## 3 Results
### Drought Impact on Phenology and Green Biomass Production
#### 3.4.2 Changes in Forest Canopy Photosynthetic Activity--3 nNDVI PC  
Identified through the 3 nNDVI PC divergent trends in photosynthetic activity of forest canopy were complementary to significant differences in forest phenology and green biomass production (Table A3, Figure 6). Stands experiencing the decline in forest 'greenness' (class 3) were characterized by low, but overall increasing CF with two intense gains in 2003 and 2009 (Figure 6a). Coupled GPP development (Figure 6b) accounted on a massive upturn in 2003, followed by an equally strong drop, and a subsequent intense variability in biomass levels between 2006 and 2012. SBD varied greatly over the whole observation period, and showed no clear change tendency (Figure 6c). Simultaneously, gradual SL prolongation was observed (Figure 6d). Drought ingress in 2003 was aligned with sooner SBD and longer growing season. Influence of biophysical factors on forest phenology and green biomass production within the 3 nNDVI class 3 drought response was moderate. Stands placed at 1400-2100 m a.s.l. showed higher CF with greater inter-annual variability (Figure 7). In contrast, elevation-dependent differences in GPP were less distinct (but statistically significant, pairwise comparison, \(p<0.005\)). Drought ingress in 2003 clearly spurred CF and GPP at higher elevations. Vegetation onset within forest growing below 1400 m a.s.l. occurred significantly sooner than SBD at higher altitudes (pairwise comparison, \(p<0.001\)), which contrasted with longer SL observed at higher elevations. Relation between exposition and forest status within regions subjected to NDVI decrease was complex (Figure 7). North slopes showed the greatest CF increase in 2003 and the highest overall seasonal production, but revealed a clear GPP decline after 2003. West-exposed stands had, in general, the highest GPP coupled with the lowest CF, as well as the smallest productivity gains associated with the ingress of drought conditions in 2003. Differences in vegetation onset analyzed among aspect classes showed strong inter-annual variability with no clear mutual relations. Season length also varied greatly among expositions, and its relation to SBD was not clear. Inclination was found to have limited impact on shaping forest response to drought conditions, and relevant results are presented in Appendix B.1. Analysis of drought response being a function of forest type was not performed due to the insufficient number of pixels associated with broadleaved and mixed stands (Table 2). Figure 6: Year-to-year variability observed within the drought impact classes 1 (in green) and 3 (in red) footprints representing respectively, negative and positive correlation to the 3 nNDVI PC: (**a**) CF; (**b**) GPP; (**c**) SBD and (**d**) SL (abbreviations as in Figure 3). Significance of class-dependent year-to-year differences was assessed with ANOVA model and is given in Table A4. Forest stands associated with the 3 nNDVI class 1, and fostering an initial drop of canopy 'greenness' in 2003 followed by a long-term increase in photosynthetic activity showed high CF with an overal upturn tendency (Figure 6a). This response was paired with gradually increasing GPP (Figure 6b). Remarkably, both biomass production measures declined in 2003. Vegetation onset occurred here on average 19.6 days later comparing with regions recognized through the 3 nNDVI class 3 response, and revealed a clear shift towards earlier SBD (Figure 6c). In contrast, SL showed no long-term trends and was roughly 32.6 days longer than observed within the impact class 3 (Figure 6d). Forest phenology and green biomass production associated with the overall NDVI increase was further shaped by local environmental conditions. Coniferous forest reported here clearly higher CF with lower annual biomass integral (Figure 8). Ingress of drought conditions in 2003 resulted in CF and GPP declines, as well as delayed season begin day observed for mixed stands. On the contrary, needle leaved woodland reported in 2003 sooner vegetation onset.