Frequency-dependent climate sensitivity of sub-daily radial growth of tree stems at the dry Arctic treeline.

Radial growth of tree stems shows remarkable variability over the year but also within individual days. Understanding the frequency-dependent growth sensitivity, i.e., the shifting responses of wood formation to meteorological conditions between annual and daily temporal scales, is essential for predicting forest growth under future climates. However, this knowledge is limited for the cold Arctic margins of species distribution. To address this gap, we monitored intra-annual and sub-daily stem radius variation of Pinus sylvestris L. for 2 years using xylogenesis microsampling and dendrometers at the cold-dry treeline beyond the Arctic Circle near Abisko, Northern Sweden. Using linear statistics, cell growth modeling and wavelet transformation, we separated individual frequencies of stem oscillations, ranked their statistical importance and identified immediate and lagged meteorological drivers of stem radius increment. Radial growth of tree stems showed overlapping oscillations at annual and daily frequencies. On an annual scale, radial growth and cell production peaked during the warm summer months. However, within the summer, stem radius increment accelerated toward the cool, moist midnight hours and ceased during the day as the temperature and solar altitude increased. Accordingly, we identified two growth-optimal intervals with peak growth rates, jointly accounting for 68% of the total growth: (i) an air temperature of 8-16 °C with a vapor pressure deficit (VPD) of <0.2 kPa, and (ii) an air temperature of 4-16 °C and VPD = 0 kPa. While correlations between peak-summer growth rates and immediate temperature and VPD were negative, these correlations switched to positive when considering lagged meteorological variables preceding growth by up to 4 days. Our results suggest frequency-dependent shifts and lagged responses of radial growth to meteorological variables at the Arctic treeline, particularly for air temperature and humidity. We propose that the midday growth reduction during summer may help explain non-linear responses of northern boreal forests to recent climate warming.