The effect of turbulence on the light environment of alfalfa

Abstract

The properties of sunflecks in alfalfa canopies and their relationship to turbulence were quantified from measurements of light fluctuations and winds. Winds were measured above the canopy at 10 Hz, while light was measured at rates as high as 27 Hz inside the canopy. Power spectral analyses were used to determine the importance of various time scales of light fluctuations. Under all wind conditions a prominent spectral peak ranging from 1 to 2 Hz was observed for photosynthetically active radiation (PAR) in the canopy at LAI of 1.7. As mean winds increased from 1 to 7 ms −1, the peak broadened and large increases were observed in the contribution of higher frequencies up to 10 Hz. The slope of spectral densities of PAR at the higher frequencies exhibited a linear relationship with the friction velocity or intensity of turbulence above the canopy. This implies that certain properties of sunflecks may be evaluated from measured properties of turbulence. The most prominent peaks in the PAR power spectra are likely due to stalk waving, which is triggered by gusts or turbulent structures. The strong damping effects of plants on the resonant interactions, as well as leaf flutter, account for the broad, flat peak under windy conditions. The frequency of leaf flutter is considered to be the main factor governing the slope of the linear portion of the PAR spectrum.