On the impact of canopy environmental variables on the diurnal dynamics of the leaf and canopy water and carbon dioxide exchange

Abstract. Quantifying water vapor and carbon dioxide exchange dynamics between land and atmosphere through observations and modelling is necessary to reproduce and project near surface climate in coupled land-atmosphere models. The exchange of water and carbon dioxide (CO₂) occurs at the leaf surfaces (leaf level) and in a net manner through the exchanges at all the leaf surfaces composing the vegetation canopy and at the soil surface (canopy level). These exchanges depend on the meteorological forcings imposed by the overlying atmosphere (atmospheric boundary layer level). In this manuscript, we investigate the effect of four canopy environmental variables (photosynthetic active radiation (PAR), water vapor pressure deficit (VPD), air temperature (T) and atmospheric CO₂ concentration (C_a)) on the local individual leaf exchange and canopy exchange of water and CO₂ at hourly time scales and the effect of atmospheric boundary layer (ABL) processes on the local exchange.

To that end, we simultaneously investigated the exchanges of water and CO₂ at leaf level and canopy level for an alfalfa field in Northern Spain during a day in the summer of 2021. We used comprehensive observations ranging from stomatal conductance to ABL measurements collected during the Land Surface Interactions with the Atmosphere in the Iberian Semi-Arid Environment (LIAISE) experiment. To support the observational analysis, we used an integrative mixed-layer atmospheric model (CLASS) that have representations at all considered levels. To relate how temporal changes of the four environmental variables modify the fluxes of water and CO₂, we studied tendency equations of the leaf gas exchange. These mathematical expressions quantify the temporal evolution of the leaf gas exchange as a function of the temporal evolution of PAR, VPD, T and C_a. To investigate the effects of ABL processes on the local exchange, we developed three modelling experiments that impose surface radiative perturbations by a cloud passage (which perturbed PAR, T and VPD), entrainment of dry air from the free troposphere (which perturbed VPD) and advection of cold air (which perturbed T and VPD).

Model results and observations matched the leaf gas exchange (with r² between 0.23 and 0.67) and canopy gas exchange (with r² between 0.90 and 0.95). The tendency equations of the modelled leaf gas exchange during the studied day revealed that the temporal dynamics of PAR were the main contributor to the temporal dynamics of the leaf gas exchange with atmospheric CO₂ temporal dynamics being the least important contributor. From the three modelling experiments with ABL perturbations, the surface radiative changes induced by a cloud perturbed the CO₂ exchange the most, whereas all of them perturbed the water exchange to a similar extent. Second order effects on the dynamics of the leaf gas exchange were also identified using the tendency equations. For instance, the decrease of net CO₂ assimilation rate during the cloud due to a decrease in surface radiation was further enhanced due to the decrease in air temperature also associated with the cloud. With this research we showcase that the proposed tendency equations can disentangle the effect of environmental variables on the leaf exchange of water and CO₂ with the atmosphere as represented in land-surface parameterization schemes and become a useful tool to analyze these schemes in weather and climate models.