## Materials and Methods
### Model  
Physical models are often used to simulate reflectance and, after inversion, can be used to extract biophysical characteristics from reflectance [13; 14; 15]. A major advantage of using a physical model is the possibility to investigate which vegetation property affects spatial patterns. An often used physical model is 'ProSail', which is the combination of the leaf reflectance model 'Prospect' [42] and the light scatter model in layers of leaves 'Sail' [51]. The combination of these two models is still improving, and it is often used due to its general robustness and because its inversion is known to perform well [52]. We used the 'HSDAR' R-package to apply the ProSail model [53]. See Table 1 for a complete overview of the model parameters. HSDAR uses the Fortran version of ProSail 5b, based on Prospect 5 and 4 Sail. Using ProSail we simulated spectra between 400 and 2500 nm. The reflectance model 'Prospect' requires detailed information on leaf structure (structure parameter, N), water content parameters (equivalent water thickness, _Cw_), dry matter content (_Cm_) and chlorophyll contents (chlorophyll a+b (_Cab_), caroten (_Car_), and brown pigment contents (_Chrown_)). The leaves collected along the transect were used to calibrate Prospect, The _Cab_ parameter was obtainedfrom the chlorophyll-a+b values. The _Car_ parameter was estimated from the carotene values. For our model brown pigments were ignored. The average water content per leaf area was calculated from the photographed and dried leaf samples. The equivalent water thickness (_Cw_) is then calculated using the corresponding formula ((FW-FD)/Area, see References [54; 55]). Average leaf equivalent water thickness (_Cw_) was found to be 0.0198 \(\pm\) 0.0043 cm (n = 50). The leaf samples were also used to estimate the dry matter content per area (_Cm_), this was found to be 0.0092 \(\pm\) 0.0025 g/cm\({}^{2}\) (n = 50). The internal leaf structure parameter (_N_) was fitted using only the 'Prospect' model, in combination with the leaf level spectra. Initially, in situ spectra showed a higher baseline than the simulated spectra, hence a first degree polynomial conversion baseline was fitted using the'spc.fit.poly' function from the 'hyperSpec' R-package [56]. The parameter \(N\) was estimated as the value of the lowest RMSE, which was 1.5. This is precisely the value that the authors of the Prospect model predicted as the best to describe monocotyledons [42] such as _Spartina_. The 'Sail' model simulates vegetation canopy reflectance from the soil water content, the Leaf Area Index (LAI) and leaf angle distribution (lidf), solar, observation zenith, and relative azimuth angle. Dry/wet fraction (_posit_) is used to scale the brightness of the soil, using a linear mixture of standard spectra of dry and wet soil, respectively. Dry/wet fraction spoil = 1 is used for dry soils, whereas spoil=0 is used for wets soils. In situ gravimetric dry/wet soil fraction (= 1-soil moisture content) were obtained from freeze-drying the in situ soil moisture content samples. Leaf area index (_LAI_) was estimated from dry above ground biomass using the conversion provided by earlier research [57]. This study reported the linear relation between biomass and LAI for _Spartina alterniflora_ as y = 634.95x + 5.4774, where y = LAI and x = biomass. _Spartina alterniflora_ is a close relative of our study species _Spartina anglica_. _Spartina alterniflora_ is a cross bread between _Spartina anglica_ and _Spartina maritima_ that naturally occurred in 1870 [58]. These species are highly similar, we therefore assume the relation between biomass and LAI is similar too. Applying this conversion allowed for the estimation of leaf area index from above ground biomass, calibrated for _Spartina_ (although a different species). _Lidf_ describes the leaf angle distribution. Based on earlier research [59], we assumed _Spartina_ to be mostly a planophile, and set the _lidf_ parameter correspondingly (_lidfa_ = 1, _lidf_ = 0). The hotspot parameter was kept at 0.