Long-term Vegetation Research Articles

Detection of vegetation coverage changes in the Yellow River Basin from 2003 to 2020

The Yellow River occupies a pivotal strategic position in the development and economic construction of China. Moreover, grasping the dynamics of change in long-term vegetation cover and predicting future trends in the Yellow River Basin could provide an empirical foundation for improved ecological protection and soil and water conservation initiatives. This study uses statistical methods such as Dimidiate pixel model, linear regression, Moran’s index, and coefficient of variation to conduct a spatio-temporal analysis of the vegetation coverage in the Yellow River Basin. The Hurst exponent is used for further analysis of the trend of change in the vegetation coverage across the study area. The results show that from 2003 to 2020, the fractional vegetation coverage (FVC) in the Yellow River Basin increased at an average rate of 0.19% per year. Furthermore, only 2.22% of the area of the Yellow River Basin shows a relative increase in FVC from 2003 to 2020; most of the increased area is located in the northwestern Loess Plateau. The Global Moran index values from 2003 to 2020 are all greater than 0.8, indicating that the vegetation coverage presents a strong agglomeration. According to the Local Moran index, the vegetation coverage of the Yellow River Basin presents a strong spatial difference. According to the coefficient of variation, 73% of the vegetation coverage in the Yellow River Basin has been highly stable over the past 18 years. In addition, the overall Hurst exponent for the FVC in the Yellow River Basin is less than 0.5, indicating a anti-persistence pattern of change in vegetation.

The 30m-NDVI-Based Alpine Grassland Changes and Climate Impacts in the Three-River Headwaters Region on the Qinghai-Tibet Plateau from 1990 to 2018

The response of long-term vegetation changes and climate change has been a hot topic in recent research. Previously, a Landsat-based fusion model was developed and used to produce a dataset of normalized vegetation index (NDVI) for the Three-River Headwater region on the Qinghai-Tibet Plateau with a spatial resolution of 30 m and the time spanning the nearly 30 years from 1990 to 2018. In this study, the NDVI was applied to an analysis of the spatial and temporal changes in the alpine grassland and the impacts from climate change using the Theil-Sen Median method and linear regression. The results showed that: (1) The regional mean NDVI was 0.39 and showed a spatial pattern of decreasing from the southeast to the northwest in the recent three decades. Among the three parks, the Lancang River Park had the highest NDVI (0.43), followed by the Yellow River Park (0.38) and Yangtze River Park (0.23). (2) An upward trending was found in the NDVI time series at a rate of 0.0031 yr–1 (R2 =0.62, P < 0.01) over the whole period of 1990–2018. The increasing rate (0.00649 yr–1, R2 =0.71, P < 0.01) in the latter period of 2005–2018 was nearly 2.3 times of that (0.00284 yr–1, R2 =0.31, P < 0.01) in the previous period of 1990–2005. In the latest periods, the three parks experienced rates that were 2.3 to 63 times the corresponding values in the early period. (3) The NDVI is correlated more positively with temperature than precipitation. The impacts of climate change decreased along with the coverage fraction from the higher, median and then lower levels. The climate change can explain 34% of the variability in the NDVI time series of the areas with a higher fraction of grassland coverage, while it was 31% for the median fraction and 20% for the lower fraction. This study is the first to use the 30 m NDVI dataset spanning nearly 30 years to analyze the spatial and temporal variability and climate impacts in the alpine grasslands of the Three-River Headwater region of the Qinghai-Tibet Plateau. The results provide a basis for assessments on the ecological management effects and ecological quality based on long-term baseline data with a higher spatial resolution.

Vegetation restoration improves soil hydrological properties by regulating soil physicochemical properties in the Loess Plateau, China

