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Totalbasal area is a very important factor in plantation forestry and it gives an indication of growth
conditions of plantations as well as the density of the trees. There are confusions among the
differentmethods used for basal area calculations and it is very important to clarify the correct
methodfor basal area estimation in plantation forestry.
Present study was conducted in the 26 year old Pinus caribaea plantation in Yagirala Forest
Reserve situated in the low country wet zone of Sri Lanka. One hectare of the plantation was
lectedfor the study. Selected one hectare area was further divided into 100 sub plots. Each tree in
eachsub plot was measured for diameter at breast height (dbh) using diameter tape and tree caliper.
Fourperpendicular measurements were taken for each tree using the tree caliper while one reading
as taken by wrapping the tape around the tree. Using different basal area factors of the angle
gaugeand spiegal relascope five stand basal area readings were also taken in side the sample plot.
Usingthe four readings ofthe tree caliper tree cross section area was traced on to a graph paper and
weightof all cross section areas was measured. Then the weight was converted in to area. Dbh
readingsobserved using diameter tape and tree caliper were converted in to tree basal area values
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usingthe equation basal area = nd /40000. Two methods were used to calculate the basal area
using caliper readings i.e. basal area calculation using one reading and two readings of the tree
caliper.Using the five readings for the angle gauge and spiegal relascope relative basal area was
calculatedfor each basal area factor.
Oneway analysis of variance test together with Tukey's pairwise comparison was used to identify
the difference between diameter tape and tree caliper. According to the test it showed that there is
no significant difference between these two instruments and there was also not a significant
differencebetween basal area calculations using one caliper reading or two caliper readings.
In order to identify the minimum plot size that have to use to collect data for stand basal area
calculationsusing sample plot method one sample t - test was used. Test mean used for this method
wasthe absolute basal area value calculated using cut and weight method. According to the test it
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.•hewed that the minimum plot size was 2500m for i.he accurate basal area calculation in sample
plotmethod. Therefore, absolute error was calculated for each basal area calculation method used
the sampleplots. According to the results it showed that the absolute error difference is decreasing
withthe increasing the plot size.
Relativebasal area values calculated for different basal area factors for two instruments were
statisticallycompared to identify the best instrument for relative basal area calculations. One
samplet - test was used for this purpose. Absolute basal area value was used as the test mean.
Accordingto the test it was revealed that both Spiegel relascope and angel gauge do not predict
relativebasal area accurately. -
Accordingto the results of the study it was concluded that use of sample plots is the best method
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forthe total basal area calculations with a minimum plot size of 2500m . It is recommended to use
thediametertape for taking the measurement for basal area calculations due to its ease of use in the
field.The error occurred due to the use of angle gauge and Spiegal relascope is very high and it is
notrecommended use both instrument to take basal area measurements in forest plantations even
thoughit is very easy to use them in the field.
