The effect of crown dimensions on transparency and the assessment of tree health
Assessment of forest health on a national scale is a difficult task. One parameter used to rapidly classify tree health is crown transparency, or the amount of sky seen through the crown. Previous investigations of sources of error in estimating crown transparency concentrated on observer experience and perception, and on lighting conditions, showing that the error can be as much as ±15%. Here we tested the hypothesis that crown dimensions, which determine the path-length of the line of sight of the observer through crowns, introduce a large bias in estimates of crown transparency. Both theoretical and empirical results show that crown transparency is highly sensitive to crown dimensions. Crowns of trees with path-lengths >10 m are always likely to be rated <30% transparent (i.e., considered healthy), although their crowns may be as unhealthy as those of trees with path-lengths <4 m, and rated >80% transparent. The misclassification of tree health is further exacerbated by the reduction in the rate at which transparency decreases per unit of path-length as the path-length increases, reflecting a lower average leaf density with increasing crown size. Thus we propose that available crown transparency data may be used to rank relative tree health within narrow intervals of path-length, thereby incorporating the effects of crown dimensions on transparency.
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