Drought intensity controls carbon allocation dynamics within experimental Scots pine-soil systems

Climate change is causing negative effects on forests and their functioning through more frequent and intense periods of drought. Repeated conditions of water limitation not only affect the growth and vitality of trees but also feed back on the cycling of carbon (C) at the plant-soil interface. However, the impact of the intensity of drought on the transfer of assimilated C belowground remains quantitatively unresolved. We assessed how increasing levels of soil water limitation affect the growth of Scots pine (Pinus sylvestris L.) saplings and performed a ¹³C-CO₂ pulse labelling experiment to trace the pathway of newly-assimilated C into needles, fine roots, soil pore CO_2, and phospholipid fatty acids of soil microbial groups. We hypothesized that increased water stress would reduce tree C uptake, and the magnitude and velocity at which newly-assimilated C is allocated belowground and further metabolized. Moreover, we expected that severe levels of soil water deficit would lead to a build-up of newly-assimilated C in fine roots. Our data indicated that with more intense water limitation, trees reduced their growth despite initially partitioning more biomass to belowground tissues under severe water stress. Moderate levels of water limitation barely affected the uptake of ¹³C label and the magnitude and transit times of C being allocated from needles to the rhizosphere. In contrast, severe water limitation increased the fraction of ¹³C label allocated to roots and soil fungi while a lower fraction of ¹³CO₂ was respired from the soil. We conclude that when soil water becomes largely unavailable, C cycling within trees becomes slower, and a major fraction of C allocated belowground is accumulated in roots or transferred to the soil and associated microorganisms without being metabolically used. Our experiment overall demonstrates the relevance of quantifying the level of water limitation at which C allocation dynamics within trees and soils are altered to inform about the trajectory of forests to the environmental pressures they face.