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The formula for calculating frequency in plant species sampling is based on the concept of occurrence within a certain number of plots. Frequency helps to quantify how commonly a particular species is found across sampled areas. By using the formula that states frequency equals the number of plots where a species occurs divided by the total number of plots sampled, researchers can derive a percentage value that reflects the prevalence of that species in the study area.

This method is especially useful in ecological and forestry studies because it provides insight into species distribution without requiring quantitative measures of abundance. Understanding how frequent a species is within a specific habitat can assist in assessing biodiversity, habitat quality, and the overall ecosystem dynamics.

The other options, while relevant to plant ecology, represent different measurements that do not pertain specifically to the calculation of frequency—plant density focuses on the number of plants per area, total cover concerns the total vegetation cover, and plant canopy cover deals with the vertical aspect of plant growth. Each serves a purpose in the broader context of ecological analysis, but for the specific question asked, the calculation of frequency is most accurately described by the correct formula.