Soil hydrological properties play a key role in soil hydrological processes. However, the effect of long-term vegetation restoration on soil hydrological properties and the corresponding influencing mechanisms remains poorly understood. Here, three soil hydrological properties including saturated water-holding capacity (SWHC), field capacity (FC) and saturated hydraulic conductivity (Ks), as well as several basic soil properties in the Zhifanggou watershed of the Loess plateau were investigated. The variations in SWHC, FC and Ks under different vegetation restoration types and their dominant influencing factors were analyzed. Moreover, we collected available Ks data from peer-reviewed publications to determine the land use with the largest Ks across the entire Loess Plateau. The results showed that SWHC FC and Ks were increased after 20 years of vegetation restoration. The higher Ks was found in shrubland and forest in the whole Loess Plateau. Compared with cropland, Ks in shrubland was increased by 87.10% at 0–20 cm, 48.89% at 20–40 cm, and 18.37% at 40–100 cm, respectively, indicating that the impact of revegetation on Ks were most obvious in the upper soil layer. Bulk density (BD), total porosity (TP), capillary porosity (CP), noncapillary porosity (NCP) and soil organic matter (SOM) had a significant effect on SWHC, FC and Ks for different land-use types (P < 0.01). Soil porosity (i.e., TP, CP and NCP) and BD, soil chemical properties (i.e., SOM and pH), and soil particle composition explained 93.8%, 59.2%, and 13.4% of the total variance in soil hydrological properties (i.e., SWHC, FC and Ks), respectively. This indicates that soil porosity and BD are the dominant factors affecting soil hydrological properties. Moreover, soil particle composition played an important role in regulating Ks, with the contribution of 38.6%. The established pedotransfer function (PTF) of Ks using BD, clay and silt content had a better performance than two existing PTFs. This research provides a more systematic and comprehensive understanding of the soil hydrological effect of vegetation restoration in the Loess Plateau.

Long-Term Vegetation Phenology Changes and Responses to Preseason Temperature and Precipitation in Northern China

Due to the complex coupling between phenology and climatic factors, the influence mechanism of climate, especially preseason temperature and preseason precipitation, on vegetation phenology is still unclear. In the present study, we explored the long-term trends of phenological parameters of different vegetation types in China north of 30°N from 1982 to 2014 and their comprehensive responses to preseason temperature and precipitation. Simultaneously, annual double-season phenological stages were considered. Results show that the satellite-based phenological data were corresponding with the ground-based phenological data. Our analyses confirmed that the preseason temperature has a strong controlling effect on vegetation phenology. The start date of the growing season (SOS) had a significant advanced trend for 13.5% of the study area, and the end date of the growing season (EOS) showed a significant delayed trend for 23.1% of the study area. The impact of preseason precipitation on EOS was overall stronger than that on SOS, and different vegetation types had different responses. Compared with other vegetation types, SOS and EOS of crops were greatly affected by human activities while the preseason precipitation had less impact. This study will help us to make a scientific decision to tackle global climate change and regulate ecological engineering.

Phytolith and simulation evidence for precipitation-modulated vegetation dynamics along the East Asian monsoon margin

An improved understanding of past interactions between terrestrial vegetation and various forcings, such as climate change, human impact, and paleofire, is crucial for assessing impacts of future global change on terrestrial ecosystems. This study seeks to find the key factor or factors that have driven Holocene vegetation change along the East Asian monsoon margin. Several high-resolution pollen records are reviewed and new phytolith-based paleovegetation reconstructions and physical geochemical datasets are presented from a peatland in northeastern China. Using 108 modern topsoil samples as a training set, canopy cover and vegetation composition are estimated for the period since 5100 cal. yr BP. Variation partitioning analysis (VPA) is used to determine the relative importance of climate change, human impacts, and paleofire disturbance. The generalized dynamic vegetation model LPJ-GUESS is forced with climate anomaly output from an atmospheric general circulation model to simulate vegetation dynamics during the mid-Holocene and the pre-industrial era. The proxy-based estimates are compared to modelling output. Results indicate that regional tree cover varied from 10% to 40% during the past five millennia. The single-core, phytolith-based reconstructions are generally consistent with stacked tree pollen z-scores calculated from different records along the East Asian monsoon margin, implying that mid-Holocene tree cover decrease was persistent and almost synchronous over extensive areas. VPA demonstrates that long-term monsoon marginal vegetation successions were mainly caused by climate effects. Numerical modelling suggests that since the mid-Holocene the retreat of forests along the monsoon margin was primarily associated with precipitation deficits. Our investigation highlights that the precipitation associated with the East Asian monsoon system has exerted a stronger influence than the westerlies on the monsoon margin climate and vegetation change. With ongoing global change, close attention to variations in precipitation patterns and amounts should be especially helpful in efforts aimed at ecological monitoring, change prediction, and restoration.

