Assessing the Distribution and Driving Effects of Net Primary Productivity along an Elevation Gradient in Subtropical Regions of China

Our research results indicate that from 2001 to 2018, the average ANPP across the entire subtropical research area in China exhibited a gradual increasing trend from north to south, a finding that aligns with previously reported trends [58,59,60]. The regional average ANPP stood at 677.17 gC m⁻² a⁻¹, coinciding with earlier research [28]. PNPP, standing for net primary productivity under conditions free from human disturbance, delineates the level of productivity of the most stable and mature vegetation that can be developed under natural conditions. This indicator is an essential tool for assessing the quality status of natural ecosystems, and it effectively distinguishes the impact of human activities on the ecological environment. In this study, the calculation of PNPP was based on the Thornthwaite memorial model. The HNPP, or human-affected net primary productivity, is calculated as the difference between the PNPP and the ANPP, quantifying the influence of human activities on vegetation productivity [52,53] The computational formulas for PNPP and HNPP are comprehensively detailed in Section 3.2. The findings revealed that the computed PNPP significantly exceeded both the ANPP and the HNPP, with the HNPP also being greater than the ANPP. The descending order of these indices was PNPP, HNPP, and ANPP. This concurred with previous research [49,61,62] and indicated the efficacy of these indicators in analyzing the study area.

The study area transitioned from the Central Plains to the southern part of China. The southern region is characterized by its richness in forest resources and a higher vegetation coverage compared to the central region. The abundance of precipitation, a suitable warm climate, and rich groundwater resources collectively contribute to favorable conditions for vegetation growth [63], resulting in a distribution trend where the vegetation is less dense in the north and denser in the south.

Areas with significant increases in the ANPP were detailed in the first section of Section 3 of this document. On a pixel scale, these regions of notable ANPP augmentation were predominantly found in the southeastern part of Yunnan Province, central Henan, the central and northern parts of Anhui, as well as near the mountains and the ecological function conservation areas within these provinces. The increase in the ANPP found within Yunnan Province was consistent with findings from previous studies [64,65]. There are mainly two reasons for this growth in ANPP in the area. The first reason is attributed to major national ecological restoration projects such as the ‘Grain for Green’ program, which led to the conversion of non-arboreal vegetation into forests in certain areas [66]. In addition, the retained farmlands, benefiting from favorable hydrothermal conditions, have also contributed to the increase in ANPP [65]. Anhui and Henan provinces are among the regions in China with a substantial distribution of farmlands, hosting a large number of croplands, where research by Luo and Xin et al. observed an increase in the area of croplands for wheat, rice, maize, etc. [67,68]; thus, the rapid increase in crop yield possibly spurred the increase in the region’s ANPP. The rise in mountainous ANPP can be attributed to the combined effects of temperature and precipitation, where, between altitudes of 1600 and 2800 m, photosynthesis in plants was positively correlated with temperature, a relationship that also applies to photorespiration and dark respiration [69]. Between 2001 and 2018, we observed an increasing trend in the total annual precipitation and the average annual temperature (Section 3.5). Abundant rainfall enriches the soil, providing more nutrients for the vegetation [69]. A naturally warm and humid environment offers improved hydrothermal conditions for mountain vegetation, promoting its growth. This accounts for the enhancement in the ANPP of the mountain vegetation. National policies can explain the significant increase in ANPP in the ecological function protection areas. Following the issuance of the “Outline of National Key Ecological Function Protection Area Planning” by the Chinese Ministry of Environmental Protection, ecological protection areas have become an indispensable part of environmental protection efforts in the country, especially the key ecological function protection areas, which are of great significance and highly valued. The designated protection areas in this study with significant ANPP increases were Southwest Karst Ecological Function Protection Area, Three Gorges Reservoir Area Ecological Function Protection Area, Qinling Mountain Ecological Function Protection Area, Dabie Mountain Ecological Function Protection Area, and Huai River Source Ecological Function Protection Area. These areas are categorized into two types: soil and water conservation ecological function protection areas and water source conservation ecological function protection. According to the “Outline of National Key Ecological Function Protection Area Planning”, both the soil and water conservation ecological function protection areas and water source conservation ecological function protection areas have established clear standards and guiding principles for vegetation protection. In the soil and water conservation ecological function protection areas, activities that destroy the forest, such as deforestation and slash-and-burn agriculture, as well as agricultural development on steep slopes, are prohibited. These measures aim to slow down vegetation degradation and maintain ecological stability. The area also proposed the need to implement soil and water conservation ecological restoration projects. For the water source conservation ecological function protection areas, the planning outline suggests integration with existing ecological conservation and development projects to enhance the comprehensive management and ecological restoration of forests, grasslands, and wetlands. Overall, these regulations strive to strengthen vegetation protection within these ecological function areas by limiting unsustainable land use activities and promoting ecological restoration projects. This way, not only is the ability to retain soil and water resources enhanced, but the net primary productivity of the vegetation within the region is also increased, thus promoting the long-term sustainability of the ecosystem.

