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Forested ecosystems represent an important part of the global carbon cycle, with accurate estimates of gross primary productivity (GPP) crucial for understanding ecosystem response to environmental controls and improving global carbon models. This research investigated the relationships between leaf area index (LAI) and leaf chlorophyll content (Chl Leaf ) with forest carbon uptake. Ground measurements of LAI and Chl Leaf were taken approximately every 9 days across the 2013 growing season from day of year (DOY) 130 to 290 at Borden Forest, Ontario. These biophysical measurements were supported by on-site eddy covariance flux measurements. Differences in the temporal development of LAI and Chl Leaf were considerable, with LAI reaching maximum values within approximately 10 days of bud burst at DOY 141. In contrast, Chl Leaf accumulation only reached maximum values at DOY 182. This divergence has important implications for GPP models which use LAI to represent the fraction of light absorbed by a canopy (fraction of absorbed photosynthetic active radiation (fAPAR)). Daily GPP values showed the strongest relationship with canopy chlorophyll content (Chl Canopy ; R 2 = 0.69, p < 0.001), with the LAI and GPP relationship displaying nonlinearity at the start and end of the growing season (R 2 = 0.55, p < 0.001). Modeled GPP derived from LAI × PAR and Chl Canopy × PAR was tested against measured GPP, giving R 2 = 0.63, p < 0.001 and R 2 = 0.82, p < 0.001, respectively. This work demonstrates the importance of considering canopy pigment status in deciduous forests, with models that use fAPAR LAI rather than fAPAR Chl neglecting to account for the importance of leaf photosynthetic potential.

Chlorophyll is the main light-harvesting pigment in leaves, facilitating photosynthesis and indicating the supply of nitrogen for photosynthetic enzymes. In this study, we explore the feasibility of integrating leaf chlorophyll content (Chlleaf) into a Terrestrial Biosphere Model 2 (TBM), as a proxy for the leaf maximum carboxylation rate at 25°C (V max 25), for the purpose of improving carbon and water flux estimation. Measurements of Chlleaf and V max 25 were made in a deciduous forest stand at the Borden Forest Research Station in southern Ontario, Canada, where carbon and water fluxes were measured by the eddy covariance method. The use of Chlleaf-based V max 25 in the TBM significantly reduces the bias of estimated gross primary productivity (GPP) and evapotranspiration (ET) and improves the temporal correlations between the simulated and the measured fluxes, relative to the commonly employed cases of using specified constant V max 25 , leaf area index (LAI)-based V max 25 or specific leaf area (SLA)based V max 25. The biggest improvements are found in spring and fall, when the mean absolute errors (MAEs) between modelled and measured GPP are reduced from between 2.2-3.2 to 1.8 g C m-2 d-1 in spring and from between 2.1-2.8 to 1.8 g C m-2 d-1 in fall. The MAEs in ET estimates are reduced from 0.7-0.8 mm d-1 to 0.6 mm d-1 in spring, but no significant improvement is noted in autumn. A two-leaf upscaling scheme is used to account for the uneven distribution of incoming solar radiation inside canopies and the associated physiological differences between leaves. We found that modelled V max 25 in sunlit leaves is 34% larger than in the shaded leaves of the same Chlleaf, which echoes previous physiological studies on light acclimation of plants. This study represents the first case of the incorporation of chlorophyll as a proxy for V max 25 in a two-leaf TBM at a forest stand and demonstrates the efficacy of using chlorophyll to constrain V max 25 and reduce the uncertainties in GPP and ET simulations.

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