The conservation patterns of grassland ecosystem in response to the forage-livestock balance in North China

HUANG Lin, NING Jia, ZHU Ping, ZHENG Yuhan, ZHAI Jun

Journal of Geographical Sciences ›› 2021, Vol. 31 ›› Issue (4) : 518-534.

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Journal of Geographical Sciences ›› 2021, Vol. 31 ›› Issue (4) : 518-534. DOI: 10.1007/s11442-021-1856-6
Research Articles

The conservation patterns of grassland ecosystem in response to the forage-livestock balance in North China

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Abstract

Being a key ecological security barrier and production base for grassland animal husbandry in China, the balance between grassland forage supply and livestock-carrying pressure in North China directly affects grassland degradation and restoration, thereby impacting grassland ecosystem services. This paper analyzes the spatiotemporal variation in grassland vegetation coverage, forage supply, and the balance between grassland forage supply and livestock-carrying pressure from 2000 to 2015 in North China. We then discuss the spatial pattern of grassland ecological conservation under the impacts of grassland degradation and restoration, and livestock-carrying pressure. Over the last 16 years, the total grassland area in North China decreased by about 16,000 km 2, with vegetation coverage degraded by 6.7% of the grasslands but significantly restored by another 5.4% of grasslands. The provisioning of forage by natural grassland mainly increased over time, with an annual growth rate of approximately 0.3 kg/ha, but livestock-carrying pressure also increased continuously. The livestock-carrying pressure index without any supplementary feeding reached as high as 3.8. Apart from the potential livestock-carrying capacity in northeastern Inner Mongolia and the central Tibetan Plateau, most regions in North China are currently overloaded. Considering the actual supplementary feeding during the cold season, the livestock-carrying pressure index is about 3.1, with the livestock-carrying pressure mitigated in central and eastern Inner Mongolia. Assuming full supplementary feeding in the cold season, livestock-carrying pressure index will fall to 1.9, with the livestock-carrying pressure alleviated significantly in Inner Mongolia and on the Tibetan Plateau. Finally, we propose different conservation and development strategies to balance grassland ecological conservation and animal husbandry production in different regions of protected areas, pastoral areas, farming-pastoral ecotone, and farming areas, according to the grassland ecological protection patterns.

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grassland degradation and restoration / forage supply / livestock-carrying pressure / grassland ecosystem conservation / North China

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HUANG Lin, NING Jia, ZHU Ping, ZHENG Yuhan, ZHAI Jun. The conservation patterns of grassland ecosystem in response to the forage-livestock balance in North China[J]. Journal of Geographical Sciences, 2021, 31(4): 518-534 https://doi.org/10.1007/s11442-021-1856-6

