Three-North Shelter Forest Research Articles

Stand characteristics regulate forest water use efficiency in the Three-North Shelterbelt Forest Program region of China

Abstract The Three-North Shelter Forest Program (TNSFP) of China, one of the largest forestry ecological projects, serves as a nature-based solution to addressing ecological, social, and economic challenges and issues. This program, mainly situated in arid and semi-arid regions, critically focuses on improving water use efficiency (WUE) - maximizing carbon sequestration per unit of water consumed - as a key strategy for optimizing water resource utilization and ensuring the long-term success of vegetation restoration efforts. However, the regulatory mechanisms of forest WUE in this region remain unclear. Here, we adopted an interpretative machine-learning method to investigate the influence of main environmental elements, topographical conditions, and stand characteristics on forest WUE in the TNSFP region from 2001 to 2022 based on remote sensing products, ground monitoring data, and forest inventory data. Our study identified soil moisture (SM) as the primary factor influencing forest WUE across the TNSFP region, with higher SM levels generally leading to improved WUE in forests. However, stand characteristics strongly mediated their relationship. Specifically, forest WUE initially increases against forest density before peaking at about 1000 trees hm–2 for needle-leaved forests and 800 trees hm–2 for broad-leaved forests, respectively, then gradually declining due to water competition. When SM is relatively adequate, moderate thinning could significantly enhance forest WUE. Furthermore, implementing management strategies to improve WUE is crucial as needle-leaved forests mature. This study emphasizes the significant impacts of stand characteristics on forest WUE in the TNSFP region, offering essential insights for optimizing water resource management in managed forests across arid and semi-arid regions.

Quantitative Assessment of the Impact of the Three-North Shelter Forest Program on Vegetation Net Primary Productivity over the Past Two Decades and Its Environmental Benefits in China

Vegetation net primary productivity (NPP) is a crucial indicator for assessing the carbon balance in terrestrial ecosystems. Qualitative and comparative research on the NPP influenced by human activities, climate change, and their interactions remains insufficient. The Three-North Shelter Forest Program (TNSFP), initiated in 1978, provides a valuable reference for such investigations. This study employs an improved residual trend method to analyze the spatiotemporal patterns, trends, and driving factors of vegetation NPP during the second phase of the Three-North Shelter Forest Program (2001–2020), as well as TNSFP’s contribution to vegetation NPP. The results indicate that (1) from 2001 to 2020, overall vegetation NPP exhibited a significant fluctuating upward trend at a rate of 3.69 g C/m−2 annually; and (2) precipitation, accounting for 1.527 g C/m−2, had a more significant impact on vegetation net productivity compared to temperature (0.002 g C/m−2). Climate factors (76%) significantly influenced vegetation NPP in the Three-North Shelter Forest region more than human activities (24%). In the last decade (2011–2020), the climate contribution rate decreased to 67%, while the human activity contribution rate increased by seven percentage points compared to the previous decade (2001–2010); (3) during 2001–2020, TNSFP contributed 10.9% to the total human activity contribution to vegetation net primary productivity, approximately 2.6% of the overall contribution; (4) After the second phase of TNSFP was enacted, PM2.5 levels decreased by an average of −0.57 μg/m−3/a−1. Concurrently, soil conservation improved from 6.57 t/km2 in 2001 to 14.37 t/km2 in 2020.

Landsat-observed changes in forest cover and attribution analysis over Northern China from 1996‒2020

