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.