Altitudinal effects on intra-annual xylem formation characteristics and environmental response in Picea crassifolia on the northeastern Tibetan Plateau

Understanding the growth-climate response of Qinghai spruce ( Picea crassifolia ) is crucial for the development of forest ecosystems on the Tibetan Plateau. With the continuous changes in global climate, how the intra-annual radial growth of Qinghai spruce responds to changing environmental conditions along the altitudinal gradient remains poorly understood. In this study, we collected microcores every 5 days along an altitudinal gradient from 2700 to 3100 m. We analyzed seasonal patterns of xylem formation and the characteristics of intra-annual radial growth rate in response to environmental factors at different altitudes. We found that 1) the start of growth in Qinghai spruce is significantly correlated with altitudes, with growth start 3 days earlier for every 100 m decrease in altitudes. However, growth end showed no significant difference across altitudes. 2) The timing of maximum growth rate differed along altitudes. For every 100 m decrease in altitude, the time of maximum growth rate advances by 5 days, with occurrence before the summer solstice at low altitude, while after the summer solstice at middle and high altitudes. 3) The growth rates at middle and high altitudes are primarily driven by precipitation and soil temperature. In contrast, growth rates at low altitude are influenced by relative humidity and mean temperature, while also being strongly affected by non-environmental factors such as tree age, non-structural carbohydrates, and human activities. Our findings provide new data on the intra-annual growth dynamics of conifers and help quantify the potential impacts of climate warming on tree growth in mountain forest ecosystems. • Earlier growth started in Qinghai spruce at low altitude. • The time of maximum growth rate is progressively delayed with increasing altitude. • Environmental factors influence Qinghai spruce growth at middle and high altitudes. • Both environmental and non-environmental factors influence growth at low altitude.