Studies of Energy and Gas Exchange within Crop Canopies (1)

Abstract

During three seasons (1965, '66 and '67) of corn plants, as part of a wide-ranging study of problems on energy and mass transfer within plant communities, comprehensive measurements of CO2 environment within a corn canopy were made with an infrared gas analyzer (Toshiba-Beckman IR-215) at Kitamoto experiment station near Tokyo. Ten-minute observation was carried out at an interval of about 1-hr over the period from 0600 to 1900. The gas analyzer was alternetely supplied with air from nine heights within or above the canopy. In this paper, the data obtained in 1966 are mainly presented.The absolute concentration of CO2 in the air immediately above the canopy has a clear diurnal cycle (daytime depression). The depression of CO2 -concentration increased with the leaf area index and finally reached about 100PPM at the time of LAI=4.2. Namely the absolute concentration decreased drastically from 370PPM at early morning (0600) to 270PPM at the time of southing (1135). It was also found that the depression is strongly affected by wind and radiation intensity above the canopy.By making reasonable assumptions, the following expression was obtained as to the daytime CO2 -cycle.Cc(t)=Ca+Ps/Da-PN0/Dasinωt, where Ca and Cc are the CO2-concentration at a reference height at which no CO2-cycle is observed and the mean CO2-concentration in the canopy, respectively, Ps the soil respiration intensity, PN0 the net assimilation intensity of the community at the time of southing, Da the integral exchange coefficient in the air above the canopy, ω=π/T the time angle for the cycle, and T the period of the cycle. Model computation shows that the depression decreases with the integral exchange coefficient, agreeing well with those presented in Fig. 1.The smooth curves of Fig. 4 were drawn to show the diurnal change in CO2 -profile shape. The change in absolute conentration was largest in a lower part of the canopy. CO2 decreased with height from the soil surface upward showing that the soil was behaving as a source. From 0700 to 1700, concentration decreased with the depth of the canopy, suggesting the downward flux CO2. CO2-profile displacement to the right after dark reflects accumulation of CO2 in the lower part of the canopy due to the release of CO2 from leaves and soil. The height of minimum concentration shows the diurnal cycle clearly. Although the height was near the top of canopy at early morning and evening, it decreases with radiation intensity.CO2-profile was approximately calculated from the following relation:C(z)=CH-∫Hz{PH-∫HzdF/dz(p∝I/a+bI-rp)dz/K(z)}dz, where CH and PH are respectively the concentration and the flux at the top of canopy, K(z) the profile of turbulent diffusivity in the canopy, p∝, a and b the parameters in light-photosynthesis curve, I the radiation intensity, and rp the leaf respiration. Model computation was made to show the influence of canopy structure, radiation intensity and soil respiration on CO2-profile shape (Figs. 6, 7 and 8). CO2 profiles computed for the canopy with Ft=4 and k=0.5 were found to be in relatively good agreement with those in the corn canopy. Particulary, the diurnal cycle of the height of minimum concentration coincides well with that observed in the canopy. The height (zm) was found to gradually decrease with displacement of the maximum level of leaf area density downward.