Net Ecosystem Exchanges of Spruce Forest Carbon Dioxide Fluxes in Two Consecutive Years in Qilian Mountains

Abstract

The net ecosystem CO₂ exchange (NEE) of spruce forest ecosystems is poorly understood by the lack of measurements of CO₂ in the Qilian Mountain of Western China. Thus, we conducted consecutive measurements of CO₂ fluxes using tower-based the eddy covariance method from 2021 to 2022. These results indicated that daily NEE of spruce forest indicated a robust temporal pattern ranging from −28.43 to 29.62 g C m⁻² from 2021 to 2022. Remarkable carbon sink characteristics were presented from late May to late September. Month accumulative NEE fluxes ranged from −336.57 to 142.22 g C m⁻² in two years. Additionally, average carbon sink was 591.51 ± 37.41 g C m⁻² in Qilian Mountain. NEE was negatively driven by vapor pressure deficit (VPD) and average air temperature (p < 0.05), as determined using the structural equation model. However, the direct effect coefficient of precipitation on NEE was weak. VPD was positively driven by air temperature and negatively determined by precipitation. In conclusion, a future warming scenario would significantly decrease the carbon sink of the spruce forest in Qilian Mountain.

1. Introduction

Forests cover approximately 4.2 billion hectares globally, accounting for 30% of the total land area [1]. Forests sequestrate carbon primarily through tree growth, which are the largest carbon sinks among terrestrial ecosystems [2]. The global forest carbon pool was estimated as 861 Pg carbon [3]. Over 100 Pg of carbon is stored in the biomass of Amazonia forests [4]. The annual land biosphere sink was estimated to be approximately 0.20–0.25 Pg carbon in China during the past decades, forests being the major carbon sink by approximately 0.18 Pg yr⁻¹ [5,6].

Global forest carbon sink gradually increased during the past decades, but the magnitude varied among different regions [6,7]. Terrestrial carbon sinks are mainly distributed in the mid- and high latitudes of the Northern Hemisphere [6]. Pine forests were a strong carbon sink by approximately 740.3 g C m⁻² in the Northeast China [8]. Annual NEE was estimated as −502.1 g C m⁻², accounting for about 32% of gross primary product in broad-leaved mixed forests [9]. Annual NEE flux of spruce forests was a robust carbon sink of 545.99 g C m⁻² on the northeast Tibetan Plateau [10]. NEE was estimated to be approximately −77 g C m⁻² yr⁻¹ in a semi-arid shrubland of Northern China [11]. Boreal forests were a minor CO₂ sink of 64.01 g m⁻² yr⁻¹ in the northeast of China [12]. However, a typical southern taiga nemorose spruce forest was found to be a source of atmospheric CO₂ [13]. Weak net CO₂ sinks in three years were reported to approximate 31.1 g m⁻² in a coniferous forest of Central Siberia [14]. Understanding land biosphere fluxes has been hampered by sparse data coverage and long-term eddy covariance observations are scarce.

Climate change and environment factors are major drivers of carbon sinks [6], such as NEE being restricted by soil drought, vapor pressure deficit (VPD) being over 1.0 kPa, and canopy conductance significantly decreased, which depressed carbon NEE [15]. The vapor pressure deficit ranged from 0.13 to 1.88 kilopascal due to extreme temperature events, which reduced boreal forest carbon uptake [12]. Air temperature was the driving factor for NEE, followed by photosynthetically active radiation in boreal forests [10]. After the surface soil temperature exceeded 1 °C, forest ecosystems became persistent net CO₂ sinks [14]. The variety of soil moisture regimes provide non-uniformity in the response reactions of the CO₂ ecosystem–atmosphere exchange concerning climate anomalies [13]. Winter warming and decreases in autumn and winter precipitation may induce spring drought, which impairs carbon sequestration of shrubland [11]. In conclusion, the determined factors were significantly distinct in different regions.

The Qilian Mountain is a vital ecological security barrier role in Western China and the Tibetan Plateau [10], as is has carbon sink and biodiversity conservation functions. Previous studies have focused on forests’ primary production variations; however, studies of net ecosystem exchange magnitude on CO₂ fluxes and its driving mechanism are extremely lacking. Thus, we consecutively monitored CO₂ fluxes through the tower-based eddy covariance method from 2021 to 2022, and two scientific hypotheses were validated. Firstly, we maintain that NEE was weak in 2022 compared with that in 2021 because of the higher air temperature in 2022. Secondly, air temperature was a more important driving factor for NEE than precipitation.

2. Material and Methods

2.1. Site Description

The field research site was located in Sigou village of Menyuan County on the southern slope of Qilian Mountains, the Tibetan Plateau (37.13° N, 102.37° E, 2520 m). The tower-based eddy covariance of CO₂ exchange was erected in a spruce forest (Picea crassifolia) with a height of 35 m on 10th August 2020 (Figure 1). It is characterized by plateau continental climate with average precipitation and air temperature of 442.0 mm and 498.3 mm and 3.78 °C and 4.41 °C in 2021 and 2022 (

Table 1. Annual climate meteorologic parameter of Qilian Mountain forests in 2021 and 2022.