Vegetation dynamics in response to climate change and human activities in the Hulun Lake basin from 1981 to 2019

Monitoring long-term vegetation dynamics is essential for developing adaptation strategy to address the challenges to the ecosystem posed by climate change and human activities. This study analyzed spatio-temporal changes of normalized difference vegetation index (NDVI) in the Hulun Lake Basin (HLB) from 1981 to 2019, and explored the relationship between climatic factors and NDVI datasets. Furthermore, the residual trend (RESTREND) method was used to distinguish between the impacts of changes in climate and human activities on vegetation dynamics. The results showed that NDVI of the basin initially increased to the 1990s, then decreased to the mid-2000s, and finally rebounded to 2019 owing to the periodic fluctuations in precipitation. There were 28.32% of the NDVI pixels significantly increasing and 13.85% of the pixels significantly decreasing. The increase proportion of vegetation coverage in forest, wetland and cultivated land was higher than other vegetation types. NDVI in the China Grass Zone (CGZ) significantly degraded by 20.39% compared with the Mongolia Grass Zone (MGZ) significantly degraded by 5.60%. NDVI was significantly correlated with precipitation and weakly correlated with temperature in the HLB, and precipitation was the principal factor influencing vegetation coverage. The results of RESTREND revealed that farmland reclamation, over-grazing, and open-pit mining led to greater browning of vegetation in the CGZ than in the MGZ. Meanwhile, the implementation of ecological protection and restoration projects have helped the greening of the vegetation in the CGZ (20.91%) compared with the MGZ (7.45%). China’s ecological restoration projects are important for Mongolia to reverse the trend of serious ecological degradation. Our results can provide scientific decision-making for the local government to formulate differentiated vegetation ecological protection and restoration measures.

Municipal solid waste landfill: Evidence of the effect of applied landfill management on vegetation composition.

Proper management of municipal solid waste (MSW) is crucial to avoid pollution, environmental impacts and threat to public health. The problem of MSW is mainly arising from inadequate landfill site management. The objective of this study was to evaluate the impact of management practices and environmental risks at two landfill sites. The landfills were subject to long-term (10 years) vegetation monitoring. The vegetation was assessed using a floristic survey of identified plant species. The vegetation analysis showed that significant differences existed between the two landfill locations, with neophytes, invasive and expansive species dominating on one of the landfill sites, which may be attributed to climatic and geomorphological differences between the two sites, but also to variations in landfill management. These environmentally problematic species can potentially spread from the landfill into adjacent ecosystems, displace native plants and degrade adjacent farmland areas. The study of vegetation monitoring data suggests that, in addition to other types of monitoring, landfills should be subjected to regular vegetation biomonitoring, too. Landfill management practices should target the regulation of unwanted species, create conditions that are favourable to native plant species and provide as early as possible the restoration of filled cells.

Vegetation structure and composition associated with Aloe lettyae in dwindling grassland fragments: Fine-scale baseline data to underpin conservation efforts

Fine-scale data on species of special concern in fragments of highly threatened vegetation types are important for conservation management. This study investigated the vegetation structure and composition associated with the range-restricted long-lived Aloe lettyae and documented mesic Woodbush Granite Grassland (WGG) fragments. Google Earth imagery was used to detect WGG remnants in the highly fragmented landscape within the original WGG polygon as outlined on the National Vegetation Map. Field-verified WGG fragments were described (size, ownership, location, condition, observed threats and surrounding land cover types) and searched for A. lettyae. In six of the currently known 13 WGG fragments boasting an A. lettyae population, including two intact large (60–192 ha), two intact small (9–14 ha) and two degraded small (9–10 ha) WGG fragments, we studied the vegetation structure and composition associated with A. lettyae. For all sampled WGG fragments combined, vegetation structure (percentage aerial cover of grass, forb and woody species, bare soil and rock, maximum height (cm) of grass, forb and woody species and standing vegetation biomass (kg/ha)) within 1 m of each A. lettyae individual (n = 378) and ≥ 5 m away from A. lettyae plants (n = 442) varied very little. However, vegetation structure variables differed significantly between WGG fragments. For the intact large WGG fragments, percentage aerial cover for grass, forb and woody species were 49–50, 26–37 and 5–15%, respectively, with the small remaining percentages being made up of bare soil and rock cover. In the intact small WGG fragments, percentage aerial cover for grass, forb and woody species was similar to that in the large fragments. In the degraded small WGG fragments, however, percentage aerial cover for grass was significantly higher (66–74%) while the percentage forb cover tended to be lower (8–13%) than in the intact fragments. WGG fragment size ranged from <1 to 192 ha, with the estimate of all currently known extant WGG fragments amounting to a total of ∼6 km2, or ∼1% of the estimated historical WGG extent. Therefore, it is imperative to conserve all remaining WGG fragments. We provide fine-scale baseline data on the vegetation structure and composition associated with A. lettyae and a description of extant WGG fragments for long-term vegetation monitoring, management and biodiversity conservation decisions and future research.

