Crown condition assessment: An accurate, precise and efficient method with broad applicability to Eucalyptus

Assessment of crown condition is a useful tool for monitoring forest health and is used widely to assess tree dieback and decline. Multiple methods used to assess eucalypt crown condition are difficult to compare across studies. Furthermore, the relative effectiveness of the available methods has not been evaluated.The objective of this study was to find an accurate, precise and efficient method for the assessment of crown condition in eucalypts. Four widely used methods were used to assess the crown condition of 516 eucalypt trees from Tasmania andWestern Australia.Two of the methods assessed individual crown condition parameters (single-parameter methods) which could then be added to give an overall score for condition (additive parameter).The other two methods use a single score to encompass many crown parameters (combined-parameter methods). A selection of trees was scored on multiple occasions and by multiple assessors to determine repeatability and reproducibility. Data were analysed by Spearman’s correlations and principal components analysis. All scored parameters were positively correlated to varying degrees. All parameters were distributed along a single-principal components analysis axis, with the parameters from the single-parameter methods having the greatest weightings. In order to address the objective of this study five criteria were developed for consideration of parameters that assess crown condition: (i) capacity to assess dieback (high correlation with other crown parameters, and capacity to indicate the presence of dieback symptoms in the crown); (ii) observer bias (sensitivity to minor change and small difference between observers); (iii) repeatability (sensitivity to minor change and small difference between years); (iv) capacity to assess different species; and (v) efficiency to score. Methods that best met these criteria used additive parameters derived from the crown parameters of primary crown dieback, epicormic growth and either crown shrinkage or dead branches. The single most useful parameter for assessment of eucalypt crown condition was primary crown dieback. This parameter was found to be the most accurate and precise measure of crown condition and is efficient to score. Primary crown dieback is recommended as the standard method for assessment of crown condition of eucalypt trees.