1 Introduction

In recent five decades, large areas of extant natural grassland ecosystems have been degraded in Inner Mongolia, the Tibetan Plateau, and on the northern slopes of the Tianshan Mountains (Zhou et al., 2005; Yang et al., 2011; Du et al., 2015). This grassland degradation, however, is characterized by regional differences. For example, in humid and semi-humid meadow grasslands, their fragmentation first occurs, followed by continuously declining coverage, until finally the degradation process of black soil beach formation is completed. In arid and semi-arid steppe grasslands, their vegetation coverage decreases continuously, eventually forming sandy land and decertified grasslands (Liu et al., 2008; Liu et al., 2011; Zhou et al., 2016). Therefore, grassland degradation is not only restrictive to lost vegetation but also entails soil degradation, and the latter is a pressing core issue (Wu et al., 2013). As the degradation of grassland is exacerbated, its vegetation coverage, productivity, aboveground and underground biomass are gradually reduced, the vegetation succession process are stalled or strongly slowed (Shang et al., 2007; Liu et al., 2009; Zhang et al., 2016). In such cases, soil organic matter, enzyme activity, and microbial community are gradually diminished (Feng et al., 2010; Wen et al., 2014; Hu et al., 2014). Other associated declines include those in grassland forage supply, carbon sequestration, water conservation, sand fixation and other services (Li et al., 2010; Xu et al., 2013; Huang et al., 2014), in addition to losses in biodiversity (Wang et al., 2001).
Grassland degradation or restoration is affected by both natural factors and human activities (Huang et al., 2013). Climate warming and changes in precipitation can lead to an earlier re-greening stage and a longer growing season (Fang et al., 2005; Ma et al., 2008; Bai et al., 2008) that affect plant species richness and soil nutrients (Ma et al., 2010; Shen et al., 2016), which together will determine overall grassland vegetation productivity. The conflict between grassland forage supply and livestock-carrying pressure caused by long-term overgrazing is the main human factor driving grassland degradation around the world (Zhou et al., 2005, 2016; Qian et al., 2007; Fan et al., 2011). In China, most of its natural grasslands now exceed their maximum carrying capacity, being overload by 27% in Qinghai, 89% in Tibet (Qian et al., 2007), and 102% in Inner Mongolia (Li and Ji, 2004). Long grazing periods and high livestock pressure lead to severe grassland degradation during the cold season (Fan et al., 2011). The aboveground and underground biomass of the temperate grasslands and alpine meadows generally decrease with an increase in grazing intensity, such that overgrazing can reduce underground biomass by up to 30% to 50% (Wang and Wang, 1999).
In the past 20 years, there has been a continuous implementation of a series of grassland ecological protection and restoration programs, such as returning grazing to grassland, grassland ecological protection subsidies and incentives, measures to prohibit grazing, rotation grazing, enclosure and natural restoration, supplementary sowing, artificial or semi-artificial grass planting, and controls of toxic weeds and rodent pests (Zhou et al., 2016; Sun et al., 2012; Ma et al., 2016). This has undoubtedly assisted in the restoration of degraded grasslands in the Xilin Gol League of Inner Mongolia, Sanjiangyuan (Three River Source Region) of Qinghai, as well as on the Loess Plateau (Huang et al., 2013; Cheng et al., 2014; Xu et al., 2017). Fencing enclosure and grazing prohibition have had demonstrated positive effects on soil, vegetation coverage, biomass, and biodiversity of degraded grassland (Li et al., 2012; Shan et al., 2012; Xiao et al., 2015), but have adversely affected ecosystem stability. Accordingly, it is necessary to determine a reasonable, and ideally optimal, enclosure time and to carry out reasonable utilization (Shan et al., 2012; Cheng et al., 2014; Xiao et al., 2015; Ma et al., 2016).
Achieving a balance between forage supply and livestock-carrying pressure would be an ideal state. This could be adjusted as needed, by maintaining a reasonable livestock carrying capacity according to local grassland production, and optimizing grazing strategies to quickly respond to short- and long-term grassland changes (Xu, 2014). Analyzing this balance between forage supply and livestock-carrying pressure can also provide a scientific basis for the formulation of grassland ecological conservation strategies (Li and Ji, 2004; Zhang et al., 2014). The traditional way to do this mainly uses grasslands’ production and utilization rates, along with estimates of livestock numbers (Huang et al., 2000). In recent years, most studies have estimated the theoretical grass yield based on a linear relationship of a vegetation index or productivity model, combined with a theoretically estimated carrying capacity of grassland determined by the number of livestock, to analyze the regional balance between forage supply and livestock-carrying pressure (Chang et al., 2012; Zhang et al., 2014; Liao et al., 2014). In addition, many studies have also considered the impacts of wild vertebrate herbivores on the balance between forage supply and livestock-carrying pressure in ecological protection areas, such as Sanjiangyuan and Aerjin (Dong et al., 2015; Yang et al., 2018).
The conservation and restoration of grassland ecosystems is extremely important for establishing ecological security barriers and modernizing animal husbandry practices in North China. To resolve the contradiction between grassland ecological conservation and resource utilization, it is necessary to shift from “emergency rescue based on ecological management” to “long-term mechanism of ecological sustainability”, and accordingly plan the grasslands’ ecological and production functions both scientifically and reasonably. Therefore, many problems require solving. How does the grassland and its provisioning of forage supply, and the livestock-carrying pressure each change over space and time? How to realize the balance between forage supply and livestock-carrying pressure to achieve the “win-win” goal of grassland ecological conservation and economic development? In this paper, the spatiotemporal changes of grassland vegetation coverage, forage supply, and balance between forage supply and livestock-carrying pressure at large scales were analyzed. The spatial pattern of grassland ecosystem conservation and restoration in North China were then discussed in the context of grassland degradation and restoration and livestock-carrying pressure. Furthermore, the targeted and sustainable strategies and suggestions for ecological conservation, coupled with relevant regions’ grassland production and utilization were put forward.

2 Materials and methods

2.1 Overview of grassland resources in the study area

In this paper, the Inner Mongolia Autonomous Region, Xinjiang Uygur Autonomous Region, Tibet Autonomous Region, Qinghai Province, and Gansu Province were selected as study areas. The grassland area of these five regions accounts for 20%, 12.6%, 26.7%, 15.7%, and 4.0% of China’s total grassland area, respectively (She et al., 2016), and correspondingly 67%, 35%, 72%, 51%, and 55% of their provincial territory. Among them, the area of available grassland for animal husbandry accounts for 81%, 84%, 88%, 92% and 90% of the provincial grassland area, respectively. We classified natural grassland into six categories: temperate grassland, temperate meadow, temperate desert, alpine grassland, alpine meadow, and alpine desert.

