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

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 latifoliumL.) 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.Wecharacterizedstructuralandfunctionalcanopydevelopmentthroughsurfaceroughness length (z 0m ; a proxy for canopy height and leaf area index) and canopy photosynthesis (F A ), 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 fromRGBbrightness levels, withz 0m andF A . 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 forz 0m and especially forF A . We calculatedCIE Yfrom pepperweedRGB brightness levels in relation to hypothetical horizontal referenceRGBbrightness levels. We obtained the latter by applying the ratio between horizontally measured and hypothetical incoming solar radiation on a vertical surface toRGBbrightness 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.