The direct flux of current assimilates from the tree canopy to the belowground compartment drives roughly half of the soil respiratory activity in boreal forests. This thesis focuses on temporal and spatial aspects of the carbon (C) flux within the tree-root-soil continuum in temperate and boreal forests. I used the stable isotopes 13C and 15N to follow C from the canopy to the belowground compartment and the flow of nitrogen in the reverse direction. The C isotope composition of photosynthate varies diurnally, but such variations could not be observed in soil-respired CO2. Labelling of small (up to 4.5 m) Pinus sylvestris trees with 13CO2 showed that it took two days for the photosynthate to reach the soil. The velocity of the phloem flux was c. 0.1 m h-1. This flux of C is absolutely vital for the production of sporocarps by ectomycorrhizal fungi, as shown by their paucity in plots with girdled trees. It is also likely to be important for other soil microorganisms; addition of a labile 13C labelled C source revealed a lack of labile C substrates in girdled plots. The reduction in the abundance of ectomycorrhizal sporocarps from the edges to the centre of girdled plots and a 15N uptake experiment showed that lateral spread of ectomycorrhizal roots was on average 4 to 5 m from the trunks. Thus, it can be expected that an area of c. 60 m2 of soil is under the influence of direct flux of current assimilates from the tree canopy of a single tree. Areas of influence of several trees overlapped. I conclude that canopy and soil processes are coupled with time lags of a few days. The direct impact of plant photosynthate should be considered more often in studies of soil. The 13CO2 labelling study demonstrated that it is now possible to follow at a very high resolution the fate of this C into the belowground system