2.2 Grassland degradation and restoration based on land use and land cover change and vegetation coverage change

Using the ecological characteristics of different grassland types, we developed a remote sensing classification system for grassland degradation and restoration in North China based on vegetation coverage change. We did this by referring to the remote sensing information extraction method for grassland degradation and restoration in the Tibetan Plateau, Inner Mongolia, Xinjiang and other regions as described by Liu et al. (2008) , Xu et al. (2017) , Gao et al. (2005), Yang et al. (2011) , Yan et al. (2011) , and according to the classification indexes of natural grassland degradation, desertification and salinization (GB 19377-2003), remote sensing interpretation and classification system of grassland degradation, restoration and changing trends. In our classification system, grassland degradation can be slight, moderate, or severe, while grassland restoration can be slight, obvious, or an extremely significant improvement (Table 1).
Table 1 Classification types of grassland degradation and restoration according to changes in vegetation coverage in North China
Grassland types Vegetation
coverage change		 Degradation and restoration types Grassland types Vegetation
coverage change		 Degradation and restoration types
Temperate grassland Decreased by 10%‒20% Slight degradation Alpine grassland Decreased by 10%‒20% Slight degradation
Decreased by 20%‒40% Moderate
degradation		 Decreased by 20%‒30% Moderate
degradation
Decreased by more than 40% Severe degradation Decreased by more than 30% Severe
degradation
Increased by 10%‒20% Slight restoration Increased by 10%‒20% Slight restoration
Increased by 20%‒40% Obvious
restoration		 Increased by 20%‒30% Obvious
restoration
Increased by more than 40% Extremely significant improvement Increased by more than 30% Extremely significant improvement
Temperate meadow Decreased by 10%‒20% Slight degradation Alpine meadow Decreased by 10%‒20% Slight degradation
Decreased by 20%‒30% Moderate
degradation		 Decreased by 20%‒40% Moderate
degradation
Decreased by more than 30% Severe degradation Decreased by more than 40% Severe
degradation
Increased by 10%‒20% Slight restoration Increased by 10%‒20% Slight restoration
Increased by 20%‒30% Obvious
restoration		 Increased by 20%‒40% Obvious
restoration
Increased by more than 30% Extremely significant improvement Increased by more than 40% Extremely significant improvement
Temperate desert Decreased by 5%‒15% Slight degradation Alpine desert Decreased by 5%‒10% Slight degradation
Decreased by 15%‒40% Moderate
degradation		 Decreased by 10%‒20% Moderate
degradation
Decreased by more than 40% Severe degradation Decreased by more than 20% Severe
degradation
Increased by 5%‒15% Slight restoration Increased by 5%‒10% Slight restoration
Increased by 15%‒40% Obvious
restoration		 Increased by 10%‒20% Obvious
restoration
Increased by more than 40% Extremely significant improvement Increased by more than 20% Extremely significant improvement
From the land use and land cover change (LUCC) database from the Resource and Environment Science and Data Center (http://www.resdc.cn/), the transferring information data between grassland and other land use types in North China from 2000 to 2015, as well as the dynamic changes of grassland with differing coverage, were extracted. The LUCC data consisted of forest, cropland, grassland, wetland, constructed land, and desert, all of which were obtained via manual interpretations of Landsat TM/ETM+, CBRS, and ENVISAT images as information sources. For this, the comprehensive evaluation accuracy reached 94.3% (Liu et al., 2018).
Using the pixel dichotomy method and the MODIS normalized vegetation index (NDVI) data processed by S-G filtering, the monthly vegetation coverage from 2000 to 2015 was calculated, at a spatial resolution of 1 km. Based on the conversion of grassland types and the dynamic changes of grassland vegetation coverage, combined with the Sen-trend degree methods and the Mann-Kendall trend test, both grassland degradation and restoration were assessed on a large scale.

