Phenology and growth responses of Fraser fir (Abies fraseri) Christmas trees along an elevational gradient, southern Appalachian Mountains, USA

Fraser fir (Abies fraseri) trees are endemic to locations above 1500 m in the southern Appalachian Mountains, and are also grown commercially for Christmas trees well below their native range (down to 600 m). To evaluate how phenology and growth of this species will respond to climate drivers associated with warming, we assessed the timing of bud-burst, shoot growth, trunk growth, as well as shoot- and leaf-level architecture, of Fraser fir Christmas trees along an elevational gradient from 664 to 1228 m. Daytime maximum temperatures and evaporative demand were highest at low elevation and cloud events and higher wind speeds occurred more often at high elevations. Bud-burst occurred 6 days sooner, new shoots ceased elongation 10 days sooner, and radial trunk growth ended 8 days later at low elevations than at high elevations, indicating a shift and lengthening of the growing season. Final shoot length did not vary among elevations, but the percent increase in trunk diameter was greatest at middle elevations. Architectural characteristics such as specific needle mass, needle packing density, and silhouette-to-projected area ratios generally did not vary with elevation. As climate change progresses, higher cloud ceilings, increased evaporative demand, and higher temperatures may further shift the timing of the growing season and reduce growth at low elevation Christmas tree farms, but farms at higher elevations may benefit from a longer growing season.