PubMed · 2026-07-01
Scots pine trees near the Arctic Circle grow fastest not at midday in summer, but during the cool, moist hours around midnight. This frequency-dependent growth pattern, driven by temperature and humidity, may explain why boreal forests aren't responding to warming as simply as climate models predict.
Stem radius increment peaked during cool, moist midnight hours (air temp 4-16°C, vapor pressure deficit near 0 kPa), with two optimal growth windows jointly accounting for 68% of total annual growth.
Immediate correlations between summer growth rates and temperature/humidity were negative, but those correlations flipped to positive when accounting for meteorological conditions up to 4 days prior.
Peak wood cell production occurred in warm summer months on an annual scale, but within summer days, midday warmth and rising solar altitude consistently suppressed radial growth.
