Flipping through field notebooks…
High-elevation ecosystems are warming faster than surrounding lowlands. In the Elk Mountain range of the Rocky Mountains, earlier snowmelt and a weakening summer monsoon are projected to lengthen the foresummer drought, the dry window between snowmelt and monsoon onset. Foresummer drought is known to control productivity in subalpine meadows, but the riparian willow (Salix spp.) shrublands that line valley bottoms have never been assessed directly: satellite studies are too coarse of scale and fine-scale mapping has been non-temporal. I combined a six-year record of drone multispectral and thermal imagery (2021–2026, 116 flights) over the Gothic townsite at the Rocky Mountain Biological Laboratory with field measurements of species identity and functional traits at 144 willow crowns to ask where willow sensitivity to foresummer drought is highest and whether topography, species, and traits explain that pattern. Drone and field measurements agreed well (R² = 0.85), and the six-year decline measured on hand-delineated crowns matched the automated segmentation (r = 0.979). Willow canopies declined dramatically since 2021: median canopy height fell 36% (81.7% of 27,991 segments declining), resulting in an implied leaf-area loss of ~20% (median LAI 2.72 → 2.17 m² m⁻²). The decline correlated with neither same-year climate (p = 0.85) nor any measured terrain variable (spatially blocked R² = 0.011); only previous-year climate response was suggestive (p = 0.082). Greenness carried almost no structural information, but showed a pheneological response: Dry years had a shift in timing of green-up by roughly two weeks (r = +0.95) while season length was unaffected and varied between (97–102 days), integrated greenness stayed essentially the same across wet and dry years. Proportional decline differed by species, with taller wet-site species losing height up to seven times faster than short dry-site species (10.9 vs 1.5% per year, p = 0.011), while no measured trait predicted decline within species. These results help to identify which willow species are vulnerable as the foresummer lengthens. Because willow species account for a large portion of summer evapotranspiration and supply the habitat and food needed for bird and beaver species, a loss of one third canopy median height in 6 years is important to continue monitoring. Furthermore, this information can be used to inform future sampling, characterization, and modeling studies.
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