What does periodic annual increment (PAI) measure?

• A. The current annual increment.
• B. The maximum height increase in a period.
• C. The annual growth rate over a single year.
• D. The average increase in tree size over a given period of years (usually 5 or 10).

Answer: (D)

Periodic annual increment measures the average growth of a tree over a defined multi-year period, smoothing out year-to-year fluctuations. It is calculated by taking the change in size (such as diameter or height) over the period and dividing by the number of years in that period. This is why the correct choice describes the average increase in tree size over a given span, typically 5 or 10 years. For example, if a tree’s diameter grows from 20 cm to 28 cm over a 10-year period, the PAI is (28 − 20) / 10 = 0.8 cm per year. This differs from the current annual increment, which looks at growth in just the most recent year, and from a single-year growth rate or a maximum height increase, which do not reflect the longer-term average growth used for planning and assessment.