Interannual variability of spring and summer monsoon growing season carbon exchange at a semiarid savanna over nearly two decades

Authors: Scott, Russell - Russ; JOHNSTON, M.R. - University Of Iowa; KNOWLES, J.F. - California State University; MACBEAN, N. - Western University; MAHMUD, K. - Midwestern University; Roby, Matthew; DANNENBERG, M.P. - University Of Iowa

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2023
Publication Date: 6/30/2023
Citation: Scott, R.L., Johnston, M., Knowles, J., MacBean, N., Mahmud, K., Roby, M.C., Dannenberg, M. 2023. Interannual variability of spring and summer monsoon growing season carbon exchange at a semiarid savanna over nearly two decades. Agricultural and Forest Meteorology. 339. Article 109584. https://doi.org/10.1016/j.agrformet.2023.109584.
DOI: https://doi.org/10.1016/j.agrformet.2023.109584

Interpretive Summary: Long-term measurements of carbon dioxide exchange between the land and atmosphere are needed characterize how agroecosystem carbon sequestration rates and how variations in the weather and management affect them. This study leverages nearly two decades of continuous measurements at a rangeland savanna site, one of thnd soil moisture control the variation in annual and seasonal gross and net carbon exchanges. Across the study period, soil moisture and vegetation increased with associated increases in the carbon exchanges. Typically, less than half of the variability in growing season photosynthesis and evaporation was captured by satellite-based estimates and land surface model simulations, highlighting the ongoing utility of long-term datasets to enable careful model testing and improvement.

Technical Abstract: Eddy covariance measurements of land-atmosphere energy, carbon, and water exchange now span multiple decades at some sites, supporting an improved understanding of flux interannual variability (IAV) and its ecophysiological and physical controls. Most eddy flux IAV studies have focused on temperate forest ecosystems, where carbon fluxes are large and flux records are longest – but also where IAV is much lower than in dryland regions, which are an essential driver of the trend and variability in the global terrestrial carbon sink. In this study, we leveraged 19 years of continuous micrometeorological measurements at the AmeriFlux US-SRM mesquite savanna site in southern Arizona, USA to quantify the IAV, trends, and drivers of carbon fluxes during the distinct spring and summer growing seasons. We also assessed the ability of modern satellite and land surface models to capture the IAV of seasonal water and carbon fluxes. Annual net ecosystem production (NEP) was small and highly variable (23 +/- 64 gC m-2 yr-1). Precipitation and associated measures of water availability determined most of the variability in NEP, largely through their influence on annual and seasonal gross ecosystem productivity (GEP) as opposed to ecosystem respiration (ER). Root-zone soil moisture captured between 73% (spring) and 85% (summer) of GEP variability and between 73% (spring) and 58% (summer) of ER variability. Across the study period, soil moisture and greenness increased with associated increases in GEP, ER and NEP. These trends were strongly influenced by very productive and wet summer growing seasons during the last two years, which were characterized by abundant understory grass cover. Typically, less than half of the variability in growing season GEP and evapotranspiration was captured by satellite-based estimates and land surface model simulations with local site forcing and calibration, highlighting the ongoing utility of long-term datasets to enable careful model testing and improvement.