In the middle and lower reaches of the Yangtze River and the Pearl River Delta urban agglomerations, we observed a significant decline in the ANPP, which is likely related to the high levels of urbanization and economic development in these areas [66,70]. Urbanization is characterized by the concentration of population and finance, along with an increase in artificial land cover, leading to a substantial reduction in vegetation [71]. In fact, as the middle and lower reaches of the Yangtze River and the Pearl River Delta are core areas of domestic economic development, their rapid economic growth and urban expansion have exerted significant pressure on the local vegetation. Therefore, the accelerating urbanization and economic activities have led to a notable decline in the ANPP in these regions.

It is noteworthy that there were differences in the annual average NPP (net primary productivity) among various forest types. In forests, evergreen broadleaf forests exhibited the highest annual average ANPP (728 gC m⁻² a⁻¹), followed by mixed forests (646 gC m⁻² a⁻¹). The other forest types were deciduous broadleaf forests (632 gC m⁻² a⁻¹) and evergreen needleleaf forests (622 gC m⁻² a⁻¹), respectively. These findings were consistent with previous research findings [72,73].

From a spatial perspective, evergreen needleleaf forests, deciduous needleleaf forests, and deciduous broadleaf forests are predominantly located in the western and northern parts of the study area. This distribution could be attributed to two main factors. Firstly, it may be due to a series of ecological forestry projects implemented in China since 1978, such as the Natural Forest Conservation Program and the Grain for Green Program [74,75]. These initiatives have promoted an increase in vegetation cover, forest area, and carbon storage. Secondly, from 2001 to 2018, there was an upward trend in the annual total precipitation and the average temperature in the study area. The increased rainfall enriched the soil fertility, providing more nutrients for vegetation, thus enhancing growth and leading to an increase in the ANPP [69]. Evergreen broadleaf forests are mainly distributed in the southeastern coastal regions of the study area and in southern Yunnan Province. In these areas, rapid urbanization has been a primary factor contributing to the decline in ANPP [76].

Different from the ANPP, which is influenced by human activities [77], the PNPP is primarily constrained by the topography, climate, and vegetation type [50]. In this study, the PNPP was significantly influenced by the temperature and precipitation, a phenomenon that can be fully explained from the perspective of algorithm construction [47]. The temporal variations of PNPP and ANPP were similar, yet the annual variation rate of PNPP was lower than that of ANPP, which might be attributed to the substantial deterioration of the vegetation level within the study area due to human activities, leading to a greater variation of ANPP relative to PNPP. This indicates that human activities have a significant impact on the variations of ANPP in this study area.

In the comprehensive analysis of altitudinal gradients, we observed a notable upward trend of ANPP within the study area as the elevation increased, gradually stabilizing with minor fluctuations before a significant decline; this finding was aligned with previous academic studies [65,78]. Additionally, the study area experienced a general decline in the average temperature and precipitation as the altitude rose (Figure 12) [65,79].

Within the altitude range of 1600 to 1900 m, the partial correlation between the temperature and the ANPP, although on the verge of shifting to negative, still presented as positive. In this elevation range, the temperature decreased with increasing altitude (Figure 12), and, concurrently, the ANPP showed a declining trend. This phenomenon could be attributed to the temperature decrease with rising altitude, which is unfavorable for the temperature conditions necessary for photosynthesis, thus limiting the rate of photosynthesis and carbon assimilation processes [78,80]. Therefore, the ANPP exhibited a downward trend under the influence of decreasing temperatures.

Further observation revealed that within the altitude range of 2000 to 2800 m, the partial correlation between temperature and ANPP was negatively correlated. In this elevation range, the temperature continued to decrease with increasing altitude, while the ANPP values showed an upward trend. This trend might be related to the increase in solar radiation within this altitude range. According to previous research, at these altitudes, the positive influence of solar radiation on the ANPP progressively intensifies, exhibiting a significant positive driving effect [65]. Consequently, within this elevation range, the favorable impact of solar radiation on ANPP may surpass the negative effects of decreasing temperature, ultimately leading to an increase in the ANPP, which peaked within this altitude range.

In regions above 2800 m, the annual average temperature dropped below 10 °C and precipitation also fell below 1100 mm, but still remained above 650 mm a⁻¹ 100m⁻¹. Both the average temperature and precipitation exhibited a decreasing trend with increasing altitude. The ANPP showed a significant downward trend, consistent with previous research findings [65]. In areas above 5000 m, the ANPP remained below 5 gC m⁻² a⁻¹.

