Defoliation by spruce budworm: estimation by ocular and shoot-count methods and variability among branches, trees, and stands

Ocular and shoot-count defoliation estimation techniques were compared for accuracy and precision at the levels of branches and crown position within trees, whole trees, plots, and stands. A shoot-count estimate of defoliation for a midcrown branch had the best relative accuracy (±7%), while the best ocular estimate was for the whole tree by an experienced observer (accuracy ±12%). Ocular estimates were biased towards ovcrestimation at low defoliation levels (error 20–30% defoliation), and previous defoliation caused the ovcrestimation of current defoliation by 30–40% on trees that had been previously severely defoliated. Observers were found to be consistent in their rating, but biased by about ±10%; experienced observers were about 5–10% more accurate than inexperienced observers. Intertree variance in defoliation was greater than intratree variance or variance between plots in a stand. It was concluded that ocular estimation of defoliation is a viable technique that can give accuracy within the limits required for surveys and many research applications, if the influence of observer experience, observer bias, and previous defoliation arc recognized, and adjusted for when necessary.