Simulating Photosynthetic 13C Fractionation at a Western Subalpine Forest for Seasonal and Multi-Decadal Time Periods with the Community Land Model (CLM 4.5)

Abstract:

Terrestrial biosphere models are an important tool to diagnose and predict land-atmosphere exchanges of carbon and energy. This is critical in order to quantify the land-carbon feedback into the climate system. Ecological observations are extremely important in order to quantify model skill and to improve techniques in which to simulate ecosystem behavior. Isotope observations of ¹³C are especially useful in diagnosing the ecosystem response to water stress, atmospheric humidity and CO₂ fertilization. We test the representation of isotopes within CLM 4.5 against site level observations of biomass and carbon fluxes measured at Niwot Ridge, Colorado. First we ‘spun-up’ the model for 1800 years to approximate site level conditions during the 21^st century. We accomplished this by imposing a site-level reconstruction of seasonally varying atmospheric δ¹³C and atmospheric CO₂ from 1850-2013 and site level meteorological observations from 1998-2013. We also imposed an empirical-downscaling of simulated photosynthesis to reproduce the observed photosynthesis. We found that the simulated δ¹³C of biomass pools was more depleted than the observed by 1-2 ^o/_oo. This finding suggests the magnitude of photosynthetic discrimination was overestimated in the model. The model reproduced observed seasonal trends in discrimination with higher values in the spring and fall but lower values in the summer. However, if nitrogen-limitation is imposed in the model the photosynthetic downscaling influences the fractionation in such a way to obscure this observed trend. This suggests an alternative approach should be taken in order to account for nitrogen limitation. During the last century the model simulated an abrupt drop in δ¹³C for biomass pools, primarily because of the concurrent drop in atmospheric δ¹³C, but also because of increasing discrimination driven by increases in the ratio of intercellular to atmospheric CO₂. Finally, we identified photosynthetic rate, and vapor pressure deficit as two primary drivers for photosynthetic discrimination.