The Properties of Sunflecks in a Flexible Plant Canopy, and Their Relationship to Turbulence

The properties of sunflecks within alfalfa canopies and their relationship to atmospheric turbulence above the canopies were quantified. Transient light environment in an alfalfa canopy was observed being highly dynamic, lasting from fractions of seconds to minutes in duration. Both probabilistic analysis and autocorrelation analysis indicate that the mean duration of sunflecks ranges from 0.1 to 1.6 seconds with most frequent sunflecks having durations of 0.2 to 0.8 seconds. Power spectral analysis suggests that winds have significant influences on the properties of sunflecks in the alfalfa canopies. Calculated power spectra of photosynthetically active radiation (PAR) time series within the canopies show clear linear relationships with the mean wind speeds and the spectra of vertical-velocity measured 1.5 m above the canopies. This implies that the properties of sunflecks may be evaluated from the measured properties of turbulence. Results also show that the total variances of the PAR time series increase with increased wind, and the enhancement in variance is more apparent in lower canopy as wind increases. This suggests that the wind-induced movement of plants can reduce the vertical light gradient with canopy depth. Also, frequency distributions of sunfleck irradiance were found to disobey the normal distribution laws, but to be skewed. The skewness is affected by both canopy depth and wind. Smaller canopy depth and stronger winds can cause larger skewness. An attempt was made to use the beta distributions to simulate the PAR irradiance distributions, and the simulated results seem acceptable. The PAR spatial series were also characterized. Spatial power spectral analysis suggests that small-scale spatial heterogeneity is a predominant feature of light environment in alfalfa canopies, and lower solar elevation seems to decrease the spatial variability. In addition, similar analyses were conducted for an aspen canopy. The power spectra of the PAR time series measured within the aspen crown indicates that wind-induced leaf-flutter can yield rapid sunflecks of 6 Hz in frequency.