Spatial patterns of global-scale forest root-shoot ratio and their controlling factors

The forest root-shoot ratio (R/S), i.e., the ratio of belowground to aboveground biomass at the stand level, is widely used in global and regional forest carbon stock estimation and in modeling of the forest carbon cycle. Despite recent advances in understanding forest R/S variations at the individual-tree level, spatial patterns of stand-level forest R/S ratio across the globe and their driving factors remain relatively unknown. Here, we compiled and analyzed an extensive dataset from 873 forest sites worldwide, analyzed and quantified the effects of major environmental and forest growth-related variables on the stand-level R/S ratio. Based on this analysis, we further mapped the spatial pattern of the global forest R/S ratio. Our results show that, globally, variations on the stand-level forest R/S ratio are largely affected by canopy height, latitude, climatic water deficit, forest type and regeneration method, which collectively explain 37% of the variations in R/S ratio. In addition, our results suggest significant intercontinental and national variations in forest R/S ratio. At the continental scale, forest R/S ratio is highest in Oceania and lowest in South America. At the national scale, Australia has the highest forest R/S ratio while Russia has the lowest values. The forest R/S ratio is generally lower in moist tropical regions, but increases when moving to the extra-tropics when seasonality in precipitation increases. The R/S ratio in temperate and boreal regions shows prominent spatial features regulated by forest species composition and regeneration method. We conclude that future changes in environmental, biotic and anthropogenic factors, such as increased climatic water deficit and forest management, might influence the forest R/S ratio, with implications for the global and regional land carbon cycle.