 33.7 

 per cent. By combining the quadrats in contiguous pairs we obtained 100 

 sub-plots of 10 X 20 m. size that have an average density of 11.88 ± 3.22 

 trees per plot and a coefficient of variation of 2 7.1 per cent. For the 50 sub- 

 plots of 20 X 20 m. and the 20 sub-plots (original strips) of 10 X 100 m. the 

 coefficient of variation fell to 19.5 per cent and 14.8 per cent, respectively. 

 The largest size sub-plot available in the present sample was 10 plots each 

 40 X 40 m. for which the coefficient of variation (using n — I in the standard- 

 deviation formula because of the small value of n) was 13.5 per cent, or 

 little different from the value for the strips. This study shows that small 

 plots commonly employed in series in many phytosociological studies are 

 not suitable for the rather heterogeneous equatorial rain forest, at least in 

 the numbers used here. 



Size of trees. Although very large trees occur in equatorial rain forest, 

 both in terra firme and varzea, the impression one gets of the forest is more 

 that of moderate to small trunks with long unbranched boles and relatively 

 small crowns — a columnar forest. The large trees which usually are emergent 

 rise above the canopy at Mucambo at about 35 to 40 m. and are usually 

 widely scattered. This impression of relatively small stature of trees, even in 

 primeval rain forest, is substantiated by the data from the Mucambo plot. 

 Our re-measurement of the trees on the plot, using a steel tape at the con- 

 stant height of 4.5 ft. above the ground, revealed only one tree with a dia- 

 meter in the 150 to 159 cm. class. Only six trees had a diameter of 1 m. or 

 more, and only 69 trees had a diameter of 0.5 m. or more, while 1,101 of 

 the stems were smaller than 0.5 m. The size classes were as follows: 150 to 



