Earlier leaf out and loss of cold tolerance for northeastern U.S. trees in response to winter warming events and early springs

O_LIWinter warming events and early springs can advance spring phenology of trees. However, these events can also decrease cold tolerance, leaving them vulnerable to subsequent refreezing. The goal of this study was to determine how spring phenology and cold tolerance of ten tree species in the northeastern U.S. responded to warming events of different timing and duration. C_LIO_LIWe exposed 300 tree seedlings of ten species to six different warming treatments. We assessed budburst, leaf out, vigor, and freezing damage on a weekly basis and measured the cold tolerance of species exposed to warming. C_LIO_LIThe timing of leaf out varied among species, and generally warming treatments advanced leaf out 5 - 41 days. Leaf out was best predicted by photoperiod for deciduous trees and thermal time for evergreen trees. Cold tolerance was reduced for most species (5 - 29 {degrees}C) when exposed to only two weeks of warming, reducing their safety margin from freezing damage. C_LIO_LIWinter warming events and early springs will drive species-specific and foliar habit responses in phenology and cold tolerance that are likely to shape competitive forest dynamics, especially at forest ecotones. C_LI