# Subarctic Vegetation under the Mixed Warming and Air Pollution Influence
## 4 Discussion
That phenomenon might be attributed in part to non-leaf (\"bark\") photosynthesis. For instance, in the case of _Populus tremuloides_, bark photosynthesis is responsible for c. 10-15% of the trees' carbohydrate pool [38]. Therefore, according to our observations, certain willow species may vegetate, even in the leafless form in a harsh habitat (e.g., in mountain tundra). Potentially non-leaf photosynthesis may be also partly responsible for the willow's high SO\({}_{2}\)-resistance. Notable \"cold period precipitation\" (i.e., snow-accumulated water) has a positive influence on the trees' growth (Table 2). At the beginning of the growth period, melted water mitigated seasonal water stress while permafrost-derived water was still not available. That was also observed in subarctic _Larix gmelini_ habitat [39]. As a drought-resistant species, larch is less dependent on soil moisture and is usually located within dryer sites, whereas spruce grows mostly on wet sites. Excessive soil moisture in summer inhibited the growth of all species (Figures 9-11). Meanwhile, decreasing temporal trends of terrestrial water storage (Figure 3d) indicate a potential improvement of moisture regime at high latitudes [11], whereas at lower latitudes vegetation is suffering from decreasing soil moisture content [40]. Together with SO\({}_{2}\) pollution, the smelters also emitted heavy metals (Cu, Ni, Cr, Co), which poisoned vegetation [41; 42; 43; 44]. In the soils of the Norilsk area, the highest concentrations of the pollutants were observed within the smelters' vicinity (c. 20-25 km). However, knowledge regarding the influence of heavy metals on vegetation in that area is still controversial and insufficient [17; 43; 44]. Alongside trees, moss and lichen communities are also severely affected by the smelters' emissions. Similar processes were described for the Southern Urals polluted areas [14; 16; 37] and Sudbury, where, since the decrease in pollution, grasses and horsetails expanded their range [44]. Unexpectedly, we found increasing trends of vegetation GPP, NPP, and NDVI indexes within a large part of formerly died larch-dominant communities (Figure 12). These increases are partly explained by the warming-driven growth of woody species (Figures 9-11; Table 2). Meanwhile, because subarctic larch forests are mostly sparse and open, tree growth itself was not the main cause of GPP and NPP increase, and mostly referred to the expansion of bushes and grass species into the \"technogenic desert\". According to [PERSON] et al. [45], SO\({}_{2}\)-killed former moss and lichen communities are being substituted by grass and bush phytocenosis. We also observed birch expansion into the pollution-damaged zone. Similar processes, i.e., certain broadleaf species expansion into the former heavily polluted territories, are reported for Monchegorsk (Peninsula Russia) [13] and Sudbury [44]. Together with warming-driving productivity increase, soil fertilization caused by NOx and SO\({}_{2}\) emissions may have stimulated vegetation growth. Since the 1970s, the volume of NOx emissions varied within c. 9000-25,000 t per year. Nitrogen is known as the main factor that limited tree growth in the subarctic, whereas industrial nitrogen emissions often caused soil fertilization (e.g., [46; 47]). Finally, it is worth noting that increasing trends of vegetation productivity in the Norilsk area coincided with prevailed positive GPP and NPP trends in the Siberian Arctic and increased carbon uptake by the forests of Siberia [39; 48].