ABSTRACT Forest dynamics provide important information on the ecological environment. The Three-North Shelter Forest Program (TNSFP) is one of the world’s largest reforestation/afforestation programs, however the actual changes in forest cover in the Three-North Regions (TNR) of China resulting from this program are highly uncertain. This study quantified changes in fractional forest cover (FFC) at 30 m using Landsat data from 1996 to 2020. Using the Google Earth Engine platform, more than 40,000 images from Landsat-5, Landsat 7 and Landsat-8 were integrated, and the annual surface reflectance was normalized based on the multi-band least squares regression and maximum normalized difference vegetation index composite method. An ensemble learning model trained using high-resolution Gao-Fen 2 satellite imagery was used to generate the FFC long time-series product. FFC showed an increasing trend with average rates of 0.022/10a in the last 25 years, and 0.03/10a after 2010 largely corresponding to the fourth and fifth phases of the TNSFP. There are significant regional differences in the relationship between FFC and air temperature ( R 2 = 0.37) and precipitation ( R 2 = 0.49). The increased air temperature in arid and less rainy areas inhibit the FFC increase, whereas the increase in precipitation had a promoting effect. FFC appeared more sensitive to changes in solar radiation and heat conditions in humid and rainy areas. The attribution analysis revealed that 34% of FFC changes were caused by climatic variables and 66% were caused by non-climatic factors. Among them, afforestation associated with the TNSFP significantly increased FFC, and forest fire is a key factor of forest change in the Greater Khingan Ranges and Lesser Khingan Ranges regions. Planting single tree species caused biological disasters in forests of Xinjiang and Inner Mongolia. Further analysis of the increased FFC using high-level satellite products demonstrated an improvement in environmental conditions with cooler land surface temperature and higher vegetation gross primary production over the TNR.

Climate Change and Anthropogenic Activity Co-Driven Vegetation Coverage Increase in the Three-North Shelter Forest Region of China

The Three-North Shelter Forest (TNSF) program is a significant ecological safety barrier in northern China, where both climate change and anthropogenic activity contribute to the increase in vegetation coverage observed. However, comprehensive effects of these factors on vegetation have not been accurately quantified yet. This study utilized the Global Land Surface Satellite (GLASS) Advanced Very-High-Resolution Radiometer (AVHRR) Fractional Vegetation Cover (FVC) data, meteorological data, and spatial distribution of ecological engineering to analyze spatiotemporal variation of FVC and climate in the TNSF program region in China during the period 1982–2018. A partial correlation analysis and residual analysis were performed to determine the relative contribution of climate change and anthropogenic activity to the FVC and the overall effect of ecological governance. Results showed that since 1982, the average FVC in the TNSF program region was 0.201–0.253, with an average growth rate of 0.01·(10a)−1. The FVC showed a significant increase in 66.45% of the TNSF region, and will continue to increase, while only 7.02% showed a significant decrease. The coefficient of variation showed a large spatial variation, with 30.86% being in very low stability regions, mainly distributed in Inner Mongolia and the Loess Plateau. A warm and wet climate is more conducive to increasing the FVC than the warm and dry climate, and ecological engineering has the largest impact on areas with an annual accumulated precipitation greater than 300 mm. A quantitative analysis revealed that climate change and anthropogenic activity contributed to the significant increase in the FVC in 15.58% and 46.81% of the TNSF region, respectively. Therefore, ecological governance projects, such as the TNSF program, play a crucial role in enhancing the FVC in this region.

Response of Vegetation Dynamics in the Three-North Region of China to Climate and Human Activities from 1982 to 2018

To tackle ecological problems, many ecological restoration projects have been implemented in northern China. Identifying the drivers of vegetation change is critical for continued ecological engineering. In this study, three typical ecological reserves in the Three-North Shelter Forest Program Region (TNSFR) were selected to identify their vegetation development characteristics and driving mechanisms using the normalized difference vegetation index (NDVI), climate factors, and land use data. The results show that (1) NDVIs increased in the range of human activities of all of the three ecological reserves, indicating an obvious effect of the vegetation restoration projects. (2) In the planting period, vegetation restoration was mainly correlated with human activities. After entering the tending period, the impact of climate changes on vegetation dynamics was enhanced. (3) Temperature and precipitation provided approximate driving effects on vegetation dynamics in Region I, while vegetation dynamics in Regions II and III were more strongly correlated with precipitation. (4) The proportion of areas with ecological measures exceeded 50% in all three regions. In short, ecological projects in the three ecological reserves dominated the quantity of vegetation restoration, while climate changes influenced the quality of vegetation restoration.

Assessing the effects of China's Three-North Shelter Forest Program over 40 years.