2.3 Estimation method of grassland forage supply

Forage supply (Y) is an important supply service of grassland and the material basis of grassland animal husbandry production (Fan et al., 2010a, 2010b). Besides the forage supply of natural grassland, there is artificial grassland, straw, silage, grain and other supplementary forage types that are also available in the study areas. Firstly, the monthly net primary productivity (NPP) of grassland vegetation was simulated by the NASA-CASA model and obtained from 2000 to 2015, at a spatial resolution of 1 km. Then, the model simulation results were verified by using the field grassland biomass data. This revealed that the NPP simulation had good accuracy (Chen et al., 2019).
Based on the ratio of aboveground to underground biomass, combined with the spatial distribution of grassland types and grassland resource survey data, the NPP data was applied to estimate the forage yield of natural grassland from 2000 to 2015 (Fan et al., 2010a; Zhang et al., 2014), by using the formula as follows:
${{Y}_{n}}=\frac{\text{NPP}}{t(1+r)}$
(1)
where Yn is the forage yield of natural grassland per unit area (kg/ha); r is the ratio of aboveground to underground biomass of grassland vegetation, and the values for different grassland types were taken from Shen et al. (2016) and Ma et al. (2014) ; t is the coefficient of biomass conversion to productivity, t=0.45.

2.4 Estimation of livestock-carrying pressure and evaluation on the balance between forage supply and livestock-carrying pressure

The grassland livestock-carrying pressure index (IP), from 2000 to 2015, was estimated as the ratio of actual livestock-carrying capacity (CS) to the theoretical livestock-carrying capacity (Cl) (Qian et al., 2007; Zhang et al., 2014), and then the balance between forage supply and livestock-carrying pressure was analyzed.
${{I}_{P}}={{C}_{S}}/{{C}_{l}}$
(2)
${{C}_{S}}=(1-{{C}_{b}})[{{C}_{n}}\cdot (1+{{C}_{h}})\cdot {{G}_{t}}]/(365{{A}_{r}})$
(3)
${{C}_{\text{l}}}=(Y\cdot {{C}_{o}}\cdot {{U}_{t}}\cdot {{H}_{a}})/({{S}_{f}}\cdot {{D}_{f}}\cdot {{G}_{t}})$
(4)
In formula (3), CS is the number of standard sheep units actually carried by the unit area of grassland (standard sheep unit/ha); Cb is the supplementary feeding rate, that is, the proportion of the total amount of forage accounted by supplementary forages, such as artificial grassland, silage and straw, which could be derived from available statistical data or literature sources (Li et al., 2014; GMR-IMAR, 2018); Cn is the number of livestock at the end of the year (standard sheep units), which is derived from county statistical data, and for which a single head of cattle is equivalent to four standard sheep units; Ch is the yearly livestock slaughter rate, which was 21% in Tibet, 45% in Gansu, 15% in Qinghai, 40% in Xinjiang, and 80% in Inner Mongolia; Gt is the grazing time on the grassland, which was 210 days per year in cold season (winter and spring) and 155 days per year in warm season (summer and autumn); Ar is the grassland area (ha), for which the distribution and area of seasonal pastures are determined according to the 1:1,000,000 grassland resource map of China.
In formula (4), Cl is the number of standard sheep units sustained by grassland; Co is the proportion of available grassland; Ut is the forage utilization rate; Ha is the proportion of edible forage, which was determined by the grassland type; Sf is the daily feed per sheep unit, set to 4 kg of fresh grass per day; Df is the ratio of dry weight to fresh weight of forage (1:3).
We estimated the IP under three scenarios: (1) the livestock-carrying pressure on the forage from natural grasslands; (2) the livestock-carrying pressure on natural grassland forage when incorporating the actual cold season supplementary feeding; (3) the livestock-carrying pressure on natural grassland forage presuming that full supplementary feeding occurs in the cold season. If IP = 1, it means that the actual livestock-carrying capacity is equivalent to the theoretical livestock-carrying capacity; hence, the grassland livestock-carrying capacity is appropriate. But, if IP > 1, it means that the grassland is being overloaded, whereas a value of IP < 1 indicates that the grassland still has the potential to sustain livestock on it.

