## 5 Results
### Par
#### 5.2.2 fIPAR and fAPAR  
The fraction of PAR absorbed by the whole ecosystem, \(fAPAR_{e}\), was calculated for four of the sites, and is shown in Figure 9. The curves display the variation in seasonal length across the different sites. As expected, the absorbed PAR is higher during the snow-free season and lower during the snow season.  
The dates of end and onset of the snow season and the length of the growing seasons vary across the sites, as a consequence of differences in latitude and vegetation type. However, the magnitudes of \(fAPAR_{e}\) during the snow-free period are very similar (_ca._ 0.93-0.98) at all the sites. Again, we have used a variable y-axis scale to highlight the subtle changes of \(fAPAR_{e}\) during this period.  
Figure 9: Fraction of ecosystem absorbed PAR at four tower sites. Note that \(fAPAR_{e}\) below and above 0.9 are shown in different scales in order to emphasize variations during the growing season.  
The intercepted PAR, fIPAR, was computed from Equation (6), and the three fAPAR quantities were computed according to Table 6. These four quantities are illustrated for the Abisko and Norunda forests in Figure 10. The fIPAR was slightly greater than the canopy-absorbed PAR within the snow-free period for the two forests. The total absorbed PAR was very stable throughout the snow-free season, and the two components, \(fAPAR_{c}\) and \(fAPAR_{g}\), varied in opposite directions during this period. This is easily seen for the Abisko forest, where the turning point on the \(fAPAR_{e}\) curve is very sharp on DOY 135. From this date until DOY 160, \(fAPAR_{c}\) and \(fAPAR_{g}\) hovered at \(\sim\)0.22 and \(\sim\)0.74 respectively. After DOY 160, \(fAPAR_{g}\) decreased and \(fAPAR_{c}\) increased at the same rates, until DOY 183. Then \(fAPAR_{g}\) leveled out at \(\sim\)0.49, and \(fAPAR_{c}\) leveled out at \(\sim\)0.46 until DOY 242. After that date their directions diverged, but with considerable noise. No obvious trends in \(fAPAR_{c}\) and \(fAPAR_{g}\) were observed after DOY 280 until the end of the year, neither in the period from the start of the year to DOY 87.  
Periods with different fraction quantities indicate different phenological stages of the forest canopy and ground vegetation, and differences in PAR absorption reflect different productivities. Variations in \(fAPAR_{e}\) indicate the seasonality status of the whole ecosystem, and the signal is composed of both variations in vegetation cover (high \(fAPAR_{e}\)) and snow cover (low \(fAPAR_{e}\)). In the growing season, increases in the absorbed PAR may imply increasing productivity, e.g., for the canopy. However, for the ground vegetation, the decrease in \(fAPAR_{g}\) does not necessarily mean that the ground vegetation productivity is decreasing. For example, during the period of DOY 160-183 in the Abisko forest, the decrease is caused by an increase in canopy-intercepted PAR.  
The seasonal pattern of land surface phenology (_i.e.,_ snow and vegetation variations) in the Norunda forest can be identified from the course of total ecosystem-absorbed PAR, and a similar seasonal pattern is also seen from the pure canopy-absorbed PAR. However, these variations do not reflect the phenology of the coniferous trees. Neither is it possible to differentiate between the phenological stages of the coniferous trees and of the ground vegetation at the Norunda forest based on our measurements (Figure 10, right), since the lack of measurements of ground reflectance precludes determination of the annual course of ground absorbed PAR.  
Figure 10: fIPAR and fAPAR at the Abisko (left) and Norunda (right) forests.