Impact of a 2°C Warmer Climate on the Fine Root System of European Beech, Sessile Oak, Scots Pine, and Douglas Fir in Central European Lowland Forests

Abstract Climate change increasingly exposes Central European forests to drought and heat stress, causing vitality decline and increased mortality of key tree species. How warming alters the size and dynamics of tree root systems is not well known. In a root coring and ingrowth core study in 24 stands, we compared fine root biomass (FRB), necromass, productivity, longevity and morphology in mature stands of European beech, sessile oak, Scots pine, and Douglas fir in two German lowland regions differing by 2 °C in mean annual temperature, which may evidence long-term thermal acclimation. FRB was significantly smaller in the warmer region in beech and Douglas fir, while cumulated fine root surface area was reduced in pine, Douglas fir, and beech, but not in oak. Both root productivity and longevity were reduced in the warmer region in Douglas fir and pine, indicating vulnerability to warming. Beech showed a non-significant productivity reduction, while longevity slightly increased. Oak tended to increase productivity and longevity, indicating highest resilience to warming. Fine root system size was more plastic than root morphology, which differed only slightly between the regions. Using reductions in root longevity and in fine root productivity in the warmer region as criteria, species are ranked for their belowground vulnerability to warming as: Scots pine > Douglas fir > European beech > sessile oak. We conclude that Central Europe’s major timber species differ largely in their belowground vulnerability to warming, with conifers being more sensitive than broad-leaved species.