Links Between Carbon and Water Cycles in Northern Ecosystems: Constraints from Stable Isotopes

Abstract

High-latitude climate change will have an impact on the carbon and water cycles in northern ecosystems. Stable isotopes in these systems can serve as indicators of changes and feedbacks. Monitoring the stable isotopic composition of Arctic river discharge provides a means to investigate integrated basin-scale hydrologic changes in remote northern regions. I measured water δ¹⁸O and δD from the Kolyma River in Siberia and local precipitation to partition the river flow into 60% snow and 40% rain inputs. Comparing this estimate with seasonal precipitation across the watershed showed a significant portion of snowmelt is retained in the soils of this permafrost dominated region, and contributes to ~40% of the growing season transpiration. The seasonal cycles of atmospheric CO₂ and δ¹⁸O-CO₂ at high northern latitudes have the potential to serve as indicators of ecological change. Effective interpretation of atmospheric observations requires an understanding of how different species and ecosystems contribute to biosphere-atmosphere exchange. By combining isotopic signatures of ecosystem water pools with measured CO₂ fluxes in three stands of an Alaskan boreal fire chronosequence (recent burn, intermediate-aged deciduous and mature evergreen forests), I compared the relative effects of stand age on the phase and amplitude of the seasonal cycles of CO₂ and δ¹⁸O-CO₂. Higher rates of mid-summer net carbon exchange and a shorter growing season at the deciduous stand resulted in the largest seasonal CO₂ amplitude and also delayed the drawdown of atmospheric CO₂ compared to the evergreen stand. Reduced levels of photosynthesis at the deciduous stand early in the growing season caused atmospheric δ¹⁸O-CO₂ to increase more slowly compared to fluxes from the evergreen stand. The distribution of stand ages in northern boreal forests is likely to determine the response of net ecosystem exchange (NEE) to future climate changes. I used three years of NEE measurements from the Alaskan fire chronosequence to determine that the sensitivity of growing season NEE to spring air temperatures and summer drought was greater at the deciduous forest than the evergreen forest. As forest fire disturbance increases due to climate warming, the shift to younger forests should increase interannual variability in atmospheric CO₂ concentrations.