## 5 Discussion
### Converging/Diverging Trends and Correlations between LOS and GSI  
This study shows that a quarter area of the NH is covered by dominant converging trends in LOS and GSI, although EA shows more areas of convergence than NA (Figure 3). Meanwhile, GSI is predominantly positively correlated with LOS in the NH, with significant positive correlations being more widespread in EA than in NA (Figure 5). Regions with converging trends in LOS and GSI are generally corresponding to areas with high positive correlations between LOS and GSI, which could be expected. However, the fact that trends in LOS and GSI are of similar direction does not a priori cause a high correlation as it depends on the magnitude and statistical strengths of the individual trends. This pattern is in accordance with the general assumption that increasing LOS that is caused by pervasive warming in middle and high latitudes of the NH has led to increasing vegetation productivity, simply because more days are available for carbon assimilation and biomass accumulation ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016). For example, [PERSON] et al. (2016) showed that LOS in a temperate deciduous forest in Midwestern US extended at the rate of about three days/year during 1998-2008 (because of delayed autumn senescence) was accompanied by an increasing trend of annual net C uptake. A comparable result for a temperate deciduous forest in Denmark was reported by [PERSON] et al. (2016), who showed that an increasing trend in C uptake during the period of 1996-2009 was partially attributed to a concurrent increasing trend in the duration of photosynthetic activity. Moreover, highly consistent with the converging trends that were shown in this study (Figure 3), [PERSON] et al. (2016) found that carbon uptake through photosynthesis has increased in temperate forests of eastern US owing to warming-induced increasing LOS. Diverging trends in LOS and GSI only cover about 6% areas of the NH, mainly in high latitudes and arid/semi-arid regions (Figure 3), and larger areas of divergence are observed in NA than EA. Additionally, GSI is characterized by low or negative correlations, with LOS primarily in high latitudes and arid/semi-arid areas and regions of significant negative correlations are more frequent in NA than in EA (Figure 5). This indicates that areas with diverging trends are corresponding to regions with low or negative correlations between LOS and GSI in the NH. This pattern may be due to distinct responses of vegetation phenology and productivity to the ongoing warming in high northern latitudes ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016). In agreement with the result from this study, [PERSON] et al. (2016) found that vegetation productivity has continuously increased during 2000-2014 compared to decreasing LOS in the boreal and arctic regions of NH. Moreover, [PERSON] et al. (2016) showed that vegetation productivity and phenology reacted differently to drought events depending on ecosystem and land cover at the continental scale of Europe. This was further confirmed by [PERSON] et al. (2017), who showed that vegetation phenology and productivity responded differently to heatwave and had contrasting recovery trajectories after a wildfire disturbance. In addition to the direct impacts from a changing climate or climate extremes on vegetation phenology and productivity, an increasingly abundant deciduous shrub cover in tundra landscapes has been reported in high latitudes and arctic regions ([PERSON] et al., 2016). Such changes in vegetation cover may also be a driver of divergent trends and changing correlations between LOS and GSI in some high latitude regions that were found in this study.