PS 32-130 - Fine root respiration of Quercus rubra: Root system activity across a latitudinal gradient in the Midwest

Background/Question/Methods

Northern red oak (Quercus rubra) is an important hardwood species and plays a key role in carbon sequestration. Latitudinally in the Midwest, mean annual temperature (MAT), growing season, and precipitation vary widely, and a species like Q. rubra, growing across latitudinal gradients, may exhibit differential growth and physiology. Across the Midwest, I investigated how latitude affects fine-root (< 0.5 mm) respiration, nitrogen concentration, and biomass of Q. rubra. At the ecosystem level, root respiration is an important component of carbon cycling, and our research aims to learn more about this process in Q. rubra and implications for a changing climate.

Six sites were established from upper Michigan to southern Illinois and Indiana, and were ranked by their MAT as cold (4.1 - 4.9 ºC), intermediate (5.4 - 9.8 ºC), and warm (11.3 - 14.2 ºC). Each site was visited in both June and July 2021, and root specific respiration was measured at ambient soil temperature beneath Q. rubra trees at six locations per site and a reference temperature of 20º C. Each sample was later assessed for dry mass and nitrogen concentration. Respiration rates were compared to root biomass, root nitrogen concentrations and site environmental variables using regression analysis.

Results/Conclusions

Results from the June 2021 sampling show higher root biomass was significantly correlated to lower specific root respiration (per unit root mass). Root nitrogen concentration also increased with increasing root specific respiration. Several of these results were also associated with MAT, with higher root biomass being found at colder sites, along with lower root nitrogen concentration. The warmer sites typically had lower root biomass and higher root nitrogen concentration.

Colder sites typically have shorter growing seasons, therefore requiring the fine-root system to do the same amount of work within less time than available in warmer climates, which could explain the increase in root biomass. This would align with the findings where higher root biomass correlates with decreased rate of specific root respiration (per unit root mass). This trade-off between biomass and respiration rate results in fairly similar allocation of carbon to fine-root respiration at the ecosystem level (i.e. per unit ground surface area) across the sites. These findings could provide insight into how Q. rubra may respond to warming temperatures and changing climates.