Climate dominance gives way to litter chemistry during long-term Scots pine needle decomposition along a boreal-to-temperate climate gradient.

The decomposition of needle litter is governed by a complex interplay between climatic conditions and substrate chemical properties, yet how their relative influence shifts during decomposition remains poorly resolved. We synthesized rate-regulating factors for long-term decomposition of Scots pine (Pinus sylvestris) needle litter across a 15-site boreal-to-temperate climatic gradient (mean annual temperature − 0.7 to 6.3 °C). Annual mass loss was analyzed across four decomposition categories, alongside concurrent measurements of acid-unhydrolyzable residue (AUR) and nitrogen, phosphorus, potassium, calcium, magnesium, and manganese in all samplings. We found that mean annual mass loss declined significantly with increasing decomposition stage, from 30.2% in the early stage to 15.8% in the late stage. Climate, particularly actual evapotranspiration, exerted a strong positive effect during early decomposition but weakened progressively and became negligible in the late stage. In contrast, the influence of litter chemistry increased with decomposition, with stage-specific effects of AUR and nutrients reflecting shifts in substrate quality and availability. The total explanatory variance of measured variables declined from ~ 75% in early stages to ~ 39% in advanced decomposition, indicating increasing complexity and reduced predictability. The relative importance of controlling factors shifted from climate dominance to substrate regulation, with different elements contributing variably across stages. These results provide empirical evidence for stage-dependent controls on litter decomposition and help reconcile contrasting views on the roles of climate and litter chemical traits. Incorporating such temporally explicit dynamics into biogeochemical models will improve predictions of litter-driven carbon and nutrient cycling. Future integration of microbial and enzymatic processes will be essential to better resolve the mechanisms underlying these dynamics and to enhance model realism under changing environmental conditions.