Long Term Observation of Fractional Vegetation Cover in Qingyang of Gansu Province and Its Response to Climate Change

Vegetation is seen as a sensitive indicator of global change because of its crucial role in connecting the atmosphere, soil, and water. Fractional vegetation cover (FVC), in turn, is an important indicator of vegetation status. Qingyang is a typically ecologically sensitive region, with a range of changes in vegetation in the last decade as a result of climatic and non-climatic factors. However, the exact impact of climate change and human activities remains unclear. Satellite observations can help to clarify that impact, allowing us to assess trends in vegetation change in the last two decades (2000–2019). In this study, daily and composite time series vegetation variations were derived from moderate resolution imaging spectroradiometer (MODIS) data and the impact of climate and human activity factors was examined for different administrative districts. By deploying multiple regression models, the research revealed that human activity has contributed 46% to the FVC variation, while the remaining 54% was led by climate factors. In areas where FVC was increasing, human activity contributed 55.89% while climate factors contributed 44.11%. In areas where FVC was decreasing, human activity and climate factors contributed 24.58% and 75.42%, respectively. The study also looks at the impacts of El Nino/IOD events in FVC dynamics in the study site. The FVC inversion result from MODIS proved capable of capturing long-term and seasonal vegetation patterns and thus provide a valuable archive for decadal-scale vegetation dynamics in the study area. Moreover, the improvement in FVC was a dual effect of climatic and human activities, while the latter owns a higher contribution especially for the implementation of ecological construction projects.

Growing-season vegetation coverage patterns and driving factors in the China-Myanmar Economic Corridor based on Google Earth Engine and geographic detector

Although previous studies have shown that the construction of the China-Myanmar Economic Corridor (CMEC) has caused critical deforestation, few studies have monitored the long-term vegetation dynamics in corridor regions. Recent studies have revealed vegetation greening trends at different scales but have failed to further explore whether these trends are due to the restoration of natural vegetation or the large-scale planting of artificial crops. To investigate the vegetation dynamics and influencing factors during the growing season between 2001 and 2020 in the CMEC area, trend analysis, the random forest (RF) classifier at Google Earth Engine platform, and the geographic detector were employed to quantitatively analyze the patterns of distribution and change in vegetation coverage and its driving factors. Note that we distinguished between natural vegetation areas and artificial crop areas using the RF classification algorithm in an attempt to improve the drawback that most previous studies confused the contribution of natural vegetation and artificial vegetation to macro vegetation change. We discovered that (1) the growing-season average fractional vegetation cover (FVC) was 0.7033, which was relatively low in central and southern regions. The FVC increased by 0.21% yr−1 from 2001 to 2020, and areas with an increasing trend in FVC were 16.67% more than those with a decreasing trend. (2) Although forests and grasslands were reduced by 0.68%, the cropland that prominently clustered in the central and southern regions contributed 50.37% to an increase in FVC, and the decreasing effects on natural vegetation were offset by the increase in artificial crops. (4) The factors that influence FVC distribution were ranked in descending order: land use/land cover (LULC) > climate > topography > anthropogenic activity; factors that influence FVC change were ranked as: anthropogenic activity > climate > LUCC. Note that the grain yield influence on the FVC distribution increased 0.31% yr -1, which was also the most important factor driving FVC change, indicating that agricultural activities increasingly facilitated vegetation greening.

