Intermediate degrees of difference between vegetation can be more reliably attributed to environmental differences.
Gross differences are self-evident.
Unfortunately, historical causes of vegetation difference interfere with the accurate establishment of correlation between present environmental factors and vegetation. If the differences are small, subjective analysis might not show them, thus only objective analysis will do the job. Remember that correlation is not proof of a causal relationship. As in all numerical work, an appropriate procedure must be used. One misguided procedure is the power line pole example.