Deterministic and stochastic components of atmospheric CO2 inside forest canopies and consequences for predicting carbon and water exchange

Atmospheric CO2 concentrations strongly influence the exchange of energy, water and carbon between the atmosphere and the terrestrial biosphere. The CO2 available to plants can be highly variable given the stochastic nature of the phenomena involved in its dynamics. However, most terrestrial ecosystem models consider CO2 concentration as a quasi-deterministic variable or use measurements taken from heights above the canopy where CO2 is usually well mixed. Therefore, in this study, we aimed to evaluate if a stochastic treatment of CO2 concentrations leads to different predictions of carbon assimilation (An) and transpiration (T) compared to a strictly deterministic approach, as well as the use of CO2 measurements from different heights. To address this goal, we used a set of observations taken in forests from the ICOS network and the ATTO research site. We applied the exponential smoothing method to decompose the time series into their trend, seasonal and stochastic (residual) components. We found that the residual component of CO2 (rCO2) follows a Laplace probability density function and its stochastic magnitude is inversely proportional to the height above the ground. To quantify the degree to which predictions of An and T would be affected by stochastic effects, we ran simulations considering the different components of the time series and Farquhar's and Penman-Monteith's models. We found significant differences in the predictions of An and T for diverse heights, with larger An (T) fluxes closer (farther) to the ground. Still, this effect is mainly due to the deterministic component that increases with decreasing height. The stochastic component tends to reduce (increase) An (T) compared to the deterministic approach. However, the difference between approaches is not large enough to compensate for the deterministic effect, suggesting that there is no merit for the consideration of rCO2 in future simulations as long as measurements taken inside the canopy are used.