The role of environmental context in mapping Lepidium latifolium with hyperspectral image data

Lepidium latifolium (perennial pepperweed) is a noxious Eurasian weed invading riparian and wetland areas of the western US. Effective management of Lepidium requires detailed, accurate maps of its distribution, as may be provided by remote sensing, in order to contain existing infestations and eradicate incipient populations. We mapped Lepidium with high spatial resolution (3m pixels) 128-band hyperspectral HyMap image data in three ecosystems of California’s San Francisco Bay/Sacramento-San Joaquin Delta Estuary (Rush Ranch in Suisun Marsh, Jepson Prairie and the Cosumnes River Preserve in the Delta; 5,500ha, 6,300ha, and 4,000ha of imagery, respectively). These sites are markedly different in terms of hydrology, salinity, and species composition. Classification tree models (CART) incorporating results of mixture tuned matched filtering (MTMF) and spectral physiological indexes were used to map Lepidium at all three sites. The technique was sufficiently flexible and robust to detect Lepidium with similar accuracies (>70%) at all sites. However, separate analyses were required at each site. Comparisons of the behavior of both the MTMFs and the CARTs between sites reveal the importance of environmental context to species mapping. Rush Ranch presents the simplest conditions for Lepidium mapping: it is the wettest of the three sites and Lepidium is spectrally distinct from co-occuring species. At the two Delta sites, many co-occurring species closely resemble Lepidium spectrally. Nevertheless, hyperspectral data provide sufficient spectral detail that, when coupled with sophisticated image processing techniques, is able to resolve Lepidium even in these challenging situations.