Print PageIt is widely cited that xeric-adapted plants have higher root-to-shoot ratios and partition relatively more of their roots deeper in the soil profile than structurally comparable mesic species. Similar patterns are also expected along soil moisture gradients within landscapes, largely based on predicted changes in resource availability. Surprisingly little work has been done to validate these assumptions at the landscape scale, and most work to date is restricted to forested ecosystems. With the widespread implementation of grassland restoration for the enhancement of ecosystem services and the provisioning of biomass, differences in carbon allocation among grasslands growing under different soil moisture conditions could significantly impact the benefits of these grasslands for carbon sequestration. Our objective was to determine the effects of soil moisture on root-to-shoot partitioning and rooting depth distributions in a temperate tallgrass prairie. We hypothesized that aboveground production would increase and root biomass would decrease with increasing soil moisture, thereby decreasing the root-to-shoot ratio. Further we hypothesized that a larger proportion of total root biomass would be allocated toward the surface as soil moisture increases. We tested these hypotheses using six plots located along a soil moisture continuum in a 30-year old restored native tallgrass prairie in Wisconsin, USA.
Results/Conclusions
We found no significant trend in aboveground biomass across the soil moisture gradient. In contrast, live root biomass was greatest near the middle of the moisture gradient, and lowest near the two extremes, illustrating an apparent optimal environmental window for root biomass. Live root-to-shoot ratios formed a weaker positive parabolic curve along the same gradient. Decay coefficients fit to the vertical distribution of live roots were not significantly predicted by plot soil moisture. Our results suggest that the tallgrass prairie supported consistent aboveground production across the soil moisture gradient, despite concurrent changes in live root biomass. However, in contrast to our expectations, we found significant investment in both total and deeper root biomass even in our wettest plots, with clear implications for soil carbon storage. This pattern may be partially explained by the high functional diversity and the concurrent changes in dominant species along the soil moisture continuum. Carbon budgets and models may need to be revised to consider the dynamic effects of soil moisture on root biomass in tallgrass prairie and other diverse grassland ecosystems.