Years	Air Temperature °C	Precipitation mm	Relative Humidity %	Wind Speed m/s	Wind Direction °
2021	3.78	442.0	32.21	1.29	163.15 (south)
2022	4.41	498.3	22.77	1.34	162.01 (south)
Average	4.10	470.2	27.49	1.32	162.58 (south)

). Average relative humidity, wind speed, and wind direction were 27.49%, 1.32 m/s, and 162.58° (south). Soil organic carbon and pH were 17.56 g/kg and 7.5 while total nitrogen, total phosphorus, and total potassium were 7.1, 1.82, and 20.47 g/kg, respectively.

The age of this secondary forest is approximately 70 years. Dominant species included Picea crassifolia, Betula platyphylla, Sabina przewalskii, Picea wilsonii, Pinus tabuliformis, Tamarix ramosissima, Larix principis-rupprechtii, Salix cupularis, and Populus davidiana.

2.2. Data Measurements

This manuscript discusses data of forest CO₂ flux from 1 January 2021 to 31 December 2022. Integrated CO₂ was automatically measured by open-path eddy covariance (CS106 Vaisala, Louisville, CO, USA) every half-hour, then we calculated daily average NEE flux. Missing data occurred only on a few winter days (less than 5%), a period when northern high-latitude forests function as weak carbon sources; therefore, these limitations have a negligible impact on the reliability of the annual results presented. Meanwhile, daily sensible heat flux, latent heat flux, vapor pressure deficit, precipitation, and air temperature were automatically recorded every half hour through meteorological sensors (CS106 Vaisala PTB110). The sampling frequency is 10 Hz, and the average 30 min flux data was collected and stored in TOB3 format.

2.3. Statistical Analysis

Half hour CO₂ flux (NEE) and meteorological sensor data (daily sensible heat flux, latent heat flux, vapor pressure deficit, precipitation, air temperature) were calculated by “tapply” function as daily data in R statistics (4.3.0 version). The driving factors of the three climate factors (latent heat flux, precipitation, air temperature) on NEE and vapor pressure deficit were analyzed by the structural equation model using the “piecewiseSEM” package. Figure 1 was drawn in Arcgis 10.4 and the other three figures were carried out in R statistics.

3. Results

3.1. Daily Net Ecosystem Exchange Characteristics of Spruce Forests CO₂ Flux from 2021 to 2022

The daily CO₂ flux indicated a robust temporal pattern ranging from −24.56 g C m⁻² to 29.62 g C m⁻² and from −28.43 to 24.84 g C m⁻² in 2021 and 2022, respectively (Figure 2). The average daily NEE fluxes were −1.69 ± 0.29 and −1.55 ± 0.39 g C m⁻² in Qilian Mountain.

3.2. Month and Annual Net Ecosystem Exchange Characteristics of Forest CO₂ Flux

Month accumulative NEE fluxes ranged from −305.62 to 69.18 g C m⁻² in July and February of 2021 (p < 0.05), and from −336.57 to 142.22 g C m⁻² in July and December of 2022 (p < 0.05, Figure 3). The spruce forest ecosystem played as a vital sink from May to September in two years. Furthermore, annual accumulative NEE fluxes were −617.96 and −565.06 g C m⁻² in 2021 and 2022 (p > 0.05); the average carbon sink of spruce forest was approximately 591.51 ± 37.41 g C m⁻² in Qilian Mountain.

3.3. Driving Factors of NEE and Net Radiation and Vapor Pressure Deficit Based on Structural Equation Model

A structural equation model was well formulated to explain the effects of climate factors on the NEE, vapor pressure deficit, and latent heat flux (Fisher’s value = 2.924, p = 0.568). The forest’s NEE was determined by VPD (p < 0.01) and average air temperature (p < 0.05). The direct effect coefficients were −0.281 and −0.209 (Figure 4). However, the direct effect coefficient of latent heat flux was 0.084 and not significant for the NEE. Meanwhile, the direct effect coefficient of precipitation on NEE was weak. The indirect effect coefficient of air temperature was 0.029 for the NEE due to latent heat flux. Therefore, the total effect coefficient of air temperature was −0.180 (Figure 4).

Additionally, VPD was driven by air temperature and precipitation, with the direct effect coefficients being 0.778 and −0.401 (p < 0.001); both factors were extremely significant (R² = 0.32). Warming increased VPD and drought stress, then drought further depressed NEE. In addition, we also determined that latent heat was significantly affected through air temperature (R² = 0.24, p < 0.001), but not through precipitation or other factors including VPD.

4. Discussion

Boreal forests, Earth’s second largest terrestrial biome, occupy 30% of the global forest area and significantly affect the global climate [16], which was currently thought to be an important net carbon sink for the atmosphere [17]. Revealing the temporal dynamics and driving factors of NEE is critical for predicting how the carbon exchange in boreal forests will change in response to climate change [18].

