Phenology advances uniformly in spring but diverges in autumn among three temperate tree species in response to warming

Predicting the length of growing season to further warming relies on both spring and autumn phenological responses of different species, but with little experimental evidence under different levels of warming. Here, the timing of spring phenology including budburst and leaf unfolding, and autumn phenology including leaf coloration and leaf fall, were examined in 2-year-old seedlings of three temperate tree species (Fraxinus mandschurica, Quercus mongolica, and Juglans mandshurica) that were grown in the field under warming conditions (W0, ambient; W2, +2°C; W4, +4°C). Our study showed that warming advanced budburst and leaf unfolding by 9 and 8 days in the W2 treatments and by 13 and 15 days in the W4 treatments, respectively, with the magnitude varied greatly among species. However, we found a delayed autumn leaf senescence only for Q. mongolica, for which leaf coloration and leaf fall delayed by 19 and 6 days and by 17 and 12 days in the W2 and W4 treatments, respectively. The temperature sensitivity of budburst declined with increasing temperature in Q. mongolica. The growing season was extended by 7–18 and 10–25 days in W2 and W4, primarily through the advancement of spring phenology. Warming increased budburst synchrony, such that J. mandshurica, which typically has the earliest leaf emergence, lost its growth and competitive advantages over the more phenologically plastic species, F. mandschurica and Q. mongolica. Our results provide new insights into the effects of elevated temperatures on phenological shifts and their potential influences on tree species recruitment and forest structure and composition.