Forests provide vital ecosystem services such as soil and water conservation, climate regulation, and carbon storage. Large-scale afforestation programs are being attempted in many countries to improve environmental conditions in deteriorated or unfavorable locations. China's Three Northern Protected Forest Program (TNSFP), accounting for 42.40 % of China's total land area, is the world's largest afforestation program to date. The TNSFP has continued providing critical ecosystem services to humans over 73 years (1978-2050) with a total investment of CNY 93.3 billion. To facilitate understanding of the TNSFP's contribution, the effects of the TNSFP for last 43 years were comprehensively evaluated by using integrated review of structured literature, bibliometric analysis, and thematic analysis. We incorporated and expanded the direct ecosystem services evidence of the TNSFP from wind and sand control, soil erosion control and carbon sequestration to indirect economic benefits, e.g. increasing crop yield and promoting economic development. We found that over the past 40 years of TNSFP construction, wind and sand hazards and soil erosion in China's Three-North areas have been effectively controlled, and forest carbon sequestration, grain production and economic output have steadily increased. The ecosystem services provided by the TNSFP are highly consistent with the thrust of the UN Sustainable Development Goals, and the TNSFP has contributed to the realization of SDG2, SDG8, SDG13, and SDG15. Although achieving tremendous ecological, economic, and social benefits, the TNSFP still has knowledge gaps in its scientific basis. And the limited local engagement and insufficient investment highly hinder the TNSFP from playing its multiple functions. We suggest several urgent actions and directions to address these limitations. This review could help researchers gain insight into key areas of ecological restoration in the TNSFP, providing a reference for future research in the TNSFP construction in China and other regions of the world embarking on similar journeys.

Multiple effects of climate changes and human activities on NPP increase in the Three-north Shelter Forest Program area

In the framework of ecological engineering, vegetation net primary productivity (NPP) has grown dramatically in the Three-north Shelter Forest Program area (TSFPA), which is influenced by both human activities and climate change, as well as their interaction. However, it remains unclear how the individual (pure effects) and interactive (shared and coupling effects) impacts affect and maintain the growing vegetation NPP trend. In this study, the monotonically increasing NPP trend from nonlinear trends was detected using Ensemble empirical mode decomposition (EEMD), and its persistence was investigated using boosted regression tree (BRT) models. We investigated the relative contributions of environmental factors and variable interactions, as well as spatially identified the multiple effects of climate change and human activities on the increasing trend and its persistence over TSFPA. The results indicated that: (1) The vegetation NPP was dominated by the monotonically increasing trend (41.08 %), and 30.04 % of all pixels can still maintain NPP increment. (2) Although soil moisture exhibited the highest interaction with the precipitation seasonality (32.21 %) on the monotonically increasing NPP trend, land-use change substantially interacted with soil moisture (26.97 %), snowmelt (23.34 %), and net solar radiation (13.43 %). (3) The monotonically increasing vegetation NPP was caused not only by the pure effects of climate change (23.57 %) and human activities (11.69 %) directly but also by their coupling effects (26.64 %) or shared effects (38.10 %) interactively. (4) The shared effects (50.94 %) dominated the persistently increasing NPP trend, whereas the coupling effects (78.78 %) dominated the non-persistently increasing NPP trend. The findings highlight the importance of the pure, shared, and coupling effects of human activities and climate change interactively for elucidating the driving of the vegetation NPP increment and its persistence over ecological engineering areas.

Stable isotopes reveal differences in climate sensitivity and physiological responses between dieback and healthy trees in a shelter forest