3 Results

3.1 Spatiotemporal variation in grassland cover in North China from 2000 to 2015

From 2000 to 2015, the net grassland area in North China was decreased by 16,300 km2, of which about 11,000 km2 were converted to grassland from other ecosystems and 27,300 km2 were converted to other ecosystems from grassland (Figure 1a and Table 2). Among them, the grassland area in Xinjiang decreased the most, by 10,800 km2. Generally, the vegetation coverage of grassland showed a trend of increasing, with that of temperate grasslands the most pronounced, at about 1.6% per year. However, the temperate and alpine meadows decreased yearly by 1.0% and 0.3%, respectively (Table 3). For the provinces, grassland vegetation coverage in Gansu, Qinghai and Inner Mongolia all increased, whereas in Tibet and Xinjiang it decreased.
Table 2 Statistics of grassland cover changes in North China from 2000 to 2015 (km2)
Grassland cover changes Region Forest Cropland Wetland Desert Others
From other ecosystems converted to grassland Tibet 12.16 0.66 3.23 1.81 0
Xinjiang 221.43 562.63 755.57 706.52 7.41
Inner Mongolia 693.09 1733.76 682.47 3668.45 63.07
Qinghai 20.78 77.02 16.35 304.43 2.14
Gansu 75.79 891.26 56.38 400.42 4.95
Total 1023.25 3265.33 1514 5081.63 77.57
From grassland converted to other ecosystems Tibet 2.67 7.8 615.29 226.4 49.29
Xinjiang 115.44 11459.14 733.74 190.90 545.71
Inner Mongolia 1816.39 2924.24 633.75 3122.95 1840.80
Qinghai 5.63 149.51 429.12 842.29 245.63
Gansu 380.03 627.20 47.68 138.77 163.94
Total 2320.16 15167.89 2459.58 4521.31 2845.37
Table 3 Statistics of grassland vegetation coverage changes in North China from 2000 to 2015
Grassland types North China Tibet
Mean (%) Mean change (%) Annual trend (%/a) Mean (%) Mean change (%) Annual trend (%/a)
Alpine grassland 18.97 0.11 0.01 15.38 -0.21 -0.06
Alpine meadow 53.35 -0.32 -0.06 45.86 -0.58 -0.13
Alpine desert 7.68 0.34 0.06 6.34 0.29 0.05
Temperate grassland 43.62 1.59 0.28 36.78 -1.16 -0.24
Temperate meadow 75.88 -0.99 -0.19 67.58 -1.31 -0.23
Temperate desert 9.41 0.52 0.10 6.03 -0.34 -0.06
Total 9.40 0.53 0.10 28.50 -0.37 -0.09
Grassland types Inner Mongolia Xinjiang
Mean (%) Mean change (%) Annual trend (%/a) Mean (%) Mean change (%) Annual trend (%/a)
Alpine grassland 14.69 0.35 0.11 25.77 -0.09 -0.04
Alpine meadow 53.27 1.74 0.35 51.60 -0.76 -0.17
Alpine desert - - - 5.41 0.38 0.07
Temperate grassland 45.03 2.13 0.36 43.71 -1.19 -0.22
Temperate meadow 85.27 0.54 0.07 76.41 -2.82 -0.55
Temperate desert 4.68 0.21 0.05 13.70 0.67 0.13
Total 29.41 1.33 0.23 28.17 -0.11 -0.02
Grassland types Gansu Qinghai
Mean (%) Mean change (%) Annual trend (%/a) Mean (%) Mean change (%) Annual trend (%/a)
Alpine grassland 17.05 1.19 0.24 29.45 1.14 0.24
Alpine meadow 75.32 -0.05 0.00 59.85 0.01 0.01
Alpine desert 16.67 0.93 0.20 16.75 0.49 0.10
Temperate grassland 40.94 3.34 0.67 37.48 2.03 0.42
Temperate meadow 80.17 0.77 0.15 65.60 0.66 0.16
Temperate desert 7.29 0.75 0.13 9.58 0.77 0.16
Total 35.07 1.19 0.23 35.07 1.19 0.23
The area of degraded grassland accounted for about 6.7% of the total grassland area in North China, being mainly in a state of slight degradation (Table 4) and primarily occurring in the temperate grasslands of central and eastern Inner Mongolia and the alpine meadows on the southeastern Tibetan Plateau (Figure 1b). The area of restored grassland accounted for about 5.4%, being mainly in a state of slight restoration (Table 4) and concentrated in temperate grassland and desert lying north and south of the Tianshan Mountains. Provincially, Inner Mongolia had the highest proportion of grassland degradation, while Xinjiang has the highest proportion of grassland restoration.
Figure 1 Distributions of grassland use change (a), grassland degradation and restoration (b) in North China

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3.2 Spatiotemporal variation of forage supply and forage-livestock balance in North China