Thirty-eight years of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

Abstract. Climate change is projected to increase the imbalance between the supply (precipitation) and atmospheric demand for water (i.e., increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able to offset increasing aridity because rising CO2 increases water use efficiency. CO2 fertilization has also been cited as one of the drivers of the widespread “greening” phenomenon. However, attributing the size of this CO2 fertilization effect is complicated, due in part to a lack of long-term vegetation monitoring and interannual- to decadal-scale climate variability. In this study we asked the question of how much CO2 has contributed towards greening. We focused our analysis on a broad aridity gradient spanning eastern Australia's woody ecosystems. Next we analyzed 38 years of satellite remote sensing estimates of vegetation greenness (normalized difference vegetation index, NDVI) to examine the role of CO2 in ameliorating climate change impacts. Multiple statistical techniques were applied to separate the CO2-attributable effects on greening from the changes in water supply and atmospheric aridity. Widespread vegetation greening occurred despite a warming climate, increases in vapor pressure deficit, and repeated record-breaking droughts and heat waves. Between 1982–2019 we found that NDVI increased (median 11.3 %) across 90.5 % of the woody regions. After masking disturbance effects (e.g., fire), we statistically estimated an 11.7 % increase in NDVI attributable to CO2, broadly consistent with a hypothesized theoretical expectation of an 8.6 % increase in water use efficiency due to rising CO2. In contrast to reports of a weakening CO2 fertilization effect, we found no consistent temporal change in the CO2 effect. We conclude rising CO2 has mitigated the effects of increasing aridity, repeated record-breaking droughts, and record-breaking heat waves in eastern Australia. However, we were unable to determine whether trees or grasses were the primary beneficiary of the CO2-induced change in water use efficiency, which has implications for projecting future ecosystem resilience. A more complete understanding of how CO2-induced changes in water use efficiency affect trees and non-tree vegetation is needed.

Harmonized site preparation and postplant herbaceous weed control for establishment of southern pine plantations on coastal bedded sites

AbstractLoblolly pine or slash pine response and vegetation colonization are summarized for a region-wide study that included five locations on coastal soils in the southern United States. The objective was to evaluate timing of postplant herbaceous weed control (HWC) treatments following preplant site preparation with imazapyr applied at two timings (August and November) and at three rates (0.56, 0.84, and 1.12 kg ha−1). All imazapyr site preparation treatments were applied after bedding. Site preparation treatments resulted in fast-growing stands without HWC at all locations with average Year 3 dominant tree height ranging from 2.6 to 3.7 m. Imazapyr plus sulfometuron was an effective HWC treatment on loblolly pine. Vegetation control and pine response varied by surface soil texture. On coarser-textured soils, the site preparation treatments resulted in &lt;10% vegetation cover in June of the first pine growing season. On these coarser-textured soils, loblolly pine growth was increased by second-year and not first-year HWC. On finer-textured soils, vegetation colonization was aggressive, with &gt;20% cover in June of the first pine growing season, such that early first-year HWC provided the largest loblolly pine response of single-pass HWC treatments. Pines were highly tolerant to imazapyr site preparation treatments as evidenced by the lack of differences in slash or loblolly pine survival and growth from the doubling of imazapyr rates for applications in either August or November. There was little meaningful residual control of herbaceous vegetation into the second pine growing season from site preparation treatments or first-year HWC regardless of location. There was no consistent pine response benefit from increasing the imazapyr site preparation rate for included treatments. Cost-effective treatments would utilize low site-preparation herbicide rates followed by the appropriate timing of HWC if longer-term vegetation control is the objective.

Revisiting the Past: Replicability of a Historic Long-Term Vegetation Dynamics Assessment in the Era of Big Data Analytics

