Giant reed is an invasive weed throughout the southern half of the United States, with the densest stands growing along the coastal rivers of southern California and the Rio Grande in Texas. The objective of this study was to use aerial photography to map giant reed infestations and to estimate infested areas along the Texas–Mexico portion of the Rio Grande. Aerial color-infrared photographs were taken along the Rio Grande between Brownsville and El Paso, TX, in June and July 2002. Based on the aerial photographs and ground surveys, the portion of the river from San Ygnacio to Lajitas, which has a river length of 898 km (558 mi), was found to be infested with giant reed. To estimate infested areas along both sides of the river, 65 (13.5%) of the 480 aerial photographs taken between Lajitas and San Ygnacio were randomly selected. The aerial photographs were digitized, rectified to Google Earth imagery, and then classified using maximum-likelihood classification techniques. The infested areas on both sides of the river, as well as water area and river length, from each photographic image were determined. Based on the estimates from the 65 aerial photos, the ratio of giant reed area to water area and the ratio of giant reed area to river length were calculated. The total giant reed area along the Rio Grande between Lajitas and San Ygnacio was estimated to be 5,981 ha (14,779 ac) with 3,714 ha or 62% on the U.S. side and 2,267 ha or 38% on the Mexican side. This study provides the first accurate estimates of giant reed infestations along the Texas–Mexico portion of the Rio Grande and will be useful for both land owners and government agencies for the estimation of water usage and economic loss and for the management and control of giant reed.
Nomenclature: Giant reed; Arundo donax L.
Interpretive Summary: Giant reed is an invasive weed throughout the southern half of the United States, with the densest stands growing along the coastal rivers of southern California and the Rio Grande in Texas. One of the most important steps for successful management of giant reed is to map its spatial distribution and determine infested areas. Because of the great expanse and inaccessibility of these areas, remote sensing provides a useful tool for mapping the spatial extent of giant reed infestations and for distinguishing it from associated plant species. This study provides the first accurate estimates of giant reed infestations along the Texas–Mexico portion of the Rio Grande based on 2002 aerial photography. The total giant reed area along the Rio Grande between Lajitas and San Ygnacio, TX, was estimated to be 5,981 ha with 3,714 ha (62%) on the U.S. side and 2,267 (38%) on the Mexican side. Furthermore, the United States and Mexico each had about 50% of the giant reed along the portion of the river between Lajitas and Del Rio, TX, whereas the United States had two-thirds of the giant reed on the portion of the river between Del Rio and San Ygnacio, TX. The results from this study will be useful for both land owners and government agencies for the management and control of this invasive weed along the Rio Grande in both the United States and Mexico. These results and the aerial photographs are currently being used for the planning and release of biological agents for the control of giant reed along the river and for the estimation of water usage and economic loss. Moreover, this study is an important first step toward the complete documentation of giant reed infestations along the river and for the long-term control and management of giant reed in the entire Rio Grande basin.
