Studying the vegetation response to simulated leakage of sequestered CO_2 using spectral vegetation indices

摘要

Measurement of spectral reflectance provides a fast and nondestructive method of stress detection in vegetation. In this shallow subsurface CO_2 release experiment to simulate CO_2 leakage of geologically sequestered CO_2, the radiometric responses of plants to elevated soil CO_2 concentration were monitored using a spectroradiometer. Spectral responses included increased reflectance in the visible spectral region and decreased reflectance in the near-infrared region and thus an altered spectral pattern of vegetation. Visible responses of vegetation include purple discoloration and eventual death of leaves at sites where the soil CO_2 concentration was very high. Derivative analysis identified two features (minimum and maximum) in the 575-580nm and 720-723nm spectral regions. The normalized difference first derivative index (NFDI) was defined based on the spectral derivative at the two bands. Four vegetation indices were analyzed with the accumulated soil CO_2 concentration to assess the accumulated impact of high soil CO_2 concentration on vegetation. Results show that with increased soil CO_2 concentration due to the surface CO_2 leakage, (1) the structural independent pigment index (SIPI) increased, indicating a high carotenoid to chlorophyll ratio; (2) the chlorophyll normalized difference vegetation index (Chl NDI) decreased, suggesting a decrease in chlorophyll content with time; (3) pigment specific simple ratios (both PSSRa and PSSRb) were reduced for stressed vegetation compared to that at the control site, indicating a reduction in both chlorophyll a and chlorophyll b; and (4) NFDI was low where plants were stressed. Changes in NFDI during the experiment were 36% and 1% for stressed and control plants, respectively. All four indices were found to be sensitive to stress in vegetation induced by high soil CO_2 concentration.