1. Introduction
Accelerated development of energy resources around the world has significantly increased forest change associated with oil and gas activities, leading to both carbon dioxide and methane emissions. The impacts of these anthropogenic indirect greenhouse gases (GHG) play a significant role on forest ecosystems at the regional and global scales [
1]. The GHG attributed up to 65 % to human activities [
2], including hydrocarbon emissions from the oil and gas infrastructures [
3] causing a global average air temperature increases by 1.4°C [
4]. Moreover, t
he extraction and refining of oil produces about 48% of hydrocarbons and 44% of carbon monoxide [
5], which can significantly impact the environment. In case of accidents or during intensive oil and gas deposit development the toxic contamination of soil can lead to reduction of air exchange in the soil hindering the water flow into the soil compared to the clean natural soils. Water shortage in the soil can lead to soil fertility and reduction of microbial activity impacting forest ecosystem negatively. Additionally, local oil and gas production delivers up to 90% of hydrocarbons, which contribute to the global GHG emissions [
6]. Due to intensive development of oil and gas depositions, oil and oil products refining the problem of their emissions locally and globally impacting the environment and humans’ health.
Recent climate change and drought-induced trees mortality affect forest ecosystems worldwide [
7]. It is well known that forest soils and permafrost are important sink for atmospheric methane emissions, where a GHG is contributing roughly 20% to the global warming [
8,
9]. Soil microorganisms remove about 30 million tons of GHG from the atmosphere annually, which is 6 - 10% of its annual flow [
10]. Living and dead trees have the potential to be CH
4 sources or sinks or both [
9]. Different tree species have different effects on the activity of CH
4 oxidation in the soil - the highest is noted under
Pinus cembra L. (27.68 mg kg
-1) in Europe, in
Larix sibirica Led. (7.98 mg kg
-1),
Pinus sylvestris L. (4.96 mg kg
-1) and the lowest - in spruce (4.62 mg kg
-1) forest [
10].
The carbon atoms fixed in tree-ring width originated from the atmospheric carbon dioxide to which the tree’s canopy is exposed [
9]. During photosynthesis several fractionation steps take place, first when CO
2 from the atmosphere (c
a) diffuses into the leaf (needle) intercellular spaces (c
i), and second during CO
2 fixation by the enzyme Rubisco [
11]. The opening and closure of stomata (g
s) determines the water control. Under warm and dry conditions trees respond to limited water resources by reducing stomatal conductance (g
s), resulting in decreasing CO
2 uptake and biomass production, in reduced intercellular CO
2 concentration (c
i). Changes in the assimilation rate of the needles will therefore influence the intercellular CO
2 concentration (c
i) through changes of the rate at which the CO
2 is utilized to form sugars and an increase or decrease in stomatal conductance will affect the rate at which this internal CO
2 (c
i) can be replenished. Trees discriminate more strongly against
13C under conditions of high (c
i), when stomata are relatively wide open, or photosynthesis is low [
11]. Under increasing CO
2 concentration, the water vapor exchange between the needles and the ambient air (c
a) is reduced and stomatal conductance decreases [
12,
13,
14]. The carbon isotopic ratio (
13C/
12C or δ
13C) in tree rings reflects signals of water availability and air humidity as a result of the impact of climate on photosynthesis. Atmospheric CO
2 is well mixed, but the sub-canopy air space can become depleted in
13C due to inputs from soil and plant interaction. Tree growth is influenced by many factors such as solar irradiance, ambient air temperature, precipitation, air humidity, soil and ground water, nutrient availability as well as water. The heavier stable carbon isotope (
13C) in tree rings is modulated by environmental parameters like temperature and moisture regime changes due to fractionation processes during CO
2 uptake as well as those related to land management, disturbances like insects [
7,
11,
12,
14,
15,
16]. Photosynthetic limitations of intrinsic Water Use Efficiency (iWUE) can be an indicator for oil refinery in case of increase, which correspond with NOx pollution [
17] and moisture changes, reducing climate sensitivity [
18]. If trees uptake a significant amount of hydrocarbon emitted from the gas and oil deposits, then we suggest that tree-ring δ
13C should also get more negative values.
Data on hydrocarbon (HC) production are available for the study area, however, it is not possible to estimate how much hydrocarbons are released from gas and oil deposits into the air, surface water, groundwater, and soil. The HC emissions most likely occur from the subsurface prior the oil and gas deposit development. It was relatively constant flow, for hundreds of thousands and even millions of years, which could change only in case of any events like earthquakes [
19]. But after the beginning of the field development, the equilibrium established during a long geological time was broken that could lead to an increase in the flow of hydrocarbons both from natural fractures and faults and due to losses during production and transportation. In the first stage of extraction, oil is extracted through natural processes. As a result, it is replaced by water. If the pressure in the reservoir does not allow oil to come to the surface, then special pumps are used to extract it. Later, secondary methods are used to extract oil. It is carried out by introducing liquids and gases into oil-bearing formations to provide the necessary amount of energy to extract oil from the earth's bowels [
20]. At this stage of production, the natural state of the reservoirs is disturbed and a significant emission of hydrocarbons into the air, water and soil
is possible. When using the hydraulic fracturing method, HC emissions can be very large, comparable to other global sources of greenhouse gases [
21]. A large increase in U.S. methane emissions over the past decade inferred from the satellite data and surface observations [
22,
23].