Open and analysis-ready data, as well as methodological and technical advancements have resulted in an unprecedented capability for observing the Earth’s land surfaces. Over 10 years ago, Landsat time series analyses were inevitably limited to a few expensive images from carefully selected acquisition dates. Yet, such a static selection may have introduced uncertainties when spatial or inter-annual variability in seasonal vegetation growth were large. As seminal pre-open-data-era papers are still heavily cited, variations of their workflows are still widely used, too. Thus, here we quantitatively assessed the level of agreement between an approach using carefully selected images and a state-of-the-art analysis that uses all available images. We reproduced a representative case study from the year 2003 that for the first time used annual Landsat time series to assess long-term vegetation dynamics in a semi-arid Mediterranean ecosystem in Crete, Greece. We replicated this assessment using all available data paired with a time series method based on land surface phenology metrics. Results differed fundamentally because the volatile timing of statically selected images relative to the phenological cycle introduced systematic uncertainty. We further applied lessons learned to arrive at a more nuanced and information-enriched vegetation dynamics description by decomposing vegetation cover into woody and herbaceous components, followed by a syndrome-based classification of change and trend parameters. This allowed for a more reliable interpretation of vegetation changes and even permitted us to disentangle certain land-use change processes with opposite trajectories in the vegetation components that were not observable when solely analyzing total vegetation cover. The long-term budget of net cover change revealed that vegetation cover of both components has increased at large and that this process was mainly driven by gradual processes. We conclude that study designs based on static image selection strategies should be critically evaluated in the light of current data availability, analytical capabilities, and with regards to the ecosystem under investigation. We recommend using all available data and taking advantage of phenology-based approaches that remove the selection bias and hence reduce uncertainties in results.

Changes in soil organic carbon and nitrogen pool sizes, dynamics, and biochemical stability during ∼160 years natural vegetation restoration on the Loess Plateau, China

Natural vegetation restoration has been reported as an effective strategy for the enhancement of soil organic carbon (SOC) and nitrogen (SON) sequestration in degraded ecosystems. However, changes in SOC and SON pool sizes, dynamics, and biochemical stability to natural vegetation restoration remain inadequately addressed. For this study, we investigated carbon (C) and nitrogen (N) contents and δ13C and δ15N values of SOC and SON pools, their labile C and N (LSOC and LSON, respectively), and recalcitrant C and N (RSOC and RSON, respectively) along 0–60 cm soil depths spanning ∼160 years of natural vegetation restoration on the Loess Plateau, China. Our results revealed that natural vegetation restoration greatly enhanced SOC, SON, LSOC, LSON, RSOC, total N, water-soluble organic C, and mineralizable organic C in the topsoil (0–20 cm), while it had little influence on the deeper soil layers (20–60 cm). The concentrations and stocks of SOC and SON in the topsoil increased significantly during the later restoration stages. The recalcitrant index for C at the 0–40 cm soil depths was highest in climax Quercus liaotungensis forest, while the recalcitrant index for N was unchanged at 0–20 and 40–60 cm soil depths between vegetation restoration stages. The δ13C values of SOC, LSOC, and RSOC, and δ15N values of SON and LSON were more enriched in the deeper soil layers relative to the topsoil, for the majority of restoration stages. The δ15N values of SON, LSON, and RSON in the topsoil were more enriched during the early restoration stages compared to the later restoration stages. Our results suggested that long-term natural vegetation restoration enhanced SOC and SON sequestration, increased biochemical stability of SOC, altered the dynamics of SOC and SON in the topsoil of degraded ecosystems through the increased inputs of plant residuals, and reduced the decomposition soil organic matter.

The spatiotemporal response of China's vegetation greenness to human socio-economic activities

Climate change and human socioeconomic activities both strongly impact long-term vegetation greenness. It is more a challenge to evaluate the impacts of socioeconomic activities on vegetative greenness than climate change, partially due to the lack of appropriate quantitative indicators of the former. Here we examined the relationship between the remote sensing nighttime light (NTL) data and the Normalized Difference Vegetation Index (NDVI), which in this study are used as the proxies of socioeconomic activities and vegetation greenness, respectively. We first eliminated the vegetation greenness changes in response to climate change and calculated the human-activities-induced NDVI (HNDVI). After explored the spatiotemporal patterns of the HNDVI and NTL data across China from 1998 to 2018, we studied the relationship between the HNDVI and NTL at the grid and county levels, respectively. Our results show that the mean adjusted DN values of the NTL data (NTLI) continuously increase (+0.2938) across our study area from 1998 to 2018, whereas the HNDVI values fluctuate with a general upward trend (+0.0018). Most grids (91.2%) with increased HNDVI were found in rural areas, particularly in the Northeast forest shelterbelt and the Loess Plateau. By contrast, the HNDVI values in rapidly urbanized areas in Chinese major urban agglomerations mainly show a downward trend, especially in the Yangtze River Delta (YRD) urban agglomeration. The relationships between the NTLI and HNDVI are inconsistent over time and across space, which could be attributed to land use conditions, afforestation projects in rural areas, and greening activities in urban areas over different periods and regions.

