Light Environments in Temperate New Zealand Podocarp Rain- Forests

Light environments in two lowland New Zealand podocarp rainforests are described using data from quantum sensors. Mean daily total photosynthetic photon flux density (PPFD) in the forest understorey varies from 2.6-5.2% incident PPFD in summer and 1.0-2.5% in autumn, and in gaps from 5.0-16.6% in summer and 6.3-8.3% in autumn. Pronounced differences in understorey PPFD occur between clear and overcast days. Overcast days tend to have a lower proportion of 2-minute periods with very low mean PPFD than clear days. In summer, 37.7-91.7% of PPFD occurs as sunflecks, but these only occur for 4.0-27.9% of the time. Most sunflecks are short duration (42.2-72.7% less than 4 minutes in summer) but some are very long (>32 minutes). Overall, light environments are similar to those found in other forests, both in New Zealand and elsewhere. However, canopy structure has a pronounced influence on spatial distribution of light environments within a forest, and differences in the size and frequency of canopy disturbances are a contributing factor to the floristic differences between the two forests studied.