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The three sampling techniques used in the virtual forest tutorial were haphazard, random, and systematic. There are advantages and disadvantages to each of the three sampling techniques. For each of the techniques 25 area quadrats were observed. For haphazard sampling 5 quadrats were selected from each of the 5 topographical regions by trying to hand select quadrats representative of the average tree density in each region. For random sampling a random list of 25 coordinates were generated and those quadrats were observed. For systematic sampling a transect which crossed the entire sampling area, passing through all topographical regions was used. Quadrats were observed on alternating sides of the transect.
The quickest sampling method was systematic sampling at 12 hours and 36 minutes. This makes sense as there is little time spent walking between quadrats as they are all located in a linear line. Haphazard (13 hours 2 minutes) and random (13 hours 13 minutes) sampling both took longer than systematic sampling due to the distance between observed quadrats.
Systematic sampling was the most accurate sampling method for the 2 most common tree species (Eastern Hemlock and Red Maple). Systematic sampling had an error of 5% for Eastern Hemlock density and 25% for Red Maple density, both lower than haphazard (14% and 51% error respectively) and random sampling (17% and 32% respectively). This result is expected as systematic sampling forces the observer to follow a preset system during sampling eliminating the possibility of the observer biasing the samples as can occur during haphazard sampling. Systematic sampling is also a more accurate predictor of the frequency of the two most abundant tree species then random sampling as 5 quadrats are selected from each topographical area. Random sampling allows for quadrats to be selected from any topographical region without any constraints on topographical regions.
Accuracy was better for all 3 sampling techniques for the higher abundance species then for the lower abundance species. Error for the density of the two least abundant species (Striped Maple and White Pine) were 60% and 100% for systematic sampling, 77% and 52% for random sampling, and 31% and 52% for haphazard sampling respectively. A small change in number of individual trees of a certain species can dramatically change the predicted density when the abundance is low. This is understandable as to get accurate results for the low abundance trees, more quadrats would need to be observed.
For predicting densities of tree species in this study it appears that systematic sampling has given the most accurate results. However, even with the use of systematic sampling, more quadrats must be observed if the densities of the lower abundance species are to be accurately predicted.