Comprehensive Assessment of Performances of Long Time-Series LAI, FVC and GPP Products over Mountainous Areas: A Case Study in the Three-River Source Region, China

Vegetation biophysical products offer unique opportunities to examine long-term vegetation dynamics and land surface phenology (LSP). It is important to understand the time-series performances of various global biophysical products for global change research. However, few endeavors have been dedicated to assessing the performances of long-term change characteristics or LSP extraction derived from different satellite products, especially in mountainous areas with highly fragmented and rugged surfaces. In this paper, we assessed the time-series characteristics and LSP detections of Global LAnd Surface Satellite (GLASS) leaf area index (LAI), fractional vegetation cover (FVC), and gross primary production (GPP) products across the Three-River Source Region (TRSR). The performances of products’ temporal agreements and their statistical relationship as a function of topographic indices and heterogeneous pixels, respectively, were investigated through intercomparison among three products during the period 2000 to 2018. The results show that the phenological differences between FVC and two other products are beyond 10 days over more than 35% of the pixels in TRSR. The long-term trend of FVC diverges significantly from GPP and LAI for 13.96% of the total pixels, and the percentages of mismatched pixels between FVC and two other products are 33.24% in the correlation comparison. Moreover, good agreements are observed between GPP and LAI, both in terms of LSP and interannual variations. Finally, the LSP and long-term dynamics of the three products exhibit poor performances on heterogeneous surfaces and complex topographic areas, which reflects the potential impacts of environmental factors and algorithmic imperfections on the quality and performances of different products. Our study highlights the spatiotemporal disparities in detections of surface vegetation activity in mountainous areas by using different biophysical products. Future global change studies may require multiple high-quality satellite products with long-term stability as data support.

Legacies of past human activities on one of the largest old-growth forests in the south-east European mountains

The Dinaric Mountains are a region considered as a hotspot for late-successional montane mixed Abies alba-Fagus sylvatica-Picea abies old-growth forests. This is likely due to historical deforestation levels being presumably lower than in other European regions. This paper provides new insights into the long-term vegetation dynamics and possible legacies of past human activities in old-growth forests in the montane zone of the Dinaric Mountains. Our extensive ground survey and the detailed land-cover types map show that the mixed A. alba-F. sylvatica old-growth forest with sparse P. abies is surrounded by almost pure F. sylvatica stands at Biogradska Gora. The well-dated stand-scale palaeoecological records (pollen, spores, stomata, macrofossils, macroscopic charcoal, and magnetic susceptibility) show that land use (cereal crop cultivation, cattle herding, and fire) during the Middle Ages caused a reduction of the A. alba and P. abies-dominated forest. After a major land abandonment around the Black Death pandemic (mid-14th century) and weaker land-use phases, F. sylvatica-dominated stands developed in the more accessible areas surrounding the old-growth forest. The legacy of past land uses is still visible as the almost pure F. sylvatica stands show less old-growth characteristics than other European beech-dominated old-growth forests. Markedly in contrast to decreasing tree cover elsewhere in the region, tree cover increased several centuries before the formal protection of the forest (1878 ce). These results support the view that historical land-use pressures played an important role for the small extent and the continuity of disturbance-sensitive A. alba and P. abies-dominated old-growth forest stands.

Enhanced surface urban heat islands due to divergent urban-rural greening trends

Satellite observations show that the surface urban heat island intensity (SUHII) has been increasing over the last two decades. This is often accompanied by an increased urban-rural contrast of vegetation greenness. However, the contribution of uneven vegetation trends in urban and rural areas to the trend of SUHII is unclear, due to the confounding effects of climate change and changes in man-made infrastructures and anthropogenic heat sources. Here we use a data-model fusion approach to quantify such contributions during the peak growing season. We show that the LAIdif (the urban-rural difference of leaf area index) is increasing (P< 0.05) in 189 of the selected 228 global megacities. The increasing trend of LAIdif from 2000 to 2019 accounts for about one quarter of the trend in satellite-derived SUHII, and the impact is particularly evident in places with rapid urbanization and rural cropland intensification. The marginal sensitivity of SUHII to LAIdif is the strongest in hot-humid megacities surrounded by croplands and in hot-dry megacities surrounded by mixed woody and herbaceous vegetation. Our study highlights the role of long-term vegetation trends in modulating the trends of urban-rural temperature differences.

