A Novel Approach for Deriving LAI of Salt Marsh Vegetation Using Structure From Motion and Multiangular Spectra

Abstract

The leaf area index (LAI) is a structural parameter that plays a major role in modeling the exchange of energy, carbon dioxide, water, and other matter between plants and their surrounding environment. Direct means of measuring LAI are time-intensive and laborious, making indirect LAI retrieval methods desirable. In this study, we developed a method for retrieving structural metrics of erectophile plants that relies on open-source structure from motion software. We achieved this using a novel goniometer system to simultaneously retrieve high-resolution digital imagery and bidirectional spectroscopy measurements of Spartina alterniflora in a laboratory environment during both ${{ peak}}$ and ${{ late}}$ growing season. Retrieved LAI metrics correlated well with directional measurements of the Modified Chlorophyll Absorption Ratio Index 2 (MCARI2). Across all planting densities, the magnitude of the MCARI2 exhibited a high degree of variance, which depended on sensor viewing angle. MCARI2 values in the peak growing season were significantly higher. This result suggests that the MCARI2 metric is sensitive to seasonal changes in plant growth and senescence. We observed a high correlation between the $\text{LAI}_{\text{contact}}$ values and the Total MCARI2 metric for the ${{ peak}}$ growth state plantings of $R^2 = 0.9$ , suggesting that summing multiple directional measurements of MCARI2 potentially provides a more accurate assessment of LAI and compensates for observed variance in MCARI2 as a function of sensor orientation. Future work will focus on validating the observed effects across a broader range of plant species.