The role of semi-arid ecosystems in the global carbon cycle
Much of my past research has shown that semi-arid ecosystems are not well represented in land surface/terrestrial biosphere models. This is crucial, given these ecosystems are in transition zones that are vulnerable to climate change and land degradation. Furthermore, recent high-profile studies have shown that semiarid ecosystems are playing a dominant role in global C cycle inter-annual variability (IAV) and its long-term trend.
However, the models used in the aforementioned high-profile studies have not yet been widely tested against in situ C and water flux data.
The first step was to check how well the models capture water availability given that is dominant control on semi-arid ecosystem processes. In the following paper, I showed that models with a mechanistic representation of soil moisture diffusion perform well at simulating daily to seasonal evapotranspiration (ET) dynamics due to more realistic fluctuations of upper layer soil moisture. Therefore, model underestimates in net CO2 flux are likely due to deficiencies in photosynthesis and phenology related processes:
- MacBean, N., R.L. Scott, J.A. Biederman, C. Ottlé, N. Vuichard, A. Ducharne, T. Kolb, S. Dore, M. Litvak and D.J.P. Moore (2020), Testing water fluxes and storage from two hydrology configurations within the ORCHIDEE land surface model across US semi-arid sites, Hydrology and Earth System Sciences, 24, 5203-5230.
In a recently submitted paper, I have shown at SW US semiarid sites that TBMs included in the TRENDY model intercomparison (used in the annual global C budget and in IPCC simulations) underestimate both the mean annual net CO2 fluxes and their IAV. Therefore, the role of semiarid ecosystems in global C cycle IAV could be more important than previously thought. You can see these results here!
- MacBean, N., R. L. Scott, J. A. Biederman, P. Peylin, T. Kolb, M. Litvak, P. Krishnan, T. Meyers, V. Arora, V. Bastrikov, D. Goll, D. L. Lombardozzi, J. Nabel, J. Pongratz, S. Sitch, A. P. Walker, S. Zaehle, and D. J. P. Moore (2021), Dynamic Global Vegetation Models Underestimate Net CO2 Flux Mean and Inter-Annual Variability in Semiarid Ecosystems. Submitted.
In this study, I showed that model water use efficiency (WUE) is too low: the GPP is not sensitive enough to pulses of moisture availability during the peak growing season. But we need to dig into more detail in the models to figure out what is causing discrepancies in these ecosystems. The subject of future work!
In a soon-to-be-submitted paper, Lab Member Kashif has optimized the C cycle related parameters at the same SW US sites and shows a dramatic improvement in the ORCHIDEE model’s ability to capture NEE, which is very exciting finding! He shows that the model improvement is mainly due to constraining parameters related to photosynthesis and phenology processes. Find out more about this great work here!
My current work (not yet published) aims to better understand which processes and intra-annual time periods are driving net CO2 IAV in semi-arid ecosystems so we can better. Analyzing eddy covariance CO2 flux data from range of semiarid sites in the southwestern US encompassing forest, shrub- and grass-dominated ecosystems, I have shown that gross CO2 uptake during the monsoon dominates the net CO2 flux (net ecosystem exchange – NEE) IAV signal. Furthermore, unlike mesic ecosystems, my analysis demonstrates that both days with peak productivity and growing season length are important controls on NEE IAV. I am collaborating in this work with ecohydrologists from the USDA and Ameriflux site PIs.
In addition to this modeling and site analysis work, and as part of this wider research goal, we have also published a paper examining how well different satellite products capture seasonal vegetation dynamics in the semi-arid southwestern US: