## 2 Materials and Methods  
### Study Site and Experimental Design  
The Mattheis Ranch is a 4977 ha working cattle ranch located near Duchess in southern Alberta, Canada, and is mostly characterized by native prairie vegetation. Recently, this site has become a research area managed by the Rangeland Research Institute (University of Alberta), offering the possibility of long-term monitoring of ecosystem dynamics under management regimes typical of this region. The climate of southern Alberta presents long winters and short, windy summers with extreme temperatures in both seasons, and peak precipitation periods occurring in early summer (June). The rest of the summer usually has lower precipitation, but summer rainfall regimes can vary markedly from year to year, with a similarly variable effect on productivity [12]. The management regime consisted of light, rotational grazing, as further described in [PERSON] _et al._[41]. Our study site centered on two eddy covariance tower locations designated \"E5\" (northern site, 50.9056N, 111.8823W) and \"E3\" (southern site, 50.8671N, 111.9045W), situated about 4.5 km apart (Figure 1). The two sites differed slightly in vegetation cover and microtopography; E5 appeared very flat and uniform, while the E3 landscape varied slightly more than E5 in microtopography and vegetation cover within the flux tower footprint [41], but was still a relatively uniform grassland-dominated site. The sites represent a typical dry mixed grass prairie community as described by [PERSON] _et al._[9]. The E5 soil is a clay loam, and the plant community is dominated by needle-and-thread grass (_Hesperostipa comata_), Junegrass (_Koeleria macrantha_), blue grama grass (_Bouteloua gracilis_), and western wheatgrass (_Pascopyrrun smithii_). Pasture sage (_Artemisia frigida_) and scarlet mallow (_Sphaeralcea occcinea_) are the most common forbs, and silver sagebrush (_Artemisia cana_) is the primary shrub species [42]. In contrast, the soil at the E3 site is a sandy loam. Dominant grasses include sand grass (_Calamovilifa longifolia_), needle-and-thread grass (_Hesperostipa comata_), and low sedge (_Carex stenophyllla_), while common forbs are scurf pea (_Psoralidium lancoelatum_), golden aster (_Heterotheca villosa_), and prickly-pear cactus (_Opuntia polyacantha_). Slight variations in topography allow forbs such as wild licorice (_Glycyrrhiza lepidota_) and golden bean (_Thermopsis rhombifolia_), and shrubs such as wild rose (_Rosa woodsii_) to flourish [42]. Due to these differences in vegetation, soil type and associated hydrology (not shown), these two sites provided a natural experiment encompassing two contrasting landscapes typical of southern Alberta rangelands. The measurements at the two grassland sites (E3 and E5) included:  
* CO\({}_{2}\) flux measurements from eddy covariance at the E3 and E5 sites,
* continuous, proxy NDVI [27; 43] from a set of 2-band radiometers, including PAR (photosynthetically active radiation) and PYR (pyranometer) sensors, comprising an \"optical phenology station,\"
* aboveground biomass samples from a 1 ha area around each flux tower site periodically collected following the sampling scheme reported in Figure 2,
* NDVI\({}_{680,800}\) measurements using a field spectrometer collected at regular intervals in the same area (1 ha) using a 10 m grid spacing (Figure 2),
* incident and reflected incoming PAR measurements at each calibration point (numbered circles, Figure 2), and
* MODIS satellite NDVI data downloaded for the study areas. Figure 1: Map of study location (**top left**), with detail of study area (**bottom left**), showing the E3 and E5 calibration sites. Further details of each site are provided on the right. Image credits: ESRI, DigitalGlobe, GeoEye, i-cubed, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swissotopo, and the GIS User Community. Each phenology station was located approximately 10 m to the southeast of its corresponding flux tower.