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Tracking the structural and functional development of a perennial pepperweed (Lepidium latifolium L.) infestation using a multi-year archive of webcam imagery and eddy covariance measurements |
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| Authors: | O Sonnentag M Detto R Vargas Y Ryu BRK Runkle M Kelly DD Baldocchi |
| Institution: | a University of California Berkeley, Department of Environmental Science, Policy and Management, 137 Mulford Hall #3115, Berkeley, CA 94720, USA b Harvard University, Department of Organismic and Evolutionary Biology, 22 Divinity Avenue, Cambridge, MA 02138, USA c Departamento de Biología de la Conservación, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, BC, Mexico d University of California Berkeley, Department of Civil and Environmental Engineering, 760 Davis Hall #1710, Berkeley, CA 94720, USA e University of Hamburg, Department of Soil Science, Allende-Platz 2, 20146 Hamburg, Germany |
| Abstract: | The continued spread of invasive weeds is threatening ecosystem health throughout North America. Understanding the relationships between invasive weeds’ key phenological phases and structural and/or functional canopy development is an essential step for making informed decisions regarding their management. We analyzed a three-year image archive obtained from an inexpensive webcam overlooking a perennial pepperweed (Lepidium latifolium L.) infestation in California to explore the ability of red (R)-green (G)-blue (B) color space information to track the structural and functional development of the pepperweed. We characterized structural and functional canopy development through surface roughness length (z0m; a proxy for canopy height and leaf area index) and canopy photosynthesis (FA), respectively, both of which we derived from eddy covariance measurements. Here we demonstrate the use of cross-correlation functions to determine the temporal lags between chromatic coordinates and two color indices, all calculated from RGB brightness levels, with z0m and FA. We found that these color metrics fail to represent the structural and/or functional state of the canopy. In contrast, relative luminance (CIE Y) appears to be a better indicator for z0m and especially for FA. We calculated CIE Y from pepperweed RGB brightness levels in relation to hypothetical horizontal reference RGB brightness levels. We obtained the latter by applying the ratio between horizontally measured and hypothetical incoming solar radiation on a vertical surface to RGB brightness levels of a vertically oriented reference of invariant light-grey color. We conclude that webcam image archives may provide an inexpensive tool for making informed decisions regarding the timing but not for assessing the effectiveness of invasive plant control measures such as mowing. |
| Keywords: | Pepperweed Near-surface remote sensing Digital camera imagery Canopy photosynthesis Surface roughness length Invasive plants |
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