# Pollution and Climatic Influence on Trees in the Siberian Arctic Wetlands
## 4 Discussion  
Norilsk industry emissions caused the biggest large-scale decline and mortality of forests beyond the Arctic circle. Tree growth in the studied northern wetlands was influenced by both air emissions (SO\({}_{2}\) mostly) and the warming climate. The beginning of industrial emissions referred to the end of 1930, whereas significant \"emissions-growth index\" correlations has been observed since 1950. The latter coincided with the first reported  
Figure 12: Mean GPP, NPP, and NDVI values (**a-c**) and time interval of the positive trends beginning (**d–f**). Period: 2001–2021. A dotted line delineates the zone of larch forests’ heavy damage and mortality. larch forests' decline and mortality [9]. The growth decrease was continuous until the end of 1990, when the tree mortality rate reached its maxima. In the following period (since 2000), larch growth experienced an increase, which was attributed to the combined effect of pollution load decrease and air and soil warming. SO\({}_{2}\) emissions (the main pollutant) decreased from c. 2.5 M t/year in 1980 to c. 1.6 M t/year at present due to technology improvements, together with nickel plant closure (2016). Some contribution to the growth positive trends may be related to the CO\({}_{2}\) fertilization effect. However, that effect is low in cold habitats [34]. In the \"background zone\", the larch growth increase correlated with air temperature only, although certain wind directions emissions partly covered that area, which indicated that emissions load was within the larch compensatory range. Since the end of 1990, elevated air temperatures in the beginning (June) and in the end (August-September) stimulated larch growth, whereas July temperatures' influence on the growth decreased. For comparison, during the previous period (1950-2000), larch GI was mostly dependent on the July temperatures (R\({}^{2}\) = 0.54), whereas June and August-September influences were insignificant. A similar effect of low growth response to July temperature was described for the Siberian pine growth within mountain forest-tundra ecotone [35]. Thus, with warming larch growth limitation by July, temperatures shifted to temperatures in the beginning and the end of growth period. Meanwhile, early (May) warms led to a GI decrease. That effect was caused by \"warming provocation\" of the phloem and cambium activity, while soil water was not still available with consequent needles and twigs desiccation, as well as the damage by late frosts. In extreme cases, larches may flash needles prior to snowmelt. Opposite to temperature, soil moisture increases negatively influenced larch growth due to the \"cold soils effect\", i.e., low soil temperatures slow down root activity with consequent tree growth decreases (Figures 10d and 11d). This effect is most pronounced in waterlogged habitats. The other negative factor is a limited root oxygen supply within poorly drained soils. However, growth limitation by water availability has also been described in permafrost zones [36; 37; 38]. Larch may experience a moisture deficit at the beginning of the growth period when elevated air temperatures are sufficient for photosynthesis, while soils are still mostly frozen [37]. For bushes (willow and alder), we observed similar larch growth and temperature shifts. Notably, that GI of willow more strongly responded to the June and September temperatures in comparison with larch and alder. That effect may be caused by non-leaf photosynthesis, i.e., photosynthesis occurred in the bark tissues. For example, for aspen, which also belongs to the Salicaceae family, non-leaf (bark) photosynthesis produced 15-20% of the total photosynthetic production [39]. In a harsh environment, willow species may grow even in the leafless form. Meanwhile, the significance of non-leaf photosynthesis for shrubs and trees surviving in the harsh habitat needs further studies.