In the Romashkinskoye UVRT field, the hydraulic fracturing method was not used, but there could also have been noticeable HC emission during the production. Methods for assessing hidden HC emission during oil field development were not developed. Usually, only accidents in which a large volume of HC is released into soil or water are evaluated.
In this study, we hypothesized that during oil and gas deposit developments and active oil production additional emission of hydrocarbons can be emitted into the atmosphere, which can be recorded in trees growing close to the oil and gas deposit site.
To test this hypothesis, we developed new tree-ring δ13C in pine wood chronologies from the oil and gas deposits (UVRT) and for the control natural reserve Raifa (Raifa) site to reveal impact of Romashkinskoye oil and gas deposit site on the pine forests.
4. Discussion
The oil and gas industry started to be developed in Romashkinskoye, the Tatarstan Republic, Russian Federation since 1943 [
19] and in 1970s reaches the maximum in oil extraction, which was recorded in tree-ring pine δ
13C
ptrw chronologies from both UVRT and Raifa sites. Recently the oil and gas production were increased by 3.5 % compared to the past. Further increase of oil and gas production is expected in the near future [
33]. Accelerating rate of GHG emissions will undouble will change the environment, biodiversity and humans’ health. Only accidents in which a large volume of hydrocarbons released into the soil or water have been evaluated so far. Therefore, our pioneering study is highlighting how trees could respond to oil and gas development infrastructure near to the deposit and in remote sites location of the region.
Our results at the tree-ring level show higher tree growth suppression during the oil extraction for the UVRT site, which is also reflected in the tree-ring δ
13C values. This can be explained by the negative impact of developed oil and gas infrastructure on the pine forest. Our finding is in line with Pickell et al. [
34], who showed anthropogenic disturbance in developed oil and gas activities on forest landscapes in the USA [
35]. The heavier stable carbon isotope (
13C) in tree rings is modulated by hydrocarbon emissions from the infrastructure site, additionally to environmental parameters like temperature, precipitation, sunshine duration, and evapotranspiration. Moreover, anthropogenic CO
2 increase due to Suess effect and global GHG emissions accumulated in tree rings, which is reflected in drastic δ
13C increase towards the recent decades.
Interestingly, pine trees from the natural reserve Raifa site, which is undisturbed by direct human impact show more positive tree-ring with variability compared to the human-induced UVRT site. However, tree-ring δ
13C, CH
4 and CO
2 in soil showed more negative values compared to the oil and gas deposits UVRT site. One of the explanations can be that pine trees from natural reserve site are more sensitive to the impact of greenhouse emissions, while pine trees from the human-induced site are already stressed by the local and global impact. Significant impact of local oil extraction and global HC emissions was revealed for UVRT site, which supports our hypothesis that both local and global hydrocarbon emissions affect local pine forests. Increasing of intrinsic water use efficiency (iWUE) along with decreasing tree-ring width chronology from the oil and gas deposits indicate on developing of drought conditions (
Figure 4) during the vegetation period, which is confirmed by the local and spatial climatic correlation analyses. Despite site-specific and species-specific differences study by Guerrieri et al. [
17] showed that NOx pollutions alter the iWUE by confirming impact of anthropogenic factors on oak trees. It is well known that variation of c
i depends on various environmental factors and on species and site conditions. Our study demonstrates site-specific response of trees to oil and gas production after 1965 showing an offset between two sites. The c
i/c
a ratio decreases toward recent decades, indicating the closure the stomata, which is also reflected in increased iWUE trends for both sites. However, higher iWUE is observed for the control natural reserve Raifa site.
Increasing evapotranspiration and extreme changes in air temperatures, less clouds and decreasing of precipitation lead to developing drier environmental conditions. Pine trees from Raifa site record more pronounced drought conditions during July over the period from 1970 to 2021, which is also recoded in δ
13C
ptrw values. Early spring temperatures can impact tree growth significantly, by shifting vegetation period to earlier dates. Such of early spring temperature shift was observed for southern part of Siberia due to early snowmelt under recent anthropogenic warming [
36,
37]. However, here is challenging to separate impact from the global and local anthropogenic emissions. Therefore, further studies are needed for complex and multi-parameter approaches.