From 2000 to 2015, the annual average forage supply of natural grasslands in North China was 33.4 kg/ha, being the highest in alpine meadow, at 50.0 kg/ha, and the lowest in temperate desert, at 14.8 kg/ha (Table 5). At the provincial level, Qinghai had the highest mean forage supply, at 46.0 kg/ha, with alpine steppe providing 57.7 kg/ha. By contrast, Xinjiang had the lowest mean forage supply, at 23.9 kg/ha, with temperate meadow of Tianshan and Altay Mountains providing 35.4 kg/ha (Figure 2a). Over the past 16 years, the forage supply of natural grasslands in North China has generally increased, at annual pace of 0.3 kg/ha (Table 5). Among them, the forage supply of temperate grassland, temperate meadow and alpine grassland types increased respectively by 0.43 kg/ha, 0.37 kg/ha, and 0.33 kg/ha, whereas the forage supply was reduced in the Tianshan Mountains and Lhasa River Basin (Figure 2b).
Table 4 Statistics of grassland degradation and restoration in North China
Types of grassland degradation
and restoration		 North China Tibet Inner Mongolia
Area (km2) Proportion (%) Area (km2) Proportion (%) Area (km2) Proportion (%)
Severe degradation 444 0.02 302 0.03 47 0.01
Moderate degradation 5820 0.21 3315 0.38 1454 0.18
Slight degradation 177093 6.51 67514 7.65 66023 8.38
Total degradation 183357 6.74 71131 8.06 67524 8.57
Slight restoration 119270 4.38 15625 1.77 32784 4.16
Obvious restoration 21471 0.79 3266 0.37 2080 0.26
Extremely significant improvement 4879 0.18 629 0.07 170 0.02
Total restoration 145620 5.35 19520 2.21 35034 4.45
Types of grassland degradation
and restoration		 Xinjiang Gansu Qinghai
Area (km2) Proportion (%) Area (km2) Proportion (%) Area (km2) Proportion (%)
Severe degradation 71 0.01 0 0.00 24 0.01
Moderate degradation 703 0.14 94 0.05 254 0.07
Slight degradation 25994 5.07 5426 3.14 12101 3.32
Total degradation 26768 5.22 5520 3.19 12379 3.40
Slight restoration 53245 10.38 11829 6.84 5748 1.58
Obvious restoration 14118 2.75 1595 0.92 404 0.11
Extremely significant improvement 3869 0.75 138 0.08 72 0.02
Total restoration 71232 13.89 13562 7.84 6224 1.71
Table 5 Statistics of forage yield for different grasslands in North China from 2000 to 2015
Grassland types North China Tibet Inner Mongolia
Mean (g/m2) Annual trend (g/m2·a) Mean (g/m2) Annual trend (g/m2·a) Mean (g/m2) Annual trend (g/m2·a)
Alpine grassland 20.23 0.13 19.72 0.10 20.52 0.56
Alpine meadow 49.98 0.33 44.04 0.17 48.19 0.79
Alpine desert 15.79 0.13 16.07 0.20 - -
Temperate grassland 42.75 0.43 36.41 -0.03 44.70 0.54
Temperate meadow 48.20 0.37 61.75 0.16 55.84 0.54
Temperate desert 14.80 0.15 11.33 0.07 12.38 0.14
Total 33.38 0.27 28.37 0.13 37.62 0.43
Grassland types Xinjiang Gansu Qinghai
Mean (g/m2) Annual trend (g/m2·a) Mean (g/m2) Annual trend (g/m2·a) Mean (g/m2) Annual trend (g/m2·a)
Alpine grassland 12.04 -0.02 10.82 0.02 28.78 0.43
Alpine meadow 25.41 -0.23 78.97 0.71 57.69 0.57
Alpine desert 9.41 0.11 13.22 -0.18 22.29 -0.24
Temperate grassland 34.06 -0.12 46.92 0.68 43.37 0.80
Temperate meadow 35.43 0.10 63.57 0.68 37.22 0.35
Temperate desert 17.24 0.14 12.93 0.15 16.80 0.22
Total 23.89 0.04 38.13 0.44 45.99 0.50
Figure 2 Distributions of annual average (a) and variation trends (b) of grassland forage supply in North China from 2000 to 2015

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The actual livestock-carrying capacity of counties was the highest in the farming-pastoral ecotone of Inner Mongolia, in the west of Xinjiang, in Gansu, and in the east of Qinghai, in all cases exceeding 10 standard sheep units per hectare. In stark contrast, livestock density was less than 1 standard sheep unit per hectare on the central Tibetan Plateau and in the west of Inner Mongolia, both of which are dominated by alpine desert grassland (Figure 3a). As Figure 3b shows, during the past 16 years, the actual livestock-carrying capacity of grasslands in North China has mainly increased.
Figure 3 The input parameters for the forage-livestock balance assessment in North China

