238 
The differences in crown counts most likely originated in the lower strata of the 
forest canopy. To obtain an indication at which height level they became 
substantial, stratified counts relative to the highest tree in the plot were 
carried out. 
STRATIFIED CROWN COUNTS ON PANCHROMATIC AND INFRARED COLOUR 
Tree height measurements are usually unfeasible on aerial photographs of the 
tropical rain forest because an unobstructed view of the ground is seldom 
available. Stratified crown counts were therefore carried out at height levels 
relative to the highest tree in the plot. Levels at 5 metre intervals were 
adopted. The interval of 5 metres was considered the smallest distance which 
could possibly correspond with the actual average height of the stories in the 
forest canopy. 
Ten one hectare plots with principal points as centres were chosen. To avoid 
mislocation of the canopy strata by differences in relief, only plots on very flat 
ground were taken. For each photo the scale was determined. The variation in 
scale was small so that an average could be used to calculate the parallax 
difference corresponding with a 5 metres interval. 
The crown counts per 5 metre stratum were performed with a Wild parallax 
bar under a Galileo Santoni scanning stereoscope. After locating the highest 
tree in each plot the floating mark was placed on the top of this tree and the 
appropriate parallax difference for the required height level was subtracted 
from the reading of the tree top. All trees with a top above the floating mark 
were then counted. The floating mark was subsequently set for heights of 5,10 
and 15 metres downwards from the top of the highest tree. Since there was no 
significant difference between panchromatic and colour, crown counts were only 
performed on panchromatic and infrared colour plots. The results are given in 
table 3. 
A "t-test" was applied to the plot means of the strata and total crown counts on 
both film types. No significant difference was found between any of the 
corresponding strata. Total crown counts were again significantly different. 
Trees below the 15 metre level obviously account for the significant difference 
and better assessment of these trees appears to be about the only advantage 
which infrared colour has to offer. Trees in the lower strata of the forest 
canopy may be expected to belong to the smaller diameter classes. 
No. 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
Total 
pho: 
On th 
covei 
was s 
out ir 
plan 1 
In the 
buttri 
to the 
of ea 
the la 
hypsc 
on th 
other 
becai 
6242 
grapl