Tree-ring stable isotopes provide insights into the drought-induced eco-physiological mechanisms of forests. We investigated the growth of poplar and Mongolian pine trees decline in the Three-North Shelter Forest, and analyzed the differences in tree-ring width (TW), δ13C, δ18O, and intrinsic water-use efficiency (iWUE) between dieback and healthy trees. An R package was used to quantify the relative contributions of climatic and physiological factors to growth. Both TW and basal area increment were lower for the dieback poplar and Mongolian pine than for the healthy trees.δ13C, δ18O, and iWUE were higher for the dieback poplar than for the healthy individuals; there were no significant differences in Mongolian pine. The measured iWUE for healthy poplar, healthy and dieback Mongolian pine were consistent with the modeled iWUE for Ci/ Ca scenario, indicating the stomata were open and Ci increased along with Ca. In contrast, the measured iWUE was higher for the dieback poplar than for the healthy individuals, which was consistent with the constant Ci scenario and partial stomatal closure. A significant decreasing Ci/Ca was detected only for the dieback poplar, reflecting the diversity in the leaf gas-exchange strategies of the two species. Pearson coefficients showed that on the annual timescale, tree growth was significantly negatively correlated with temperature, VPD, and iWUE, but positively correlated with SPEI. Moreover, the relationships between TW and stable isotopes and stomatal regulation, which differed in the two species, were affected by the dieback process. Furthermore, iWUE andδ18O had the highest and lowest relative contributions to TW, respectively. The relative contribution of meteorological factors (Tem, VPD and SPEI) to the growth was significantly lower than that of physiological factors (iWUE and δ18O). Compared with the healthy controls of both species, the dieback trees were less sensitive to climate conditions, but the effects of physiological activities on growth were greater. These results provide a better understanding of the climate sensitivity and physiological responses of trees under long-term drought conditions.

Key Strategies Underlying the Adaptation of Mongolian Scots Pine (Pinussylvestris var. mongolica) in Sandy Land under Climate Change: A Review

Forest degradation and mortality have been widely reported in the context of increasingly significant global climate change. As the country with the largest total tree plantation area globally, China has a great responsibility in forestry management to cope with climate change effectively. Mongolian Scots pine (Pinus sylvestris var. mongolica) was widely introduced from its natural sites in China into several other sandy land areas for establishing shelterbelt in the Three-North Shelter Forest Program, scoring outstanding achievements in terms of wind-breaking and sand-fixing. Mongolian Scots pine plantations in China cover a total area of ~800,000 hectares, with the eldest trees having >60 years. However, plantation trees have been affected by premature senescence in their middle-age stages (i.e., dieback, growth decline, and death) since the 1990s. This phenomenon has raised concerns about the suitability of Mongolian Scots pine to sandy habitats and the rationality for further afforestation, especially under the global climate change scenario. Fortunately, dieback has occurred only sporadically at specific sites and in certain years and has not spread to other regions in northern China; nevertheless, global climate change has become increasingly significant in that region. These observations reflect the strong drought resistance and adaptability of Mongolian Scots pines. In this review, we summarized the most recent findings on the ecohydrological attributes of Mongolian Scots pine during its adaptation to both fragile habitats and climate change. Five main species-specific strategies (i.e., opportunistic water absorb strategy, hydraulic failure risk avoidance strategy, water conservation strategy, functional traits adjustment strategy, rapid regeneration strategy) were summarized, providing deep insights into the tree–water relationship. Overall, the findings of this study can be applied to improve plantation management and better cope with climate-change-related drought stress.

A dataset of soil conservation capacity assessment of the Three-North Shelter Forest Programme (2000–2020)

A dataset of soil conservation capacity assessment of the Three-North Shelter Forest Programme (2000–2020)

Per- and polyfluoroalkyl substances (PFAS) in the Three-North Shelter Forest in northern China: First survey on the effects of forests on the behavior of PFAS

Per- and polyfluoroalkyl substances (PFAS) are a family of anthropogenic chemicals, that have attracted increasing attention since the early 2000 s. Although forests have been shown to act as a filter and important sink for nonpolar persistent organic pollutants (POPs), relevant reports on PFAS are lacking. Air, soil, and leaf samples were collected inside and outside the forest from two regions of the Three-North Shelter Forest in northern China between 2017 and 2018. Twenty-seven PFAS were analyzed to study the effect of forest on the transport and fate of PFAS. The average ratios of PFAS in the air outside to inside the forest (Qair) ranged from 2.83 ± 0.78–10.6 ± 3.1. A significant positive correlation was found between Qair and the n-octanol−air partition coefficient of individual PFAS (p = 0.041). Higher Qair values for most ionic PFAS were found in broad-leaved forests than in coniferous forests. Soil samples outside the forests showed higher PFAS levels than those inside. The measured concentrations of 8:2 fluorotelomer alcohol, a volatile neutral PFAS, in leaf samples were two orders of magnitude higher than those estimated using the equilibrium leaf−air partition of nonpolar POPs, indicating that it may not fit the case of PFAS with surface activity.

