I recently published new method to estimate foliage clumping from optical gap fraction measurements is available here: http://www.nrcresearchpress.com/doi/abs/10.1139/cjfr-2018-0213

The major advantage of the new method is that it is insensitive to zero-gap fraction measurements, making it particularly suitable for denser forest canopies.

The method can be implemented in LAI-2000/2200 Plant Canopy Analyzer and digital hemispherical photography.

The article is available from myself upon request