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Keywords:

  • canopy openness;
  • canopy-scope;
  • crown illumination ellipses;
  • crown illumination index;
  • forest dynamics;
  • hemispherical photography

Summary

1.  The high spatial and temporal variability of forest understorey light environments requires lengthy and/or extensive sampling in order to characterize it by direct measurement. As this is often impractical, a number of surrogate measures have been developed that estimate light availability from assessments 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) the crown illumination ellipses method, which compares the size of canopy gaps 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 dots printed on a transparent screen to assess canopy openness, but is more robust 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 illumination 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 advantage of giving ratio scale measurements that can be used in parametric statistics. The crown illumination ellipses can be used to score the illumination of crowns that are above head height.