Consistency of Forest Damage Assessments

The applicability of forest damage assessments depend on many different factors. Crucial factors are sampling design, the consistency over time and accuracy of the assessments. Visual observations, as forest damage assessments, are affected by weather, and visibility conditions and the status of target object. However, the assessments also largely depend on the experience and visual perception of the observer. For the best result measurements should be clearly defined and thoroughly calibrated. Experiences from Sweden, in the past twelve years, often show small, and not significant, differences in assessments of defoliation when tested at national calibration courses (0.0 – 5.9 %) and control surveys (0.1 – 2.2 %). Yet, measurements of agreement reveal some limitations in the assessments. Defoliation on Scots pine (Pinus silvestris) and Norway spruce (Picea abies) were assessed with fairly good agreement (Kappa statistics 0.16 – 0.58), where pine in general was assessed with a weaker agreement. Well known damage symptoms were measured with good agreement (Kappa statistics 0.5 – 0.7) while more diffuse damage symptoms are found with a weaker agreement. Clearly defined symptom descriptions are needed for the best results. Low intensity, sparse and irregular damage attacks are found with a weaker agreement. Due to sample design sparse and irregular damages also are scarcely detected. The interpretation of results from forest damage assessments requires a consideration to the accuracy