A fingerprint of climate change across pine forests of Sweden

Needle traits of coniferous forests reflect environmental conditions and influence tree physiology and growth. Given the sensitivity of needle traits and tree growth to climate, temperature warming of ≈1°C in the past century may have influenced structure and function of high latitude forests across the globe. Here we show that throughout a ≈1,000 km transect in cold, high latitude Scots pine (Pinus sylvestris L.) forests in Sweden, which has warmed by ≈1°C in a century, needles today (2012-2017) are 9-19% longer and have 30-31% shorter life-times than in 1914-15. These century-scale shifts in needle traits were detected by sampling 74 sites from 2012-2017 along the same transect first assessed at 57 sites in 1914-1915. Geographic variation in temperature strongly explained spatial patterns of needle length and longevity in both the early 20th and 21st centuries, with Scots pine from warmer sites further to the south having longer needles and shorter needle life-times than at colder, more northern sites. Moreover, the warming of climate along the transect in the past century has likely driven the temporal shift towards longer needles with shorter life-spans. The spatial and temporal variation in needle traits is both cause and consequence of variation in Scots pine tree growth rates, which tend to be higher in warmer times and places. These century-scale changes in Scandinavian Scots pine needles represent a fingerprint of climate change on a fundamental biological element, the leaf, which are likely to be mirrored by similar changes for evergreen conifers across the boreal biome.