## 5 Results
### Par  
#### 5.2.1 Reflectance and Transmittance  
We demonstrate the effect of decomposing ecosystem PAR reflection into flux reflected from pure canopy and flux from the ground at two forest sites: Abisko birch forest and Norunda coniferous forest (Figure 8). We also show the \"true\" PAR transmittance, \(\tau PAR\). This quantity is smaller than the  
Figure 7: Daily trajectories in reflectance, PAR, and NDVI at Stordalen mite from May 13 (DOY 133) to May 16 (DOY 136). Daily NDVI increased by 0.2 on May 14 and May 15. The reflectance in red, NIR and PAR decreased simultaneously with the largest decrease occurring for the red reflectance. The labels are shown at the 12:00 location of the x-axis, and the tick interval is 6 hours. measured \(\tau PAR_{m}\), which contains a portion of the ground reflectance. The difference between \(\tau PAR\) and \(\tau PAR_{m}\) was substantial during the snow season for the Abisko forest, but there was no obvious difference for the Norunda forest. We will not show \(\tau PAR_{m}\) results here. PAR transmittance at Abisko forest showed a clear seasonal pattern, with the lowest values appearing in the middle of the growing season (DOY 183-242). At Norunda forest, the seasonality of PAR transmittance was considerably weaker, with no clear minimum. PAR reflectance was lowest during DOY 135-249 at the Abisko forest, and during DOY 110-229 at the Norunda forest. In the Abisko forest, the subtle but regular variations in PAR reflectance during these periods largely corresponded to the fluctuation in PAR transmittance (Figure 8, left). In the Norunda forest, strong seasonal patterns were only observed from PAR reflectance of the whole ecosystem and of the canopy, and these two quantities closely followed each other (Figure 8, right). The period of high _vs._ low reflectance at Norunda forest clearly indicated the snow period (before DOY 64, and after DOY 315), and the snow-free period (DOY 110-229). During the snow period, the reflectance of the canopy was persistently higher (over 0.2), but the variability of the two reflectances was quite large due to the coexistence of green canopy and snow on the branches. In the Abisko forest, the snow season can be easily identified from the ground PAR reflectance, \(rPAR_{g}\), which approximates to one for the snow period (before DOY 87 and after DOY 293), and falls below 0.05 during the snow-free period (DOY 135-280). As a contrast, the pure canopy reflectance, \(rPAR_{c}\), should always hover around a low value, since both green leaves in summer and bare branches in winter have low reflectance. However, we observed relative higher \(rPAR_{c}\) for some intermittent periods in winter. These high reflectances were most likely caused by short periods of snow on the branches. Figure 8: The annual courses of canopy PAR transmittance, ecosystem PAR reflectance, canopy PAR reflectance, and ground PAR reflectance at the Abisko (left) and Norunda (right) forests. Note the varied scale for the y-axis which is used to amplify the reflectance and transmittance changes during the growing season and transition periods snow-to-vegetation and vegetation-to-snow. The seasonal trajectory of the fraction of transmitted PAR, \(\pi PAR\), reflects foliage conditions of the tree canopy. In the deciduous Abisko forest, \(\pi PAR\) fluctuates regularly, with low values during the full-foliage stage (DOY 183-242), and high values during the bare-branch stage (before DOY 183 and after DOY 280). This pattern of fluctuation is less obvious at the Norunda forest, since the canopy there is evergreen. At this site, \(\pi PAR\) variations are clearly more influenced by snow on the branches than by plant phenology variations. Turning points in all of the PAR curves at Abisko forest nearly match those of the NDVI curves in Figure 6, e.g., at DOY 135, 160, 183, 242, and 280. At Norunda forest, only ecosystem and canopy reflectance data match those in the NDVI curve, e.g., at DOY 64, 106, 198, 235, and 305.