Number of growth days and not length of the growth period determines radial stem growth of temperate trees

Radial stem growth dynamics at seasonal resolution are essential to
understand how forests respond to climate change. We studied daily radial
growth of 160 individuals of seven temperate tree species at 47 sites
across Switzerland over eight years. Growth of all species peaked in the
early part of the growth season and commenced shortly before the summer
solstice, but with species-specific seasonal patterns. Day length set a
window of opportunity for radial growth. Within this window, the
probability of daily growth was constrained particularly by air and soil
moisture, resulting in intermittent growth to occur only on 29 to 77 days
(30 to 80 %) within the growth period. The number of days with growth
largely determined annual growth, whereas the growth period length
contributed less. We call for accounting these non-linear intra-annual and
species-specific growth dynamics in tree and forest models to reduce
uncertainties in predictions under climate change.