Global (G), photosynthetically ative (PAR), and near infrared (NIR) radiation were measured above and below the canopies of three amazonian ecosystems: main forest, campina, and campinarana, during august-september. The main forest site is located at the Reserva Ducke (2° 57' S; 59° 57' W), which is 23 km from Manaus, where the average tree height was 35m, with some trees reaching 40m, and the measurements were taken at 45m. The campinarana and campina sites are located at the Campina Biological Reserve (2° 34' S; 60° 02' W), 46 km from Manaus, being the measurements heights 17m for the campinarana (average tree height was 14m) and 10m for the campina (average tree height was 7m). Within the vegetation 32 sensors were used at 0.5m above the soil surface in an approximated square grid of 5m x 5m. Every three seconds the signal of each sensor was recorded by a datalogger to give an average for each minute. Plant area index (PAI), estimated by hemispherical photographs taken during the experiments, were equal to 5.44 m2 m-2 for the forest, 2.49 m2 m-2 for the campinarana, and 2.18 m2 m-2 for the campina.
An asymmetry on the daily variation of the albedo was found at all sites and for the three wavebands, with larger afternoon values. PAR albedo showed the least dependency to the solar angle, and NIR displayed the largest asymmetry. Taking half-hour averages, the global and NIR albedo was well represented by a second degree polynomial function of the solar angle, but PAR albedo showed a very weak linear relationship and can be assumed constant.
Fluxes densities can be represented non-dimensionally by the reflection (r), absorption (a), and transmission (t) coefficients expressed as percentage, and ensemble averages results were:
i) Forest - rg = 11.3, ag = 85.5, tg = 3.2; rnir = 21.2, anir = 73.5, tnir = 5.3; rpar = 2.0, apar = 96.7, tpar = 1.3.
ii) Campinarana - rg = 10.4, ag = 75.0, tg = 14.6; rnir = 20.2, anir = 54.8, tnir = 25.8; rpar = 1.5, apar = 94.7, tpar = 3.8
iii) Campina - rg = 11.0, ag = 71.8, tg = 16.9; rnir = 22.0, anir = 52.8, tnir = 25.2; rpar = 1.4, apar = 89.8, tpar = 8.8.
Characterization of the solar regime below vegetated canopy is difficult due to the spatial distribution of the sunflecks, and by their dynamic nature throughout the day. For solar angle greater than 60°, sunfleck frequency increased substantially and lasted, on average, from 2 to 4 min in the forest, and up to 10 min at the campinarana and campina. Sunflecks peak bursts can reach up to 10% of full sun at the forest, 18% at the campinarana, and 30% at the campina. Ensemble average smoothes out fluctuations which are important from the ecological aspect, and the a and t coefficients vary with the sampling scheme.