In line with previous studies [51,81,82,83], we observed a negative response of ANPP to precipitation in high altitude areas, potentially due to the fact that in subtropical regions, despite precipitation decreasing with lower altitudes, the values remain between 600 and 800 mm a⁻¹ 100m⁻¹ (with a mean value of 1271.27 mm a⁻¹ 100 m⁻¹ in low altitude areas), possibly exceeding vegetation growth requirements. When precipitation surpasses the vegetation growth demands, the photosynthesis of vegetation reacts adversely to the reduced radiation and increased relative humidity. Hence, from low to high altitude areas, as precipitation values approach the vegetation growth needs, the adverse reactions induced by radiation and relative humidity decrease, causing an increment in the ANPP values. However, the influencing factors for ANPP in high altitude areas are not limited to precipitation; studies demonstrate that temperature remains the dominant factor for ANPP in these regions [84,85]. In this study, the ANPP in high altitude areas predominantly showed a positive correlation with temperature. Consequently, despite the reduction in precipitation with increased altitude causing some increase in the ANPP, the overall trend of ANPP was significantly downward due to the dominant effect of temperature. Within elevation intervals divided into 100 m units, different factors dictated the changes in ANPP at various altitudes. The variation in ANPP along the elevation gradient illustrated the influence of several meteorological elements, such as precipitation, temperature, and solar radiation, coupled with human activities, on the altitudinal distribution of ANPP.

The alteration in ANPP is primarily induced by human activities and climate fluctuations. Numerous preceding studies have utilized ANPP, HNPP, and PNPP to differentiate these two elements’ effects on ANPP [18,48,49,61,62,86]. Our research findings indicate that a combined influence of climate change and human activities has led to the increase in ANPP in the study area, with their combined effect being greater than the individual impacts of climate change or human activities on the vegetation’s ANPP. This increase can be ascribed to the escalating temperatures and heightened precipitation in the region, which create conducive climatic conditions in most areas. These factors boost photosynthesis and carbon storage in vegetation, ultimately contributing to the elevation of ANPP [87,88]. Additionally, human-driven efforts towards vegetation restoration, including grassland conservation policies and diverse ecological restoration initiatives, have markedly enhanced vegetation coverage and alleviated the detrimental effects of human activities on ANPP [89,90]. Thus, in most of the areas within the study region where there is an increase, the growth in ANPP is dominantly driven by both climate change and human activities [62].

The increase in ANPP in certain mountainous regions of the study area can be attributed solely to climate change. This positive effect is likely due to two factors: the lack of human activity in these secluded mountainous regions and the favorable conditions created by increased temperatures for vegetation growth. Temperature is a critical factor that limits plant growth. Research has indicated [88,91] that in areas with an elevation of approximately 1000 m, photosynthesis in vegetation is often hindered at temperatures below 20 °C, which results in a lower ANPP in many mountainous areas due to the cooler climate. Nevertheless, as per the research presented in Section 3.5 of this study, the study region experienced a general upward trend in annual average temperatures from 2001 to 2018. The gradual increase in temperature promotes vegetation growth [89,92], especially in mountainous areas, leading to a positive influence of climate change in these regions.

Regions where solely human interventions have resulted in a rise in the ANPP were predominantly concentrated in southern Henan Province, northern Hubei Province, northern Anhui Province, central-southern Yunnan Province, and central-northern Guizhou Province. These areas are primarily characterized by farmlands and the karst regions of China. In these regions, the impact of climate change on ANPP is relatively minor compared to human activities. As analyzed in Section 4.1, the areas of southern Henan, northern Hubei, and northern Anhui are abundant in farmlands, where human activities, closely linked to agricultural practices, play a significant role. Factors such as expanded farmland area or enhanced efficiency of land use are largely due to human involvement [67,68]. In Yunnan and Guizhou Provinces, located in the Southwest China karst region, ecological protection projects, ecological migration, and initiatives such as converting farmland back to forest or grassland have created favorable conditions for the accumulation of ANPP [93]. Numerous studies have revealed an increase in vegetation cover and a growing trend of ANPP in the Southwest China karst region [89,93,94,95]. As a result, human interventions have resulted in a rise in ANPP in these areas, exerting a more significant effect than that of climate change.

