Improving the Simulation of Carbon Cycle Fluxes in Arctic-Boreal Vegetation in the Community Land Model

Ongoing monitoring of CO₂ fluxes has revealed an increase in the seasonal amplitude, which is magnified in the Arctic-Boreal, coinciding with the amplified warming. The drivers of changes in the seasonal carbon cycle fluxes are complex, but in the terrestrial Arctic, can be attributed to increased plant productivity in the growing season and/or increased respiration through the shoulder seasons and winter. Since the scientific community lacks a full understanding of the underlying mechanisms, and the trend is spatially heterogeneous, we investigate circumpolar carbon cycle changes using the Community Land Model 5 (CLM5) at a half degree resolution. We benchmark model results using data from satellites and flux tower observations with a focus on representing plant functional type (PFT) specific seasonal CO 2 fluxes. We find a consistent tendency for the onset of productivity of deciduous plants to be late, the offset of productivity to remain abnormally productive in fall, and the overestimation of productivity of grasses and shrubs. We focus model development of alternate phenology and photosynthesis schemes at flux tower sites, chosen for a dominant vegetation type matching PFTs in CLM5. In particular, the onset of productivity is more accurate when based on soil temperatures, air temperature, and snow cover, rather than a soil temperature threshold. Then, allowing offset to vary with latitude, instead of a global offset timing, improved fall leaf offset. Finally, we adjust the temperature scaling of V _cmax and J _max , which are not optimized for the Arctic, biasing both the maximum productivity and phenology. These model adjustments all reveal that models tuned to more temperate or even tropical observations can easily be inaccurate when extrapolated to the Arctic. Thus, many land surface models may not capture the changes in CO₂ amplitude from high latitude terrestrial processes. With a more accurate seasonal cycle and maximum productivity, we can better examine the underlying mechanisms for the increase in the amplitude of the carbon cycle in the Arctic-Boreal. We outline a suite of land surface experiments, including changes in snow extent, fire regimes, and PFT distributions, to understand the Arctic-Boreal spatial variations and the critical shoulder season contrasts (spring vs autumn) in the carbon cycle.