Frost-Associated Defoliation in Populus tremuloides Causes Repeated Growth Reductions Over 185 years

Climate change is causing a shift to earlier spring budburst in deciduous temperate trees, which may predispose them to greater risk of damage from growing season frosts. Populus tremuloides may be particularly vulnerable to growing season frosts, which can defoliate entire canopies and damage woody tissue. These growing season frost events may shift in frequency under continued climate change and alter a disturbance regime that disproportionately impacts deciduous, cold-sensitive vegetation. We used Populus tremuloides tree cores from 14 sites across the montane and subalpine forests of Utah, USA, to identify characteristic tree rings, namely ‘white rings’, associated with growing season frost damage over the period 1767–2016. Early growing season frost was estimated using a frost intensity index using local data sourced from 22 long-term weather stations and was correlated with white ring occurrence. Over the last 150 years, the frequency of white rings was between 5 and 6% of all years. Growing season frost occurrence was similar in northern and southern Utah and highly synchronous across stands indicating widespread defoliation. Populus tremuloides radial growth declined nearly 40% during years of frost-associated defoliation. Our results highlight a severe disturbance that disproportionately impacts Populus tremuloides . Our methods are broadly applicable in identifying historical frost-induced defoliation in deciduous trees and emphasize the potential for discrete weather events to impact tree health Graphical Abstract