In coastal areas where ANPP has declined, human activities have been the primary factor driving the reduction, with their negative impact being more significant than that of climate change, aligning with the findings of previous research [76,87,88]. This is likely due to the rapid urbanization in these regions. Studies have shown that accelerated urbanization leads to a deterioration in the quality of forest ecosystems and a significant decrease in vegetation cover. Urban expansion encroaches upon the habitats vital for vegetation, resulting in substantial reductions in vegetation cover and ANPP. Although previous research indicates [87] that in highly urbanized areas, low productivity vegetation is gradually replaced by higher productivity types, the increase in ANPP due to this vegetation transformation was insufficient to offset the decline caused by the expansion of impervious urban surfaces. Moreover, according to a study by Xin et al., the adverse effects of nocturnal warming on vegetation growth in the Fujian and Guangdong regions also contributed to the decline in ANPP [76]. Therefore, in these regions, human activities predominantly contribute to the reduction in ANPP.

Temperature and precipitation have always been the two most commonly used factors in studying the driving models between ANPP and meteorological elements [49,64,76,96,97,98,99], as they largely affect the growth and distribution of vegetation. However, previous studies have not reached consistent conclusions regarding the impact of these two meteorological factors on ANPP. Studies reporting both positive and adverse effects of these factors on ANPP have been documented [48,62,65,96]. This study largely agrees with the conclusion that temperature has a positive effect on ANPP, with 65.14% of the regions showing a positive correlation between temperature and ANPP, and a trend of temperature increase was observed in areas with positive correlations. Therefore, in these regions, the increase in temperature leading to a rise in ANPP might be due to the fact that, under the global warming trends in the study area, a temperature increase before reaching the optimum growth temperature can enhance soil microbial activity, improve photosynthetic efficiency, and promote organic matter accumulation in plants [97,98], thereby improving productivity. Another possible reason is that the rise in temperature advances the phenological phase of the vegetation, extending the growth period and accelerating the photosynthesis process, which promotes the carbon assimilation process and biomass accumulation [100]. However, in areas within the research region where the temperature was negatively correlated with ANPP, these were mainly distributed in the central and northern parts of Yunnan Province, the central high altitude areas of Taiwan Province, and the central and southern parts of Hubei Province.

We observed a trend of temperature increase in the middle and upper areas of Yunnan Province and the central high-elevation areas of Taiwan Province, implying that in these areas, the rise in temperature has a negative effect on ANPP, leading to a decrease in ANPP. This might be because global warming increases the evapotranspiration of the ecosystem, which intensifies soil water shortage, restricting vegetation growth and reducing ANPP [76,99]. Another part of the reason could be that in high altitude areas, global warming causes snow and ice to melt, increasing soil moisture at plant root zones, causing plants to be in an anaerobic state, which in turn leads to a decrease in ANPP [101]. In the southern part of Hubei Province, we observed a trend of temperature decrease, which in this area promotes vegetation growth, resulting in an increase in ANPP. The Han River Plain, located in the heart of Hubei Province and extending southwards, encompasses a vast expanse of farmland and stands as one of the key grain-producing bases. Studies have shown that the arable land area in the Han River Plain is gradually expanding [67,68], and with the continuous upgrading of agricultural technology, its yield is expected to increase. The rapid increase in crop yield is likely the primary reason for the increase in ANPP in this area, and its impact outweighs the negative effects of a temperature decrease on ANPP, possibly being the reason why the ANPP increased in this region despite the downward trend in temperature.

Changes in precipitation can influence the growth of plant root systems and the supply of moisture [102]. In our study, precipitation was found to be a more significant enhancer of ANPP, as approximately 60% of the regions exhibited a positive correlation between precipitation and ANPP. These areas have experienced a yearly rise in precipitation, suggesting that the increase in rainfall positively influences vegetation growth in these regions. Precipitation represents the maximum amount of water available to regional vegetation [51]. The level of precipitation directly affects the soil moisture content, which is a key factor linking precipitation and ANPP. Precipitation drives dynamic changes in soil inorganic nitrogen within the system because the main source of new nitrogen in the system is net deposition. The increase in precipitation enhances soil moisture content, promoting plant activities and improving the efficiency of photosynthesis in vegetation, allowing for the accumulation of more organic matter [18]. However, the remaining 40% of areas with a negative correlation and those with a positive correlation exhibited considerable spatial heterogeneity; the former were often found in the eastern coastal areas closer to the sea, while the latter were more often distributed inland. In coastal areas, we also observed a trend of increasing precipitation; the negative correlation led to a decrease in ANPP in these regions. This might be because the increase in precipitation was accompanied by an increase in cloud cover, thereby reducing solar radiation, altering the oxygen environment in the root system, suppressing the photosynthesis of vegetation, and leading to a decline in productivity [82,83]. An increase in precipitation can also lead to a reduction in soil organic matter, accelerating the rate of soil erosion and causing flood disasters, destroying the environment for vegetation growth, and reducing ANPP [103].