Due to the observed climatic shifts, the problem of correct estimates of the carbon sequestration capacity of forests and its possible temporal dynamics is being actualized. In plant ecology, tree root systems are the least studied. The purpose of this study was (a) to investigate, based on measurements carried out at 1782 sample plots, whether the effect of the law of the limiting factor is manifested when modeling the ratio of the belowground to the aboveground live biomass, i.e. the root-to-shoot ratio (R:S) of five forest-forming species in Eurasia as it relates to the geographically determined gradients of temperature and precipitation; (b) to show to what extent the climate-dependent models of R:S dependence on temperature and precipitation may be used to predict changes in R:S in temporal gradients based on the principle of space-for-time substitution and (c) to obtain the mean R:S values for forest-forming tree species (genuses) of Eurasia and perform their ranking. It has been established that, in cold regions, R:S increases with increasing precipitation, whereas upon transition to warm regions, one limiting factor (heat deficit) is replaced by another one (heat excess), and R:S dependence on precipitation changes to the opposite trend. In humid regions, R:S decreases with increasing temperature, whereas upon transition to dry conditions, one limiting factor (moisture excess) is replaced by another one (moisture deficit), and R:S begins to increase. The comparison of the above patterns with previously published ones, which relate to the aboveground biomass, suggest that they are directly opposite, i.e., the factors that limit the amount of the aboveground biomass are stimulatory for the R:S ratio, and vice versa. Our estimates of the typical R:S values for 24 Eurasian tree species range from 0.11 for dipterocarpus in Malaysia to 0.37 for ash in Europe.