A key vegetation parameter important to maintaining Golden-cheeked Warbler (hereafter warbler) breeding habitat is the amount of broadleaf versus Ashe juniper tree cover on a site. Mixed composition woodlands with mature Ashe juniper trees provide enhanced foraging opportunities through high arthropod diversity and supply juniper bark used for nesting. FWS scientists sought to map broadleaf and juniper tree canopy at 1-meter (m) scale across the entire 85,500 square kilometer (km2) breeding area in central Texas to model warbler density from point count data. High spatial resolution (0.5-m to 1.0-m pixels) leaf-on (2016) and leaf-off (2014 to 2015) digital color infrared aerial photography is periodically collected for Texas through the National Agriculture Imagery Program (NAIP) and Texas Orthoimagery Program (TOP).
The FWS team used Random Forest classifications tree (RF) models optimized with Recursive Feature Elimination (RFE) to select a subset of important predictor variables from seasonal images. Spectral features included blue, green, red, and near-infrared bands, the normalized difference vegetation index (NDVI), and the difference between leaf-on and leaf-off NDVI values (dNDVI). Total, Ashe juniper, and broadleaf percent tree cover estimated at 1-hectare spatial scale showed a strong positive relationship (r2 = 0.93, r2 = 0.83, r2 = 0.73, respectively) with independent canopy intercept data collected within 100-m x 100-m field plots at a 10-m spacing with a moose-horn densitometer. Field plots (n = 98) collected in 2012 were limited to Balcones Canyonlands National Wildlife Refuge, Texas. The FWS team is collecting additional data from across the warbler’s breeding range to evaluate and refine digital tree canopy data that will be used to estimate warbler density. Mapped tree cover and warbler density data will help prioritize critical habitat areas for conservation and habitat improvement efforts in central Texas.
Tree cover mapped from multi-date digital color infrared aerial photography across central Texas counties. The area encompasses 85,500 km2 of potential breeding season habitat for Golden-cheeked Warblers.