VERTICAL DISTRIBUTIONS OF CANOPY FOLIAGE AND BIOLOGICALLY-ACTIVE RADIATION IN A DEFOLIATED REFOLIATED HARDWOOD FOREST/

Vertical profiles of foliage area and solar irradiance in the ultravio let-B (UVB, 280-320 nm), photosynthetically active (PAR, 400-700 nm), and total spectral regions were measured simultaneously in a partially refoliated mixed oak forest, previously defoliated by gypsy moth, usi ng canopy analyzers and broadband radiation sensors mounted on an auto -levelling platform of a mobile, up-down lifting tower. Measurements w ere taken at ten locations in the stand; at nine vertical positions in each location. Temporal variations also were evaluated in a second ex periment with the same protocol at a fixed location. Downward cumulati ve leaf area index was fit to the Weibull cumulative distribution func tion. Good agreements were found between the data and their Weibull re presentations, with nonlinear R(2) value averaged at 0.98 for the ten fittings of the spatial samples and greater than 0.99 for the means. B oth the scale and shape parameters of the Weibull cumulative distribut ion function were significantly correlated and decreased with the cano py leaf area index. As an indicator of the internal consistency of the canopy analyzers, the temporal variation of the leaf area measurement s at the fixed location was about 10% for solar zenith angle in the ra nge of 30-45 degrees. The irradiance of UVB, PAR and total solar radia tion within the canopy were all found to attenuate with downward cumul ative leaf area index, and their vertical distribution could be reason ably well described by Beer's law of attenuation. The attenuation rate was greatest for UVB, smallest for total spectral region and intermed iate for PAR. Extinction coefficients were 0.86, 0.79 and 0.64 for UVB , PAR and total solar energy, respectively. Ratios of UVB to PAR, UVB to total spectral region and PAR to total spectral region also were sh own to decrease with cumulative leaf area index. The use of long-term change in the flux ratio of UVB to PAR to monitor the forest adaptatio n to, and damage level from, increased exposure to UVB was recommended .