Vertical distributions of canopy foliage and biologically active radiation in a defoliated/refoliated hardwood forest

Abstract

Vertical profiles of foliage area and solar irradiance in the ultraviolet-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, using canopy analyzers and broadband radiation sensors mounted on an auto-levelling platform of a mobile, up-down lifting tower. Measurements were taken at ten locations in the stand; at nine vertical positions in each location. Temporal variations also were evaluated in a second experiment with the same protocol at a fixed location. Downward cumulative leaf area index was fit to the Weibull cumulative distribution function. Good agreements were found between the data and their Weibull representations, 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. Both the scale and shape parameters of the Weibull cumulative distribution function were significantly correlated and decreased with the canopy leaf area index. As an indicator of the internal consistency of the canopy analyzers, the temporal variation of the leaf area measurements at the fixed location was about 10% for solar zenith angle in the range of 30–45°. The irradiance of UVB, PAR and total solar radiation within the canopy were all found to attenuate with downward cumulative leaf area index, and their vertical distribution could be reasonably well described by Beer's law of attenuation. The attenuation rate was greatest for UVB, smallest for total spectral region and intermediate 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 shown 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 adaptation to, and damage level from, increased exposure to UVB was recommended.