Seasonal controls of canopy chlorophyll content on forest carbon uptake: Implications for GPP modeling

Abstract 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. Key Points Start of season maximum leaf chlorophyll and LAI values differed by 41 days Daily GPP showed the strongest relationship with canopy chlorophyll content LAI overestimated GPP at the start of the growing season