## 2 Materials and Methods
### Calculating the Normalized Difference Vegetation Index  
The NDVI [26] is a widely used remote sensing index that exploits the characteristic contrast in reflectance between the red and NIR wavelengths by vegetation. The NDVI has been used to estimate the biophysical properties of vegetation, such as biomass or photosynthetic activity [38]; however, the NDVI can be impacted by sun-sensor geometry [39] and variations in soil background, particularly in low biomass environments [40]. The NDVI is calculated as the ratio between the difference and the sum of the NIR and red bands in an image (Equation (1)):  
\[\text{NDVI}=(\text{NIR}-\text{red})/(\text{NIR}+\text{red}) \tag{1}\]  
The NDVI was calculated from the ArborCam image using band 4 (Red-1) and band 7 (NIR). The NDVI was calculated for all 12 PlanetScope images using the red band 3 and NIR band 4. We used the ArborCam NDVI to explore the relationship with the aboveground carbon content derived for the quadrats. We used the PlanetScope NDVI to analyse the saltmarsh seasonal response (both described below). Investigating the Relationship between ArborCam Normalized Band Differences and Aboveground Carbon Content  
We undertook an initial exploratory analysis to understand the relationship between the aboveground carbon content for each quadrat and the ArborCam NDVI. The NDVI is sensitive to vegetation biomass [38], and because we calculated the aboveground carbon for each quadrat from established, species-specific fractions of vegetation biomass (Table 1), we therefore expected the NDVI to be positively correlated with aboveground carbon content. The mean ArborCam NDVI was calculated for each quadrat location using all pixels falling within or overlapping a quadrat polygon, with partial pixels given a fractional weighting. Pearson's correlation coefficient (r) was used to explore the covariance between the mean ArborCam NDVI values and the aboveground carbon content for all quadrats. Our initial exploration found that quadrats dominated by a rush species, _Juncus kraussii_, were relatively high in aboveground carbon content but were correspondingly low in NDVI compared to other quadrats, resulting in a relatively poor correlation between the aboveground carbon content and NDVI (r = 0.51, \(p\) < 0.01). Quadrats were therefore partitioned into two groups: the 6 outlier quadrats dominated by _J. kraussii_, and the remaining 41 quadrats representing all other saltmarsh species (herbs, grasses, and sedges). \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Acquisition (UTC)** & **Local Time** & **PlanetScope Image ID** & **Tide (m)** \\ \hline
17 April 2020 23:40 & 18 April 2020 09:40 & 20200417\_234006\_101f & 0.6 \\
18 May 2020 23:39 & 19 May 2020 09:39 & 20200518\_233928\_1032 & 0.7 \\
15 June 2020 23:00 & 16 June 2020 09:00 & 20200615\_230004\_0f46 & 0.7 \\
13 July 2020 00:10 & 13 July 2020 10:10 & 20200713\_001012\_26\_1064 & 0.7 \\
25 August 2020 23:38 & 26 August 2020 09:38 & 20200825\_233805\_1009 & 0.6 \\
27 September 2020 & 27 September 2020 & 20200927\_001210\_14\_1061 & 0.7 \\
00:12 & 10:12 & 2020 October 2020 22:39 & 20201022\_223957\_02a & 0.8 \\
25 November 2020 & 26 November 2020 & 20201125\_234809\_79\_105c & 0.8 \\
23:48 & 10:48 & 20201224\_001518\_69\_105d & 0.7 \\
24 December 2020 00:15 & 24 December 2020 11:15 & 20201224\_001518\_69\_105d & 0.7 \\
21 January 2021 00:03 & 21 January 2021 11:03 & 20210121\_000350\_37\_2416 & 0.8 \\
20 February 2021 00:17 & 20 February 2021 11:17 & 20210220\_001743\_67\_105d & 0.7 \\
24 March 2021 00:18 & 24 March 2021 11:18 & 20210334\_001859\_67\_105e & 0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Monthly PlanetScope images used in this study with tide level (m) at time of acquisition. Additional bands are available in the ArborCam image, particularly in the red-edge region, that may provide suitable normalized-difference (ND) band combinations in place of the red and NIR bands typically used in the NDVI.