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 pa rameter 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, wh ich determine the path-length of the line of sight of the observer through crowns, introduce a large bias in estimates of crown transparency. Both the oretical and empirical results show that crown transparency is highly sensi tive to crown dimensions. Crowns of trees with path-lengths > 10 m are alwa ys likely to be rated < 30% transparent (i.e., considered healthy), althoug h 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 furt her exacerbated by the reduction in the rate at which transparency decrease s per unit of path-length as the path-length increases, reflecting a lower average leaf density with increasing crown size. Thus we propose that avail able crown transparency data may be used to rank relative tree health withi n narrow intervals of path-length, thereby incorporating the effects of cro wn dimensions on transparency.