# Subarctic Vegetation under the Mixed Warming and Air Pollution Influence
## 4 Discussion  
Within the spread of Norilsk's plumes, subarctic tree growth was determined mostly by the combined influence of sulfur dioxide emissions and thermal regime. Chronic (since the 1940s) SO\({}_{2}\) pollution led to the largest forest community decline and mortality worldwide. Since the first documented larch stands' mortality (c. five thousand ha in 1970 [35]), the approximate area of weakened and damaged vegetation cover spread to c. 24,000 square km [7], although that area might be overestimated. The spatial pattern of emission transfer and, consequently, vegetation damage is determined by prevailing winds and terrain orography. Thus, northwestern winds prevailed in the summer, whereas southeastern ones prevailed in the winter (Figure 5c). Vegetation mortality spread mostly in the southeast direction along the valley bounded by hills, at a distance of up to c. 100 km, whereas the northwest direction of mortality spread to c. 20 km only [7; 24]. However, mixed larch, spruce, birch (LSB) forests survived in the vicinity (c. 5 km northeast) of the semelters due to relief-sheltering (Figures 2c and 7). Contrary to earlier studies [17], we found that larch is more SO\({}_{2}\)-resistant compared to spruce. Thus, GI dependence on SO\({}_{2}\) for spruce was much higher than for larch (Figure 9; \(r^{2}\) = 0.54 vs. \(r^{2}\) = 0.2, respectively). Meanwhile, spruce survive due to the mitigation of pollution influence by the inflection of wind direction. Winter winds themselves caused serious damage to spruce by desiccation and snow abrasion. Actually, the spruce forest in question is the most northward refugium of spruce species. That refugium is a seed-source for spruce northward warming-driving migration. In a broader aspect, the heterogeneity of the terrain orography provided sheltered sites for vegetation within the area of SO\({}_{2}\) influence. Even in the impact zone, trees can survive on leeward slopes, along river valleys, or within the depression around the numerous lakes. It is noteworthy that even within the impact zone larch regeneration can be found within local depressions, although [PERSON] et al. [9] reported zero larch seedlings within the larch mortality zone. Certainly, regeneration location, abundance, vitality, and species composition dynamics need further studies. Vegetation  
Figure 12: NDVI (10\({}^{-4}\)/year), GPP (kg/ha/year), and NPP (kg/ha/year) significant (\(p\) < 0.05) trends (2001–2021). The zone of significant productivity trends is shown by the oval. The dotted line approximated the area of the larch communities’ decline and mortality (adopted from [34]). sheltering by relief features was also reported for Karabash (Russia) and Sudbury (Canada) polluted territories [36; 12]. What is noteworthy is the high SO\({}_{2}\) resistance of birch and especially willow (Figure 11b,c; Table 2). This indicates a potential use of both species for the reclamation of polluted areas. In the impact zone, the larch growth decrease lasts until the mid-1990s (Figure 6). In the mixed LSB zone, similar decreases were insignificant for all tree species (Figure 7). In the current period of warming, the growth indices of all woody species are increasing (Figures 6-8). That increase coincided with current warming (Figure 2a) and a minor (approximately 13%) decrease in SO\({}_{2}\) volume (Figure 6c). Actually, the combined effect of those two factors led to tree growth increase (Figures 9-11; equations in Table 2). A strong positive response of all species to elevated air and soil temperatures refute the hypothesis of warming-increase trees' vulnerability to the pollutants [19]. In Sudbury, Canada, and South Urals, Russia, more vigorous growth of certain surviving species has also occurred since pollution decreased [12; 37]. Meanwhile, no analysis of climate warming was carried out in those works. In addition, the willow's strong growth response to September temperatures is remarkable, i.e., when leaves' photosynthesis is partly or completely inhibited (Figure 10a; Table 2, Equation (2)).