Long-Term Effects of Climate and Competition on Radial Growth, Recovery, and Resistance in Mongolian Pines.

Understanding the response of tree growth and drought vulnerability to climate and competition is critical for managing plantation forests. We analyzed the growth of Mongolian pines in six forests planted by the Three-North Shelter Forest Program with tree-ring data and stand structures. A retroactive reconstruction method was used to depict the growth-competition relationships of Mongolian pines during the growth period and their climatic responses under different competition levels. Drought vulnerability was analyzed by measuring the basal area increment (BAI) of different competition indices (CIs). In young trees, differences in BAIs in stands with different CIs were not statistically significant. After 15–20 years, medium- and high-CI stands had significantly lower tree-ring widths (TWs) and BAIs than the low-CI stands (p < 0.05). The standardized precipitation evapotranspiration index (SPEI), precipitation, relative humidity, and vapor pressure deficit were major factors affecting tree growth. On a regional scale, climate outweighed competition in determining radial growth. The relative contribution of climatic factors increased with the gap in SPEI between plantation sites and the native range, while the reverse pattern of the competition-growth relationship was observed. Drought reduced TWs and BAIs at all sites. Stands of different CIs exhibited similar resistance, but, compared with low-CI stands, high- and medium-CI stands had significantly lower recovery, resilience, and relative resilience, indicating they were more susceptible to drought stresses. Modeled CI was significantly negatively related to resistance, resilience, and relative resilience, indicating a density-dependence of tree response to drought. After exposure to multiple sequential drought events, the relative resilience of high-CI stands decreased to almost zero; this failure to fully recover to pre-drought growth rates suggests increased mortality in the future. In contrast, low-CI stands are more likely to survive in hotter, more arid climates. These results provide a better understanding of the roles of competition and climate on the growth of Mongolian pines and offer a new perspective for investigating the density-dependent recovery and resilience of these forests.

Considerations on Forest Changes of Northwest China in Past Seven Decades

Forests cover four billion hectares (31%) of the Earth’s landmass and contain over 75% of all carbon in vegetation. They provide renewable raw materials and natural amenities, protect land and water resources, harbor biological diversity, and mitigate climate change. However, due to less precipitation, the forest coverage rate is only 5.86% in Northwest China. The forests in these arid areas are mainly distributed in alpine areas, which play a key role in runoff regulation and ensure the ecological and economic development of the middle and lower reaches of the inland basins. In the past several decades, China had experienced large-scale deforestation and reforestation. What were the changes of the few forest areas and growing stock in arid Northwest China? Has forest quality been restored? Changes of that had been analyzed in this article. The results showed that the forest area and growing stock decreased greatly from the 1950s to 1970s; the artificial forest area increased clearly; the natural forest area stabilized from the 1980s, growing stock of forest per unit area was still lower than that in the 1950s; and the forest quality had not yet been restored. Results of the analysis of influencing factors showed that the policy-led human activities in different periods were the efficient cause of forest changes in Northwest China. With the development of forestry science, more and more attention has been paid to forest ecological restoration. “Three-North Shelter Forest Program (TNSFP)” in 1978 had made great progress on forest areas by afforestation, and the ecological benefits and economic benefits also increased remarkably owing to the increase in the artificial forest area. In recent years, great progress on forest areas has been made by afforestation, and the forests also contribute significantly to the ecology and economy. However, large-scale afforestation in the Loess Plateau had caused a phenomenon of large investment and little effect on water conservation in some afforestation areas, which showed that afforestation initiated under different polices, which were not always scientifically based, resulted in unintended consequences. Clear ecological principles should be used to ensure best environmental and forest ecology outcomes.

