## 2 Materials and Methods  
### Site Description and Flux Tower Observations  
Our study sites consisted of two EC flux sites in northern Wisconsin, USA: Park Falls (US-PEA, 45.945\({}^{\circ}\)N, 90.273\({}^{\circ}\)W) [48; 49] and Willow Creek (US-WCr, 45.806\({}^{\circ}\)N, 90.080\({}^{\circ}\)W) [50]. Both sites are located in the Chequamegon-Nicolet National Forest, and are characterized by an interior continental climate [49]. Park Falls is dominated by a mixed landscape of upland forests (mainly deciduous but with a significantiferous coverage) and open wetlands [49]. The Willow Creek site is located about twenty km to the southeast of Park Falls. Willow Creek is a 80-100-year-old deciduous broadleaf forest (DBF) stand consisting mainly of sugar maple, basswood, and green ash, with a closed canopy of about 24 m in height and a leaf area index (LAI) of 5.3 [50].  
We used EC and meteorological measurements from these two forest sites for the period 2014-2017 to constrain the SCOPE model, and investigated the relationships between satellite-derived SIF and flux tower GPP for the same period. EC flux and meteorological measurements have been made continuously at Park Falls [48] and Willow Creek since 1996 and 1998, respectively. We used the hourly GPP and meteorological data at Park Falls and half-hourly data from Willow Creek for the period from 2014 to 2017 for constraining the SCOPE model, and also tested the model with the data since 2003 at Park Falls. The net ecosystem exchange (NEE) measurements obtained from the EC technique were partitioned to GPP and ecosystem respiration using a non-linear regression of nighttime NEE to surface soil temperature [51]. The details of the flux data processing procedures of these two sites have been described in previous studies [51; 52]. We also used meteorological data such as wind speed, air temperature, incoming solar radiation, specific humidity, and soil moisture as the driving data of the SCOPE model. The canopy height was 19 m and 24 m at Park Falls and Willow Creek, respectively, while wind speed was measured at the height of 30 m and 29.6 m, respectively. The roughness length and zero plane displacement were calculated from vegetation height and LAI in the SCOPE model as described in [53].  
Solar-Induced Chlorophyll Fluorescence (SIF) Observations from Orbiting Carbon Observatory-2 (OCO-2)  
The OCO-2 has been collecting SIF measurements with a repeat frequency of approximately 16 days since September 2014. The OCO-2 SIF was retrieved using a simplified forward transition model exploiting the infilling of the Fraunhofer lines at 757 nm and 771 nm wavelengths from the O\({}_{2}\) A-band observation records [47]. Although the spatial resolution of OCO-2 is 1.3 km \(\times\) 2.25 km in the nadir mode, the total swath width is only 10.3 km. With the sparse coverage of the instrument, temporally dense SIF data are not available for most locations over the globe. The Park Falls site is a very tall tower and a validation site for the OCO-2 mission [2], and OCO-2 has been collecting SIF data in the target mode, leading to temporally well-spaced retrievals of SIF measurements near the flux tower site (Figure 1).  
We obtained the level 2 SIF Lite product (OCO2_L2_Lite_SIF, version 8r) from the Land Processes Distributed Active Archive Center (LP DAAC). This product contains bias-corrected SIF along with other selected fields from the IMAP-DOAS algorithm [47]. For each site, we extracted all the OCO-2 SIF observations that were available within the 2.5 km \(\times\) 2.5 km area surrounding the flux tower during the study period. The instantaneous SIF observations were calculated by averaging all the available samples within the surrounding area. Then, we converted the instantaneous SIF measurements to daily SIF using the daily correction factor contained in the product [47].