Roots have transformed Earth’s surface for millennia by acting as material conduits and weathering agents. Deep roots remain understudied in spite of increasing recognition that they represent disproportionately large contributors to ecosystem functions. Deep root networks are transitioning globally for two reasons. Roots are shallower where perennial, deeply rooted species are replaced with more shallowly rooted, annual crops during land conversion to agriculture. Additionally, climate and land management changes are prompting shifting species ranges that can induce changes in a region’s rooting depths, as well as changing rooting depths of extant species. In spite of well-quantified changes in global land cover known to be associated with distinct rooting depths, no estimates of rooting depth change and the spatial extent of its potential consequences exist.
We quantify for the first time the extent to which potential rooting depths are truncated or extended in response to global change factors. Using spatially explicit estimates of potential and contemporary vegetation and predicted rooting distributions, we estimate changes in rooting depth from pre-agricultural times to today, and project rooting depth changes by the end of this century. Depending on analyst assumptions, we observed up to a 12% decrease in average global rooting depth between potential vegetation and contemporary land cover, equivalent to a shallowing of as much as 20 cm. This change is the net result of widespread root truncation of ~34% where potential vegetation is replaced with crops, and regional root expansion of ~57% due to woody encroachment into grasslands and tundra. Under four Representative Concentration Pathway scenarios (i.e., RCP 2.6, 4.5, 6.0 and 8.5) we estimate that global average rooting depth may become up to 1.3% yet shallower by 2100, but could increase by 1.2 cm under a stabilization scenario. Given the influence of deep roots on C cycling, hydrology, and soil development; anthropogenic impacts on deep roots have meaningful implications for Earth systems in the Anthropocene. The direct effect of land cover change has already initiated deep subsurface shifts, and these trajectories will be driven more indirectly by perturbations such as climate change in the coming decades.
Hauser, Emma, Pamela L Sullivan, Sharon A Billings (2019): Rooting Depths Change on a Global Scale in the Anthropocene. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.
This Paper/Book acknowledges NSF CZO grant support.