Boundary decisions, or edge effects, refer to the problem of deciding whether to include plants intersected by the sample unit boundary. Boundary decisions are more frequently encountered in vegetation with abundant plant populations, and are a less significant issue when sampling areas with sparse plant abundance. The probability of having to boundary decision problems is also influenced by sample unit size and sample unit shape because these characteristics determine the actual boundary length relative to the area encompassed by the boundary of the quadrat or transect.
A consistent personal inclination to admit or reject boundary plants will compromise sample accuracy, either overestimating (due to consistent admission) or underestimating (due to consistent omission) the attribute being measured. The accuracy of vegetation attributes measured by counting, such as frequency or density, are particularly susceptible to bias arising from boundary decisions. Data obtained by direct methods to determine biomass can also be strongly biased by boundary decisions, because all vertical sides of the quadrat must be considered.
Clearly defined ground rules help mediate some of the inaccuracies caused by boundary decisions. Some people decide to admit plants with half or more of their basal area within the sample unit. Others may decide to include all plants intersected by the right and upper boundaries of the sample unit, while rejecting plants touching the two remaining sides.
References and Further Reading
Sukhatme, P.V. 1947. The problem of plot size in large-scale yield surveys. Journal of American Statistics Association 42:297-310.