Xs. Yang et al., VERTICAL DISTRIBUTIONS OF CANOPY FOLIAGE AND BIOLOGICALLY-ACTIVE RADIATION IN A DEFOLIATED REFOLIATED HARDWOOD FOREST/, Agricultural and forest meteorology, 67(1-2), 1993, pp. 129-146
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
.