Figure 8 . Diurnal cycles of commonly used vegetation indices
(NDVI, NIRv, PRI, CCI), PAR, and SZA collected from PhotoSpec at NEON
Delta Junction, AK for three vegetated understory targets. Diurnal
cycles are the average cycle over the 20 day period indicated and shaded
error bars are the standard deviation of measured quantities over the 20
day period.
The interaction of orbital mechanics with diurnal and seasonal variation
in vegetation indices results in possible bias due to overpass timing of
spaceborne instruments (Xiao et al., 2021). As shown in Figure 8, some
indices are diurnally invariant (e.g., NDVI, NIRv, and CCI) whereas PRI
is not. Consistent observation in the morning versus afternoon may
result in discrepancies in comparing observations from multiple
instruments and platforms. Relying exclusively on observations from
either morning or afternoon may obscure important diurnal processes at
work that govern plant productivity (see discussion of xanthophyll cycle
dynamics, above). On a seasonal basis, the extended diurnal photoperiod
experienced by high latitudes provides an opportunity for higher
frequency observation (i.e., multiple per day) of vegetation spectra by
spaceborne instruments during the peak season; however, this potential
advantage of deploying sensors that can collect multiple daily
observations rapidly diminishes in the shoulder seasons and winter when
photoperiod is abbreviated or non-existent (i.e., polar winter, Figure
8). Therefore, interpreting time series vegetation indices from arctic
tundra requires careful accounting for overpass timing, photoperiod, and
all associated responses to diurnal physiology and viewing/ illumination
geometry.