4.1. Comparative Analysis of Forest NEE Flux Across Different Sites

There was considerable variation in the forest CO₂ flux between years and sites [19]. Annual total carbon fluxes of pine forests were −756.84, −834.73, and −629.37 g C m⁻² based on the eddy covariance system in 2015, 2016, and 2017 in Northeast China [8], indicating that boreal forests were robust carbon sinks in relative high latitude regions. A strong carbon sink of −660 g C m⁻² was indicated with small interannual variability in Western Germany spruce forests from 2011 to 2017 [20]. Furthermore, net exchanges of taiga CO₂ between spruce and the atmosphere were −327 and −174 g C m⁻² in 2013 and 2016 in Russian [21]; spruce and pine forests NEE were reported as −262 and −246 g C m⁻² in Sweden [16]. However, arctic treelined forest was a weak sink of −37.73, −85.36, and −130.36 g C m⁻² from 1997 to 1999 in Canada [19].

In this study, we discovered that Qilian Mountain spruce forests were a robust carbon sink of approximately 591.51 (−617.96 and −565.06 in 2021 and 2022) g C m⁻² on the northwest Tibetan Plateau. This carbon sink capacity was higher than 545.99 g C m⁻² in an annual study [10]. The carbon sink was a little lower than those in Northeast China and Western Germany, but it was significantly more robust than these in Russian, Sweden, and Canadian forests [6,8]. In addition, the annual net ecosystem exchanges of alpine meadows and shrubs were −79.3 and −77.8 g C m⁻² in the same region of Qilian Mountain [22]. In conclusion, spruce forests were vital carbon sinks across the different ecosystems.

Furthermore, data gaps were caused by equipment failures or environmental conditions, along with low-quality data spikes exceeding 1% of raw data in each 30 min period in Swedish forests [16]. Half-hour average CO₂ fluxes were computed after spike removal, two-dimensional coordinate rotation, time lag compensation, and Webb–Pearman–Leuning corrections for density fluctuations, with minimal data gaps contributing less than 3% to the net ecosystem exchange (NEE), making them negligible [22]. EddyPro software (version 7.0) was used to screen the 30 min flux data, eliminating values associated with instrumental errors, resulting in an average retention of 68% of the original data [8]. Similarly, our study also demonstrates high reliability, as data gaps were limited to a few winter days (less than 5%), a period when high-latitude northern forests act as minor carbon sources, having minimal impact on the annual carbon flux calculations.

4.2. Determining Driving Factors of Spruce Forest CO₂ Flux in Pine Forests

Assessing the carbon cycle response to climate change is important for predicting future climate in northern high latitude regions because warming and precipitation variation may be pronounced [23]. In this paper, two years of NEE data were measured to verify the driving mechanism of the carbon dynamics. These results indicated that both drought and warming significantly depressed forest carbon sink in the Qilian Mountain spruce forest ecosystem, rather than latent heat flux and precipitation (Figure 4). Similarly, the potentially increased carbon sink of the spruce forest will be at least partly offset by a concurrent increase in VPD in interior Alaska [23]. VPD offset potential positively impacts warming on high-latitude vegetation productivity but also amplifies the negative effect of soil drying in the Northern Hemisphere [24]. Alpine ecosystem NEE was strongly regulated by atmospheric vapor pressure deficit [22]. In conclusion, VPD significantly hindered high-latitude forest ecosystem carbon sink.

In this study, the average air temperature negatively regulated spruce forest NEE; the direct and indirect effect coefficients were −0.209 and −0.180, respectively. Previous results showed that Europe coniferous forest (pine and spruce) carbon uptake decreased suffering from elevated temperature during summer in 2018 because the temperature anomaly introduced a result of gross primary production being higher than the decrease in ecosystem respiration [25]. Similarly, the mean daily NEE of the spruce forest decreased at a high air temperature and a low amount of precipitation in the beginning of the growing season [21]. The boreal forest was a relatively small carbon sink, mostly due to the colder climate and shorter growing season at higher latitudes [12]. NEEs of larch forests were −76.5 and −103.6 g C m⁻² during the growing season of 2013 and 2015 because of greater precipitation in the summer of 2015 [18]. The maximum CO₂ uptake of the larch forest was observed in July, primarily due to high photosynthesis rates influenced by optimal soil moisture conditions and air temperature in Central Siberia [18]. Forest NEE was unaffected by water content change and earlier snowmelt and greater heat accumulation produced a larger growing season sink [19].

5. Conclusions

Daily, monthly, and yearly net ecosystem exchange characteristics were studies of spruce forest CO₂ fluxes from 2021 to 2022 in Qilian Mountain on the northeast Tibetan Plateau. Therefore, annual accumulative NEE fluxes were −617.96 g C m⁻² and −565.06 g C m⁻², respectively. The average carbon sink of the spruce forest was approximately 591.51 ± 37.41 g C m⁻², and this was indicated as a robust carbon sink. Both VPD and average air temperature significantly decreased carbon sink function, but both latent heat flux precipitation weakly increased carbon sink. VPD was remarkably determined by air temperature and precipitation, with positive and negative direct effect coefficients. Our results would help to accurately evaluate carbon sink capacity and predict its variation trend in spruce forests in Qilian Mountain.