Soil pH changes in a small catchment on the Chinese Loess Plateau after long-term vegetation rehabilitation

Soil pH is a fundamental parameter of soil chemical properties, and changes in soil pH (CIPs) alter hydrogeochemical processes in the Earth's critical zone. Afforestation has long been reported to change surface soil pH at the site scale or regional scale, while few studies have discussed the effects of afforestation and microtopography on soil pH at the catchment scale. We hypothesize that both afforestation and microtopography affect surface soil pH in a catchment. To test this hypothesis, two neighboring catchments in which one catchment was restored with natural vegetation and the other was afforested in 1954 were selected and surface soils were sampled. The soil pH, soil water content (SWC), total nitrogen (TN), soil organic and inorganic carbon (SOC and SIC) were analyzed for those soil samples. Moreover, trees coverage, LAI and grass cover were investigated in the two catchments. The results showed that over 60 years of afforestation in the catchment reduced the soil pH values at the depth of 0–80 cm in only the topography of the downhill gully, and the CIPs ranged from −0.02 to −0.15. In the topography of the upper hillslope, afforestation showed no significant impacts on soil pH. The reduction in soil pH in the gullies of the forestland catchment had a significant linear and negative relationship with TN and SOC. Moreover, microtopography showed significant impacts on soil pH. In the forestland catchment, the downhill gully showed lower soil pH values than those in the upper hillslope, while the opposite difference was detected in the grassland catchment. The high coverage of vegetation in the forestland gully and frequent gravity erosion in the grassland gully could explain the contrasting trends of soil pH differences between the two landforms in the catchment. The results of this study deepen our understanding of the effects of afforestation and catchment topography on soil pH and the related critical zone processes.

Analysis of the Spatial and Temporal Changes of NDVI and Its Driving Factors in the Wei and Jing River Basins.

This study aimed to explore the long-term vegetation cover change and its driving factors in the typical watershed of the Yellow River Basin. This research was based on the Google Earth Engine (GEE), a remote sensing cloud platform, and used the Landsat surface reflectance datasets and the Pearson correlation method to analyze the vegetation conditions in the areas above Xianyang on the Wei River and above Zhangjiashan on the Jing River. Random forest and decision tree models were used to analyze the effects of various climatic factors (precipitation, temperature, soil moisture, evapotranspiration, and drought index) on NDVI (normalized difference vegetation index). Then, based on the residual analysis method, the effects of human activities on NDVI were explored. The results showed that: (1) From 1987 to 2018, the NDVI of the two watersheds showed an increasing trend; in particular, after 2008, the average increase rate of NDVI in the growing season (April to September) increased from 0.0032/a and 0.003/a in the base period (1987–2008) to 0.0172/a and 0.01/a in the measurement period (2008–2018), for the Wei and Jing basins, respectively. In addition, the NDVI significantly increased from 21.78% and 31.32% in the baseline period (1987–2008) to 83.76% and 92.40% in the measurement period (2008–2018), respectively. (2) The random forest and classification and regression tree model (CART) can assess the contribution and sensitivity of various climate factors to NDVI. Precipitation, soil moisture, and temperature were found to be the three main factors that affect the NDVI of the study area, and their contributions were 37.05%, 26.42%, and 15.72%, respectively. The changes in precipitation and soil moisture in the entire Jing River Basin and the upper and middle reaches of the Wei River above Xianyang caused significant changes in NDVI. Furthermore, changes in precipitation and temperature led to significant changes in NDVI in the lower reaches of the Wei River. (3) The impact of human activities in the Wei and Jing basins on NDVI has gradually changed from negative to positive, which is mainly due to the implementation of soil and water conservation measures. The proportions of areas with positive effects of human activities were 80.88% and 81.95%, of which the proportions of areas with significant positive effects were 11.63% and 7.76%, respectively. These are mainly distributed in the upper reaches of the Wei River and the western and eastern regions of the Jing River. These areas are the key areas where soil and water conservation measures have been implemented in recent years, and the corresponding land use has transformed from cultivated land to forest and grassland. The negative effects accounted for 1.66% and 0.10% of the area, respectively, and were mainly caused by urban expansion and coal mining.