# Pollution and Climatic Influence on Trees in the Siberian Arctic Wetlands
## 3 Results
### Trees' Growth Index Responses to Pollution Impact and Climate Variables  
The dynamics of larch growth index (GI) in the impact and control zones until 1950 was highly synchronous (correlation coefficient \(r=0.9\); Figure 6). In the second half of the 20 th century, that synchronicity decreased (\(r=0.79\)) due to changes mainly in the low-frequency component. Since 2000, a GI increase has been recorded for trees in the impact zone mostly, and during the last decade (compared to the same pre-warming period of 1990-1999) larch GI increased by 1.6 times; for comparison, within the control zone, GI increased 1.2 times (Figure 6b). Figure 5: Wind-dependent smoke plums orientation. Emissions from the Nadezhda copper and nickel plants (the latter closed in 2016) indicated by numbers 1, 2, and 3 at figures; (**a**)—Landsat 5 scene (14 July 1997), (**b**)—Landsat 5 scene (30 August 1997), (**c**)—Landsat 8 scene (18 July 2019), (**d**)—Landsat 7 scene (1 September 2001). The dynamics of radial increment (a decrease in GI until the second half of the 1990s with a subsequent increase at the beginning of the 21 st century) and the dynamics of SO\({}_{2}\) emissions are in antiphase (an increase and subsequent decrease in emissions; Figure 7). Although the local minima of the radial increment during this period correspond to similar minima recorded for other wood species and are likely to be of a climatic origin, the increase in the radial increment in the impact zone (low frequency component; Figure 6b,c) coincides in time with a decrease in SO\({}_{2}\) emissions. According to dendrochronological analysis, the onset of tree mortality dates back to the late 1950s. The maximum mortality of larch trees was observed in 1980-1990 against the background of an increase in sulfur dioxide emissions (Figure 8). Comparative analysis of the GI and sulfur dioxide emissions dynamics for the period 1985-2020 (i.e., for period of reliable SO\({}_{2}\) emissions data) indicates significant dependence of GI on emissions (\(r=-0.56\) for larch and \(r=-0.41\) for alder, \(p<0.05\); Figure 9). No significant correlations between GI and sulfur dioxide emissions were found for the larch trees in the control zone and for the willow in the impact zone. Figure 6: Growth index chronologies of larch ((**a**)—chronology for entire region; (**b**)—chronologies for impact zone, i.e., dead and survived trees, and for control zone) and shrubs (willow and alder; (**c**)). Black arrows indicate oil spill date. Figure 8: Pre-mortality growth index dynamics of dead trees (solid line) and tree mortality dynamics (gray bars; % of all sampled dead trees). Figure 7: Comparing of survived larch trees’ growth index with sulfur dioxide emission values (black solid lines indicate 5-year moving average). Figure 9: [PERSON]’s correlation coefficient of growth index vs. sulfur dioxide values (1985–2020): * (star) indicate significant correlations with \(p<0.05\). on climatic changes between the impact and control zones. It should be noted that the observed prolongation of the growth period, due to increased temperature in June and September, stimulates radial growth increase. Meanwhile, there is significant negative correlation between GI and the spring warmth (temperature increases in May; Figure 10a). The multiple regression analysis indicates the volume of SO2 emissions and air temperature as the main factors influencing the radial increment of larch trees in the impact zone. At the same time, the air temperature in June-July has a stimulating effect on radial increment, and the May temperature has an inhibitory effect (Equation (1)). \[\text{GI}=0.65\times\text{T}_{\text{JJ}}-0.43\times\text{SO}_{2}-0.35\times \text{T}_{\text{MAY}}-0.04, \tag{1}\]  
where GI--growth index, TJJ--June-July temperature, TMAX--May temperature, SO2--values of SO2 emissions. Explained dispersion: R\({}^{2}\) = 0.7. Period: 1985-2020.