Nighttime Warming Enhances Tree Growth in Temperate Tree Species

Asymmetric warming in temperate regions differentially impacts tree growth depending on whether daytime or nighttime temperatures increase. To elucidate the underlying mechanisms and species-specific responses, we investigated five temperate broadleaf tree species (Juglans mandshurica, Phellodendron amurense, Fraxinus mandshurica, Betula platyphylla and Tilia amurensis) exhibiting contrasting water status regulation strategies and differing in mycorrhizal types. Seedlings of the five species were subjected to five temperature treatments: ambient control, daytime warming (+2°C, DT+2 and +4°C, DT+4) and nighttime warming (+2°C, NT+2 and +4°C, NT+4). Radial growth and xylogenesis were monitored throughout the growing season. Notably, only NT+2 significantly increased ring width (+99.42%) and theoretical hydraulic conductivity (+260.58%) across all species. Nighttime warming enhanced radial growth by increasing the number of cambium and radially enlarging cells, while daytime warming extended the xylem formation period. Furthermore, mean ring width (MRW) increased significantly with radially enlarging cells (maxEC) under nighttime warming. This response pattern was particularly pronounced in arbuscular mycorrhizal and isohydric species, contrasting with ectomycorrhizal and anisohydric species. Collectively, our results demonstrate that the effects of asymmetric warming on tree growth are contingent upon both the amplitude and timing of temperature increases. These findings provide critical mechanistic insights into how asymmetric warming influences tree growth and forest productivity, aiding predictions of carbon sequestration potential under climate change.