An improved method for the rapid assessment of forest understorey light environments

1. The high spatial and temporal variability of forest understorey light en vironments requires lengthy and/or extensive sampling in order to character ize it by direct measurement. As this is often impractical, a number of sur rogate measures have been developed that estimate light availability from a ssessments of forest canopy structure. 2. The subjective crown illumination index developed by Clark & Clark (1992 ) was compared with Garrison's (1949) moosehorn and two new methods: (i) th e crown illumination ellipses method, which compares the size of canopy gap s with a series of standard area ellipses printed on a transparent screen: and (ii) the canopy-scope that, like the moosehorn, uses an array of 25 dot s printed on a transparent screen to assess canopy openness, but is more ro bust and portable, measuring the largest canopy gap visible from the point of measurement rather than canopy openness overhead. 3. The new measures were more highly correlated with canopy openness in the range 0-30%, measured from hemispherical photographs, than the crown illum ination index, and showed lower levels of between-observer variability. 4. The canopy-scope has the potential to be widely used for the simple and rapid assessment of forest understorey light environments. It has the advan tage of giving ratio scale measurements that can be used in parametric stat istics. The crown illumination ellipses can be used to score the illuminati on of crowns that are above head height.