Intense tree growth monitoring reveals increasing moisture limitation in a former treeline stand

Warming shifts isotherms upwards and previously cold-limited stands can reveal signs of moisture-limited growth. However, the pace of this transition and its characteristics are unclear. To bridge this gap, we present outputs of an intense monitoring effort of tree growth and phenology from a Picea abies stand located originally at the treeline in the Krkonoše Mts. at the Czech-Polish border. This site has been gradually lagging behind the advancing treeline isotherm. During a 12-year period between 2014 and 2025, we collected xylogenesis data, measured stem expansion and shrinkage using dendrometers, and monitored microclimate and leaf phenology. For each year, we derived critical growth dates (start, end, peak growth date), and mean and maximum growth rates for both xylogenesis and dendrometer data. In addition, we evaluated the time series of the tree water deficit. Our results show that despite high inter-annual variability, there was a trend towards a longer duration of xylogenesis, mainly associated with the extension of the cell wall thickening phase. Secondly, we found a reduction in the mean daily rate of cell formation. These trends observed at the cellular level were consistent with observations from dendrometers. The period of growth extended towards the end of the summer and the mean growth rates slightly decreased over time. Interestingly, tree water deficit increased over time with more frequent summer periods with negative climatic water balance and strongly negative soil water potentials. This was reflected in stem growth mainly in the driest years (2018, 2019, 2024) when growth cessation was the earliest within the entire period of monitoring. Our intense growth monitoring witnesses a transition from a strictly cold-limited treeline stand towards tree growth with occasional signs of moisture limitation. Although tree growth was unambiguously affected by drought only during the warmest and driest years, mean growth rates have been slightly declining due to increasing latent tree water deficits.