A Preliminary Study of the Impacts of Shelter Forest on Soil Erosion in Cultivated Land: Evidence from integrated 137Cs and 210Pbex Measurements

The Three-North Shelter Forest Program (TNSFP) has played an important role in increasing the forest coverage and decreasing the wind erosion rate. Fallout radionuclides (FRNs), such as caesium-137 (137Cs) and excess lead-210 (210Pbex), have extensively been used as fast and economical tools to estimate erosion rates over a broad range of spatial and temporal scales. However, the 137Cs and 210Pbex conversion models used to estimate annual soil erosion rates under steady state conditions have not been used to predict corresponding rates following reforestation due to the land-use changes that have occurred since 1963 in China. This study investigated the influence of shelter forest on soil erosion of cultivated land (CL) by using 137Cs and 210Pbex in northern China. Opposing trends in the spatial variation were found between the 137Cs and 210Pbex inventories, which were due to their differing sources and mass balances within the soil profile. Exploiting this finding, we propose a conceptual 137Cs and 210Pbex variation model to detect depth variations between before and after reforestation. Wind erosion rate was 29.11 ± 14.42 Mg ha-1 yr-1 in the CL when estimated with 137Cs. However, estimation by the revised 210Pbex mass balance model showed that wind erosion rates were 68.55 ± 22.11 and -8.52 ± 47.32 Mg ha-1 yr-1 before and after the surrounding forest matured, respectively. The spatial variation in the 137Cs inventories mainly reflects the soil erosion patterns before the surrounding shelter forest matured, while the spatial variation in the 210Pbex inventories mainly reflects the soil erosion patterns after the surrounding shelter forest matured. Furthermore, the soil erosion rates that were deduced from the 210Pbex conversion model confirmed that the TNSFP played an important role in decreasing the soil erosion rates and intercepting sandstorms in northern China. Thus, the advantages and limitations associated with the use of FRNs for assessing wind erosion in the study area were also proposed. More attention should also be paid to the limitation of estimating soil erosion rates by using FRNs measurements in the TNSFP region of China.

LAI-indicated vegetation dynamic in ecologically fragile region: A case study in the Three-North Shelter Forest program region of China

The Three-North Shelter Forest Program (TNSFP) region of China is an important ecological region covering more than 42.4% of China’s land area. Several ecological restoration projects have been implemented in this region, but evaluation work is relatively limited partly due to the selection of suitable remote sensing products. In this study, leaf area index is proposed as a suitable indicator to evaluate the ecological restoration situation as it is an indicator reflects the growth of green vegetations. The Global Land Surface Satellite (GLASS) LAI dataset and land cover dataset from 2000 to 2015 were adopted and analyzed in this study. The forest cover area slightly increased, while the grassland area decreased. Although large-scale forestation movements were carried out across the entire TNSFP region, the forest growth conditions mainly improved in regions with an annual precipitation greater than 400 mm. Furthermore, LAI-precipitation correlations were very high (r greater than 0.6) in the regions across 400 mm isohyets, but lower (r < 0.4) far away from 400 mm isohyets. In relatively arid regions, forest growth did not show an obvious increase trend and sometimes a decrease trend. The increase in the LAI of these regions was mainly due to the restoration of grassland. Therefore, although LAI in most of the TNSFP region showed increasing trends, it is still not sufficient to state that these trends were caused by forestation projects in China.

Analysis of Runoff Trends and Drivers in the Haihe River Basin, China.

