SPATIAL AND TEMPORAL VARIATION IN THE LIGHT ENVIRONMENT OF DEVELOPING SCOTS PINE STANDS - THE BASIS FOR A QUICK AND EFFICIENT METHOD OF CHARACTERIZING LIGHT

Spatial and temporal variations in photosynthetic photon flux density (PPFD) at 1 m above the forest floor were measured under and between dominant pines and in 21- to 52 m(2) gaps under developing Scots pine (Pinus sylvestris L.) stands in southern Finland. For each stand, PPFD data were collected continuously every 10-s and then averaged for every 5-min period during 4-6 days under completely clear and overcast sky conditions. Measurements were made during 102 consecutive days within two different areas in each of 10 different Scots pine stands ranging in age from 7 to 105 years. Mean daily percent PPFD in the understory varied from 7.5% in the 20-year-old stands to 38% in the 7-year-old stands, and did not differ significantly between completely clear and overcast sky conditions. Mean daily percent PPFD did not vary significantly between the different sensor locations within closed pine stands, indicating that at high latitudes mean daily percent PPFD does not vary much spatially under closed Scots pine forests. Measurements of PPFD made at different periods during the day under clear sky conditions, one of the main approaches used in the literature, were not representative of the whole-day conditions. Two different results suggest that PPFD can be better characterized under completely overcast sky conditions. First, the 5-min percent PPFD in the understory was fairly constant between 06:00 and 20:00 for all stand ages under completely overcast sky conditions. Second, there was an almost perfect 1:1 relationship of the mean daily percent PPFD measured at 40 different locations in the 10 stands between clear and overcast sky conditions. These two results suggest that an instantaneous measure of percent PPFD obtained under completely overcast sky conditions is representative of the mean daily percent PPFD for both sky conditions. These results provide the basis for a quick and efficient method of estimating the mean daily percent PPFD in the understory of closed forests.