Overlooking the canopy: The importance of canopy structure in scaling isoprenoid emissions from the leaf to the landscape

Isoprene and monoterpenes are highly reactive organic compounds, emitted by most plant species, which play an important role in air chemistry and air pollution. Different leaf-scale isoprenoid emission models are available. These models are scaled to the canopy through coupling them to terrestrial biogeochemical models and thus used to generate regional emissions inventories. Although the leaf scale models have been shown to perform similarly, large unexplained differences exist in regional emissions inventories. This may be explained in part by the complete lack of inter-comparisons of emission model estimates when scaled from the leaf to the canopy. In this paper we address this problem by coupling four different isoprene emission models (Guenther et al. model, Niinemets et al. model, BIM2 and the Martin et al. model) to two terrestrial biogeochemical model platforms (MoBiLE, GOTILWA+) that describe canopy structure differently. Simulations of isoprene emissions for the Puechabon Mediterranean holm oak stand are analysed, with both canopy photosynthesis models constrained using FLUXNET measurements. The results demonstrate that even with constrained canopy level photosynthesis, large model platform dependent within canopy differences can exist in both modelled photosynthesis and emissions. This results in large differences in modelled isoprenoid emissions, due to the relatively higher sensitivity of emissions to canopy microclimate, in particular temperature. This is the first time emission results from two biogeochemical platforms have been compared, and demonstrates that different canopy descriptions can lead to larger differences in modelled emissions than that attributable to the difference between the emission models themselves. This is an important aspect that has not been acknowledged by the emission modelling community.