## Materials and Methods
### In situ Measurements  
Independent sets of in situ data were collected for calibration and validation in May 2015. To calibrate the model we collected data along three transects. Each transect started at the vegetation edge, adjacent to the water and was extended landward perpendicular to the water line, until _Spartima_ no longer occurred. Additionally, we verified that the species no longer occurred over the next 10 m landward of the final point of each transect. Along the transects, a 1 \(\times\) 1 m plot was placed every 5 m for sampling. The location of the center of the plot was recorded with a differential gps (dGPS). In each plot, vegetation cover, vegetation biomass, soil moisture content, reflectance, and chlorophyll content were measured. The vegetation cover was estimated for each plot in percentages using expert judgement. Alongside the 1 \(\times\) 1m plot above ground vegetation biomass was sampled using a square area of 20 \(\times\) 20 cm (outside the 1 \(\times\) 1 m plot). The biomass samples were fresh weighed (FW), dried for a minimum of 4 days in an oven at 65 \({}^{\circ}\)C and weighed again (dry weight, DW). Soil-moisture content was measured by taking samples of the top three cm of the soil using a syringe with the nozzle cut off. For each plot, this was sampled 3 times, samples were pooled and processed. These samples were fresh weighed (FW), freeze-dried for 72 hours, and weighed again (DW), following Reference [46]. Reflectance was measured using a TriOS Ramses 842D spectroradiometer. This device measures electromagnetic radiation between 320 and 950 nm, sampling every 3.3 nm with 0.3 nm accuracy. Measurements are performed with a specially designed rig to hold the spectroradiometer stable at 2 m above the sediment surface. The measurement radius on the ground then becomes a circle with a diameter of 20 cm. This was done five times per plot, in a quincunx (dice five) pattern to avoid overlap and hence pseudo-replication. A reference measurement was taken prior to each measurement with a piece of white Styrofoam. Styrofoam is known to have a stable reflectance that represents incoming radiance [47]. We attempted to minimize the time between measurement and reference. If the light intensity changed noticeably between reference and measurement, both were taken again. All measurements were manually checked for errors, reference measurements were used to correct for irradiance, following Reference [47]. Similarly to the reflectance measurements at the canopy level, additional spectral measurements were taken at the leaf level. To do so, 10-30 top leaves were randomly collected within each plot. This set of leaves was arranged to form a surface of about 8 \(\times\) 10 cm. Special care was taken to ensure leaves were not upside down, and were clean. This surface was placed at exactly 16 cm from the spectroradiometer. At this distance, only the center circle with a 1 cm diameter was measured. Every 'leaf surface' was measured 5 times, moving the measuring area to avoid overlapping of the measurements and taking a reference measurement in between two measurements. Further processing was similar to the measurements at the canopy level. After the leaf surface measurements, the leaves of every leaf surface were immediately frozen and brought to the lab for chlorophyll analysis. The chlorophylls a, b and a-carotene were extracted using high-performance liquid chromatography (HPLC). For the HPLC procedure we followed Reference [48]. To measure chlorophylls the leafs were first freeze dried, and treated with a 10 mL 90/10 acetone/water solution to extract a sample. After centrifugation 50 \(\upmu\)l of this extract was separated for pigments on a C18-column with use of reversed phase chromatography. Separation was based on the interaction of pigments between column and the tertiary gradient used. After separation, the pigments were detected by a Photodiode Array (PDA) and a fluorescence detector.