COS 165-5 - Fine root ecosystem respiration in northern red oak declines across a Midwest, USA latitudinal gradient

Abstract: The terrestrial carbon sink sequesters carbon dioxide from the atmosphere in plants and soils, countering anthropogenic emissions. However, plant respiration is a significant carbon flux out of this sink, and in the absence of acclimation or adaptation, this flux is typically predicted to increase more than photosynthesis as the climate warms. Support has commonly been found for thermal acclimation of leaf respiration, but whether this is possible in fine roots has been less clear. Beyond physiological changes, temperature responses may be moderated by fine root traits, which are hypothesized to follow a trade-off between nutrient conservation and acquisition, or biomass shifts. This study aimed to understand: (1) if respiratory responses to temperature are moderated by root economics spectrum trade-offs; and (2) whether northern red oak (Quercus rubra), a widespread and economically important species, can conserve carbon by downregulating its specific fine root respiration rates in response to higher Mean Annual Temperature (MAT). To assess differences in fine root traits and respiration across temperatures and to determine if downregulation is present, we sampled Q. rubra fine roots from 14 sites across the Midwest of the USA, representing a MAT gradient of 4-14 °C. Specific respiration rates at ambient and constant temperatures, root morphological traits, and root nitrogen content were measured on excised fine root samples. Fine root biomass measurements were then used to scale up to ecosystem fine root respiration. Relationships between fine root traits and respiration agreed with the root economics spectrum trade-offs, with the acquisitive traits of specific root length and area, fine root nitrogen, and specific respiration rates highly correlated with each other and opposite to the conservative trait measured, root tissue density. Though specific respiration rates themselves did not show a simple downregulation response to MAT, a shift toward more conservative traits and a decline in biomass from north to south resulted in lower fine root ecosystem respiration at our warmest sites. Earth system models generally assume that plant respiration will exponentially increase with temperature at a fixed rate, leading to the prediction that trees will retain less carbon for growth and that forest carbon sinks will lessen with warming. However, results presented here suggest that changes in biomass and nutrient foraging strategies, rather than solely in intrinsic respiration rates, may in some cases combine to prevent this outcome.