Assessment of Oak Wilt Threat to Habitat of the Golden-Cheeked Warbler, an Endangered Species, in Central Texas

A major epidemic of oak wilt, caused by Ceratocystis fagacearum (Bretz) Hunt, has been killing trees in Central Texas for at least 40 years. This has created large and expanding canopy gaps in the vast, homogenous live oak woodlands (Quercus fusiformis Small) in the Edwards Plateau region of Texas. The changes in stand structure may have detrimental consequences for an endangered migratory songbird, the golden-cheeked warbler (GCW, Dendroica chrysoparia Sclater & Salvin). More information is needed to assess the direct impact of oak wilt on the GCW and how oak wilt control measures might affect bird populations. In our study, two surveys with different objectives were conducted at the Fort Hood Military Installation in Central Texas. In 2001, IKONOS 1-meter pan-sharpened satellite imagery was used to assess the incidence and severity of oak wilt. The disease was found to be the cause of mortality in 69 percent of the sampled plots. Only a small proportion of the oak wilt centers (12 percent) were located in designated GCW habitat. A second survey was conducted in 2003-04 to determine the key characteristics of GCW nesting sites and how they compare to those of oak wilt centers. This systematic survey was based on randomly selected cluster sample plots stratified in 5 resource categories based on the presence or absence of oak wilt, GCW habitat, or GCW nesting sites, or both. Stand densities ranged from 90 trees/ha (GCW habitat, no oak wilt) to 1298 trees/ha (GCW habitat, nesting site). Juniper to oak ratios ranged from 0.24:1 (GCW habitat, no oak wilt) to 6.57:1 (GCW habitat, no oak wilt). Classification tree analysis was conducted to identify independent variables associated with the presence of nesting sites in GCW habitat. Key variables in the resulting model included road density, selected Landsat and SPOT 10 satellite imagery bands, elevation, and distance to roads. In terms of tree mortality, the impact of oak wilt on GCW home ranges may be minimal. Further analyses are needed to evaluate the impacts of other site disturbances caused by oak wilt, such as fragmentation and alterations in stand composition. The results of this project will be used to aid natural resources mangers when conflicts occur between endangered species management and oak wilt control.