During the past decades, runoff has been highly influenced by climate change and human activities in Haihe River basin, and it is important to analyze the runoff trends and the drivers of its change to guide water resources management. The Mann–Kendall method and Pettitt test were conducted to analyze the hydrological and climate trends. Data from six sub-basins were used, including runoff at six representative hydrological stations and precipitation and air temperature at 49 meteorological stations. We used multiple-regression analysis and policy review to explore the influence of climate change and human activities on the runoff change at six sub-basins. According to the results, annual runoff showed a significant downward trend at six hydrological stations (p < 0.05), and the most probable change points at all stations showed up during the period from the late 1970s to the early 1980s. Moreover, the middle and late 1990s could be another probable abrupt change point at Luan River and Chaobai River. The declining trend of the annual mean precipitation at the six sub-basins was insignificant (p > 0.05), and there were no significant abrupt change points except the Zhang River area (p < 0.05). Compared with the precipitation trend, the annual mean air temperature exhibited a significant increasing trend at all stations, and the period from the late 1980s to the early 1990s might be the most probable abrupt change points at all four sub-basins. The trend analysis and the abrupt change point analysis suggest that mean air temperature is the main climate factor that will lead to the decline in the runoff time-series, while the insignificant downward trend of the precipitation might accelerate the downward trend of the runoff data. Through elevant policy measures, including land-use reform and the construction of the Three-North (north, northeast, and northwest China) Shelter Forest, China started to implement a family-contract responsibility system and initiated the first stage of construction of the Three-North Shelter Forest Program in 1978. The land-use reform policies greatly stimulated the peasants’ initiative for land management and significantly changed the land use pattern and water use quantity in the Haihe River basin in a short time. Besides, the precipitation decreased and the air temperature rose, so an abrupt change in runoff occurred from the late 1970s to the early 1980s. The abrupt change in the runoff in the middle and late 1990s highly tallied with the construction time of the Three-North Shelter Forest Program. After near 20 years of construction of the Three-North Shelter Forest Program, the forest area increased, the forest quality had been improved, and the vegetation coverage on the underlying surface had been changed significantly, so the construction of the Three-North Shelter Forest Program was an important cause of runoff change in the middle and late 1990s. Also, change in precipitation and air temperature enlarged the effect of change in the runoff.

Payoff from afforestation under the Three-North Shelter Forest Program

Large-scale afforestation is a primary tool for combatting desertification and global warming. However, the consequences for future ecosystems are complex and poorly understood. To judge the payback from afforestation in arid regions, we assessed the effects of the Three-North Shelter Forest Program, which is the world’s largest ecological restoration program, in predominantly semi-arid to arid regions in China. Our analysis revealed a net loss of 230.8 × 109 RMB annually for afforestation, suggesting that tree planting is a poor choice in much of northern China. However, establishment of fruit tree plantations and protecting vegetation to allow natural recovery produced large annual benefits (235.5 × 109 RMB and 447.43 × 109 RMB, respectively) due to having the highest product value and lowest restoration cost, respectively. Because the net benefits were negative for artificial afforestation, and some net benefits were negative for some restoration programs in some regions, it is essential to change the proportions of the different restoration methods both to improve the restoration success and ensure the survival of the residents in areas affected by the program.

Tree-Ring Analysis Reveals Density-Dependent Vulnerability to Drought in Planted Mongolian Pines

Population density influences tree responses to environmental stresses, such as drought and high temperature. Prolonged drought negatively affects the health of Mongolian pines in forests planted by the Three-North Shelter Forest Program in North China. To understand the relationship between stand density and drought-induced forest decline, and to generate information regarding the development of future management strategies, we analyzed the vulnerability to drought of planted Mongolian pines at three stand densities. A tree-ring width index for trees from each density was established from tree-ring data covering the period 1988–2018 and was compared for differences in radial growth. Resistance (Rt), recovery (Rc), resilience (Rs), and relative resilience (RRs) in response to drought events were calculated from the smoothed basal area increment (BAI) curves. The high-density (HDT) group showed a consistently lower tree-ring width than the border trees (BT) and low-density (LDT) groups. The BAI curve of the HDT group started to decrease five years earlier than the LDT and BT groups. Pearson correlation analysis revealed that the radial growth of all of the groups was related to precipitation, relative humidity (RH), potential evapotranspiration (ET0), and standardized precipitation evapotranspiration index (SPEI) in the previous October and the most recent July, indicating that Mongolian pine trees of different densities had similar growth–climate relationships. Over the three decades, the trees experienced three severe drought events, each causing reduced tree-ring width and BAI. All of the groups showed similar Rc to each drought event, but the HDT group exhibited significantly lower Rt, Rs, and RRs than the BT group, suggesting that the HDT trees were more vulnerable to repeated drought stress. The RRs of the HDT group decreased progressively after each drought event and attained &lt;0 after the third event. All of the groups showed similar trends regarding water consumption under varying weather conditions, but the HDT group showed significantly reduced whole-tree hydraulic capability compared with the other two groups. From these results, HDT trees exhibit ecophysiological memory effects from successive droughts, including sap flux dysfunction and higher competition index, which may prevent recovery of pre-drought growth rates. HDT trees may be at greater risk of mortality under future drought disturbance.