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Considering only the forage supply of natural grassland under scenario 1 (Figure 4a), the average livestock-carrying pressure index is about 3.8. The livestock-carrying pressure of natural grassland in the west of Inner Mongolia, Hexi Corridor, Junggar Basin and those in surrounding areas of Tarim Basin in Xinjiang, are all mostly in a state of extreme overload, yet the natural grasslands in the northeast of Inner Mongolia and the central Tibetan Plateau still have the potential of supporting livestock.
Figure 4 Patterns and trends in livestock-carrying pressure on natural grasslands in North China from 2000 to 2015

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Under scenario 2 of actual cold season supplementary feeding (Figure 4b), the average livestock-carrying pressure index is about 3.1. This significant reduction in livestock-carrying pressure on the natural grasslands arose from the high supplementary feeding rate in central and eastern Inner Mongolia. In other regions, however, the changes in their livestock-carrying pressure index are very small due to the lower supplementary feeding rates there.
Under scenario 3, assuming the full supplementary feeding occurs in the cold season (Figure 4c), the average livestock-carrying pressure index is about 1.9, with most of the natural grasslands entering a state of insufficient livestock-carrying pressure both in Inner Mongolia and on the Tibetan Plateau. However, it is difficult to relieve the livestock-carrying pressure in moderate and severe overloaded areas due to higher livestock-carrying pressure in the warm season.
In the last 16 years, the livestock-carrying pressure on natural grasslands has continued to increase, being only mitigated in Xilin Gol (Figure 4d).

3.3 Spatial patterns of grassland ecosystem conservation and restoration in North China

Based on our overlay analysis of grassland use change, grassland degradation and restoration, and forage supply for the past 16 years, coupled with the assessment on the balance between forage supply and livestock-carrying under the different scenarios, the spatial pattern of different modes for grassland ecosystem conservation and restoration in North China were analyzed. This was done according to the principle of balancing grassland ecological protection with sustainable development of animal husbandry (Figure 5).
Figure 5 Spatial patterns of grassland conservation and restoration in North China

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Five main implications emerged from this analysis. (1) Strict natural grassland protection system is needed for nature reserves, ecologically fragile areas, key ecological function areas at the source of rivers, and alpine desert areas. For local grassland degradation caused by human factors, this may be improved via natural restoration measures such as enclosure and grazing prohibitions and halting the development of artificial grasslands.
(2) The degraded and overexploited natural grassland in the pastoral area should be allowed to lie fallow, free of grazing pressure, so as to carry out grassland restoration. Taking Yinshan Mountain, Ordos Plateau, and Lhasa River Basin as typical examples, it is necessary to restore the productivity of their natural grasslands via paused grazing, but without damaging the original vegetation, and to also make use of near-natural grass reseeding to promote the restoration of grassland vegetation and reduce the risk of invading poisonous weeds. Other worthwhile interventions include promoting the construction of artificial grassland in those areas with good water and thermal conditions; strengthening the allocation of forage resources among regions to expand the sources of forage; promoting off-site fattening of livestock, in tandem with the goal of paused grazing without halted raising livestock, reduce the number of livestock without decrease livestock production.
(3) It is imperative to implement rotational grazing and grassland improvement measures for natural grasslands that are degraded or unchanged yet characterized by insufficient livestock-carrying in pastoral areas. We should also promote natural grassland restoration via rotational grazing and grassland improvement. While protecting natural grassland, it is crucial to scientifically define the suitable and unsuitable areas for development and construction of artificial grasslands. Finally, improvements and upgrades to the planting technology of artificial grassland are recommended, to promote grazing type of artificial grassland and reduce cutting type of artificial grassland.
(4) Those natural grasslands found degraded and overloaded in the farming areas, and farming-pastoral areas ought to be the focus of future improvement. We suggest the following actions: Develop high efficiency grassland animal husbandry vigorously through the adjustment of industrial structure; improve the forage species and yield of supplementary feed by introducing grassland and farmland rotations, and changing the grain to feed for livestock. In order to achieve sustainable and high-yield planting, we should also strive to improve the sowing mode, water and fertilizer management methods, cutting or eating time, as well as the frequency and total amount of artificial grassland established. Accelerating the industrialization, scale and intensification of modern animal husbandry, so as to greatly enhance the efficiency and economic benefits of animal husbandry production is another worthwhile aim.
(5) For other natural grasslands currently undergoing recovery and under low livestock-carrying pressure, their livestock numbers should be strictly determined according to the forage supply. Taking both the eastern Tianshan Mountains and Xilin Gol as representative examples, we suggest accurately monitoring the annual seasonal, and even monthly, dynamic changes of forage yield and livestock numbers is warranted. Much, too, could be learned from animal husbandry and short-term fattening production systems in farming areas. This knowledge could be applied towards gradually increasing the amount of supplementary feed of refined materials in the cold season, so as to achieve the goal of sustaining livestock numbers and securing the long-term improvement of grasslands.

