Sun-Sensor-Surface Geometry Considerations - Decagon Devices SRS Operator's Manual

3 THEORY SRS Sensors
will absorb more radiation as a consequence of saturation of chloro-
phyll centers. A normalized difference of reflectance at 570 nm that
remains unchanged despite changes in light saturation to 531 nm
will indicate the level of xanthophyll absorbance and the efficiency
of plant light use. Because increased xanthophyll absorbance is not
solely correlated with LUE, researchers have investigated many other
relationships too.
Numerous studies correlate PRI to various ecophysiological variables
including the epoxidation state of xanthophyll, maximum photo-
chemical efficiency of photosystem II, effective quantum yield, maxi-
mum photosynthesis rate, electron transport under saturating light,
non-photochemical quenching, and chlorophyll to carotenoid con-
tent ratio (Sims & Gamon, 2002; Garrity et al., 2011; Garbulsky
et al., 2011; Porcar-Castell et al., 2012). Garbulsky et al. (2011)
and Porcar-Castell et al. (2012) provide excellent overviews of what
has been done with PRI including analyses of PRI correlations with
several of these variables at the leaf, canopy, and ecosystem levels.
We encourage our customers to use these references as a starting
resource.
3.5

Sun-Sensor-Surface Geometry Considerations

Spectral reflectance measurements are inherently variable due to ra-
diation source, reflecting surface, and sun-sensor-surface geometry.
Hence, it is not uncommon for a time series of NDVI or PRI to con-
tain high amounts of variability. Consider changes in daily NDVI
measured at four-day intervals in a subalpine meadow. (Figure 6)
Figure 6 shows the control treatment green-up under drought con-
ditions. The well-watered treatment in Figure 7 includes the same
time period but has already undergone initial green-up.
There are three things to notice about these data. First, canopy
green-up is clearly visible in the time series of the control (upper
graph) treatment. Second, the track of daily NDVI is generally con-
cave, which indicates that sampling across a consistent sun angle (e.g.
60 elevation angle (Ryu et al., 2012)) or around solar noon is advis-
able when summarizing an entire day to a single value. If comparing
12