## 5 Results
### Relationship with Phenology and Environmental Data  
In addition to the optical measurements at the Abisko forest, we have analyzed other observations of vegetation phenology and productivity, including eddy covariance measurements, air temperature records, digital camera photographs, and manual observations by observers at the Abisko Scientific Research Station (_ca._ 1.3 km away from our site). These data are shown in Figure 12. GPP was estimated at this site from net ecosystem exchange (NEE) flux measured by an open-path infrared CO\({}_{2}\) analyzer (Li-Cor 7500, Li-Cor Inc., USA) and a three-dimensional sonic anemometer (Gill R3, Gill Instruments, UK). The partition algorithm is described by [PERSON] _et al_. [PERSON] et al. (2009). Air temperature was measured at the height of 2 meters using a Vaisala WXT 510 Weather Transmitter (Vaisala Oyj, Finland).  
The site is complex in the sense that it consists of sparse deciduous canopy on top of a partly evergreen understorey layer. As explained in the previous sections, the seasonality of snow, understorey, and overstorey are confounded, creating a complex annual reflectance curve. GPP starts to increase above zero at almost the same date in spring as the temperature does, and it decreases to zero in the autumn, slightly before the temperature. The timings of phenophase transitions from ground observations and phenology camera images are drawn as vertical lines (labeled 1-6 in Figure 12). These dates match the optical observations of PAR and NDVI well. The period of snow melting (1-2) is characterized by a very rapid increase in both \(fAPAR_{g}\) and NDVI. During this period GPP increases quite slowly. The period after the main snow melt, when the buds of the birch trees are developing (3-4), is identical with the NDVI \"shoulder\" period. During this period GPP increases slowly, probably due to a low rate of understorey photosynthesis. \(fAPAR_{g}\) has reached its maximum, and \(fAPAR_{c}\) is relatively constant. In mid-June (5) the birch leaves are fully developed, and both GPP, NDVI and \(fAPAR_{c}\) increase rapidly. They reach a maximum at DOY 183 (NDVI four days earlier than GPP), and level out during the middle of the peak season. In the autumn, GPP starts decreasing ca. 18 days earlier than NDVI and \(fAPAR_{c}\). Leveling out of GPP matches the date of yellowing of leaves (6), and NDVI reaches its minimum value. After this date, NDVI increases again, concurrently with \(fAPAR_{g}\), and probably as a consequence of exposure of the green understorey due to shedding of the birch leaves. GPP remains low but positive throughout this period, indicating some understorey vegetation productivity. When the temperature falls below zero degrees NDVI drops rapidly to below 0.2. During the winter season the optical data sets are very noisy. During the growing season, however, it can be noted that the NDVI curve is less noisy than any of the other curves, including GPP. The green fields in Figure 12 denote the vegetation period according to the optical measurements: (A) the period dominated by understorey growth; (B) the period from bud-burst to full leaf development; (C) the period of leaves developing into a full canopy; (D) the period of mature canopy; (E) the period of discoloration and start of defoliation of the canopy; and (F) the period of re-exposure of ground vegetation due to defoliation.