A SIMPLE AND EFFICIENT METHOD TO ESTIMATE MICROSITE LIGHT AVAILABILITY UNDER A FOREST CANOPY

This study presents a new, simple, and efficient method for estimating microsite light availability in the understory of a forest. The perce ntage of above-canopy photosynthetic photon flux density (%PPFD) trans mitted above 16 microsites in the understory of a mixed conifer-broadl eaf forest was measured every minute between 07:00 and 19:00 for both a completely overcast and a cloudless day. Instantaneous measures of % PPFD were also taken at different times on 3 overcast days. The instan taneous measures of %PPFD were strongly and directly related (P < 0.00 1) among themselves and with mean daily %PPFD values. These results de monstrate the usefulness of using an instantaneous measure of %PPFD ta ken under overcast sky conditions for estimating the mean daily %PPFD at any microsite under a forest canopy.