4 Discussion and conclusions

4.1 Discussion

The actual overloaded state of the natural grassland in North China discerned in this study is basically consistent with the findings to date for Inner Mongolia, Qinghai, and Tibet (Wang and Wang, 1999; Yang et al., 2000; Li and Ji, 2004). Based on this, we also determined the distribution of grasslands livestock-carrying state on the 1-km grid scale. Nonetheless, this study did have some uncertainties: (1) It was difficult to detect the information on grassland soil degradation in the form of toxic weeds and black-soil beach, based solely on changes in vegetation coverage. (2) The average amount of livestock numbers at the county scale was evenly distributed on the 1-km grid, which fails to reflect the spatial heterogeneity of the actual livestock-carrying capacity. (3) The spatial resolutions of input parameters were relatively coarse, such as the slaughter rate of livestock, grazing time of grassland, utilization rate of forage, and ratio of edible forage. They were assigned by regional average or grassland type, but this could be made more precise and localized geographically. (4) The amount of supplementary feeding varied among pastoral households, from counties to provinces, across different years. This paper estimated the supplementary feeding rate according to the edible forage reserves in cold season, but overlooked any flows in forage trade. (5) The forage supply and the balance between forage and livestock were examined in terms of yearly changes and coarsely classified into “cold season” and “warm season”. Without considering the monthly dynamics, it may be difficult to effectively solve the problem of seasonal overexploitation in pastoral areas (Fan et al., 2010b; Zhang et al., 2018). (6) In considering only the domestic livestock, any population expansion of wild herbivores was ignored, such as that of the Tibetan wild ass, Tibetan antelope, and wild yak in Sanjiangyuan, Qiangtang, Altay, Arkin, Taxkorgan and other protected areas, which has exacerbated grassland degradation because they compete with domestic livestock, leading to overgrazing (Dong et al., 2015; Yang et al., 2018).
In North China, a vast landscape dominated by an arid and semiarid climate, the relationships among grassland productivity, forage supply, and precipitation have the strongest correlation (Fang et al., 2005; Bai et al., 2008; Ma et al., 2008). Grazing affects grassland dynamics and composition via selective grazing and trampling, and aboveground and underground biomass both decrease as grazing intensifies (Fan et al., 2011). Alternatively, measures such as returning grazing land to grassland, returning farmland to grassland, leaving pasture ungrazed, and rotation grazing, and prohibition of grazing are able to restore the local grasslands. According to the spatial and temporal differences in grassland degradation and restoration and livestock-carrying pressure, we have refined different models of grassland ecological protection, production and utilization. Specifically, we take the conservation and restoration of vegetation as the focus, to change the mode of economic development as the main line, to vigorously develop the “decompression and efficiency” of sustainable ecological animal husbandry. We need to focus on balance between grassland forage and livestock pressure, through the accurate acquisition of forage supply information, we can make adjustments to herd sizes, use precise grazing, and apply knowledge and lessons learned from ecological livestock breeding. Efforts should also be made to promote ecological protection, as well as improve people’s livelihoods and coordinate development of regional economies.

4.2 Conclusions

In this paper, the degradation and restoration of grasslands, as well as the spatio-temporal changes of forage supply and livestock-carrying pressure, were analyzed in North China from 2000 to 2015, with corresponding strategies proposed for grassland ecological conservation, production, and utilization. The results show that the grassland area in North China has undergone a net decrease, but the forage supply per unit area of natural grassland has generally increased, and the livestock-carrying pressure continues to increase. The comparison of the three grass-livestock balance scenarios reveals that supplementary feeding can alleviate the livestock-carrying pressure of natural grassland in central and eastern Inner Mongolia, but this beneficial effect was not apparent in other regions because of their lower rate of supplementary feeding. Assuming full supplementary feeding in the cold season, the livestock-carrying pressure index can be substantially reduced, by about half. Based on our comprehensive analysis of grassland degradation and restoration, in light of striking a balance between grassland forage supply and livestock-carrying pressure, this study offers different strategies for achieving grassland ecosystem conservation, as well as the production and utilization of protected areas, pastoral areas, farming-pastoral areas, and farming areas.

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Funding

The Second Tibetan Plateau Scientific Expedition and Research Program, No(2019QZKK0404)
Strategic Priority Research Program of the Chinese Academy of Sciences, No(XDA20020401)

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