Factors Contributing to Efficient Forest Production in the Region of the Three-North Shelter Forest Program, China

Forests are the most important renewable resources and provide critical ecosystem services worldwide, especially the provisioning service, making a great contribution to human well-being. The Three-North Shelter Forest Program (TNSFP) is a large-scale ecological project aimed at improving ecological environments and consolidating economic construction in China through the development of artificial forests. In our study, stochastic frontier analysis was adopted to estimate forest production efficiency (FPE) by using dynamic panel data. Based on the FPE of 13 provinces located within the TNSFP region during the period 2000–2016, the effects of the natural and socioeconomic influencing factors on FPE were further explored by using the Tobit regression model. The estimated results confirmed the validity of the constructed model and revealed an increasing trend of the mean annual FPE value, which ranged from 0.3147 in 2000 to 0.5681 in 2016. The FPE was declining from the eastern region to the western region in 2000. However, this spatial distribution characteristic changed enormously in 2016; regions with low FPE were in the center of the TNSFP region, surrounded by the regions with high FPE. Moreover, the following factors positively influenced FPE: average temperature (1.4476), total annual rainfall (0.0800), per capita GDP (0.0882), the education levels of forestry employees (0.2120), the number of forest technology stations in townships (0.0149), and disease and pest control areas (0.0190). However, the impacts of the policy relating to the Natural Forest Protection Program on FPE were insignificant. These influencing factors had differential effects on FPE within the TNSFP’s three sub-regions during the period 2000–2016. These findings can contribute to more efficient forest management and strengthen resource and environment management.

Impacts of the Three-North shelter forest program on the main soil nutrients in Northern Shaanxi China: A meta-analysis

Vegetation restoration in arid and semi-arid areas plays an extremely important role in improving the ecological environment and preventing soil erosion. The construction of the Three-North Shelter Forest (SF) in Northwestern China commenced several decades ago to increase forest productivity, improve soil fertility, and achieve better ecological benefits in arid and semi-arid zone. To date, many studies have researched the effect of SF on the soil nutrients in Northern Shaanxi, China, but their results have often been contradictory. Therefore, we could comprehensively review these studies to evaluate the impacts of SF on soil nutrients from the following three aspects: soil layer, planting year and tree species. This study compiles the results of 2308 observations from studies ranging from 1978 to 2018 to assess the effects of SF on the distribution of the soil organic carbon, total nitrogen, available phosphorus, and available potassium contents in Northern Shaanxi, China. Compared with farmland or abandoned land, the results showed that the greatest increases in the soil organic carbon (118.57%) and total nitrogen (86.19%) contents were observed in the 0–20 and 20–40 cm soil layers, respectively. However, compared with farmland or abandoned land, the greatest increases in both the soil available phosphorus (14.17%) and available potassium (71.13%) contents were observed in the 0–20 cm soil layer. Planting Pinus Sylvestris var. mongolica (PS) is more conducive for increasing the soil organic carbon (120.86%) and available phosphorus (74.33%) contents. Additionally, Pinus tabuliformis (PT) is more beneficial for increasing the soil total nitrogen content (73.82%). Meanwhile, Robinia pseudoacacia (RP) and Caragana korshinskii (CK) are more effective for increasing the available potassium (72.93%) content. This study demonstrated that soil organic carbon (104.07%), total nitrogen (130.28%), and available potassium (125.60%) contents were most affected in planting years >30 years, and the available phosphorus content was most affected in planting years 0–10 years (27.33%). In conclusion, SF can significantly increase soil nutrient contents in areas of Northern Shaanxi, China. Planting shelter forest is recommended to improve the soil environment and achieve the greatest soil fertility benefits.