Grazing alters the phenology of alpine steppe by changing the surface physical environment on the northeast Qinghai-Tibet Plateau, China

A high-resolution gridded grazing dataset of grassland ecosystem on the Qinghai-Tibet Plateau in 1982-2015

Grazing intensity, characterized by high spatial heterogeneity, is a vital parameter to accurately depict human disturbance and its effects on grassland ecosystems. Grazing census data provide useful county-scale information; however, they do not accurately delineate spatial heterogeneity within counties, and a high-resolution dataset is urgently needed. Therefore, we built a methodological framework combining the cross-scale feature extraction method and a random forest model to spatialize census data after fully considering four features affecting grazing, and produced a high-resolution gridded grazing dataset on the Qinghai-Tibet Plateau in 1982-2015. The proposed method (R2 = 0.80) exhibited 35.59% higher accuracy than the traditional method. Our dataset were highly consistent with census data (R2 of spatial accuracy = 0.96, NSE of temporal accuracy = 0.96) and field data (R2 of spatial accuracy = 0.77). Compared with public datasets, our dataset featured a higher temporal resolution (1982-2015) and spatial resolution (over two times higher). Thus, it has the potential to elucidate the spatiotemporal variation in human activities and guide the sustainable management of grassland ecosystem.

Warming delays but grazing advances leaf senescence of five plant species in an alpine meadow

Leaf senescence is the final stage in the life cycle of leaves and is critical to plants' fitness as well as to ecosystem carbon and nutrient cycling. To date, most understanding about the responses of leaf senescence to environmental changes has derived from research in forests, but the topic has been relatively neglected, especially under grazing conditions, in natural grasslands. We conducted a 3-year manipulative asymmetric warming with moderate grazing experiment to explore the responses of leaf senescence of five main species in an alpine meadow on the Qinghai-Tibetan Plateau. We found that warming prolonged leaf longevity through earlier leaf-out and later leaf senescence, and grazing prolonged it through a greater advance in leaf-out than first leaf coloration for all plants. Warming did not affect leaf nitrogen (N) content or N resorption efficiency (NRE), but grazing increased N content in coloring leaves for P. anserine and P. nivea and decreased NRE for K. humilis, P. anserine under no-warming, and for P. nivea under warming. The interactive effects of warming and grazing on leaf phenology and leaf traits depended on species identity and year. There were positive relationships between leaf-out and leaf senescence mainly derived from grazing, and positive relationships between NRE from old leaves and leaf senescence for three out of five plant species. Therefore, our results indicated that earlier leaf-out could result in earlier leaf senescence only under grazing, but depending on plant species. Delayed leaf coloring increased NRE from old leaves for some plant species measured under warming and grazing. Our results suggested that alpine plants may develop strategies to adapt to warming and grazing to assimilate more carbon through prolonged leaf longevity rather than increased NRE through earlier leaf coloring in the alpine meadow.

Effects of cadmium stress on fruits germination and growth of two herbage species

Cadmium (Cd) pollution is a global environmental problem. It is of great significance to find a kind of pasture that can grow normally in a cadmium environment, especially in the Tibetan Plateau. We studied the fruit germination and fruit growth of Elymus sinsubmuticus S.L. Chen and Elymus tangutorum (Nevski), native plants of the Tibetan Plateau, in different cadmium environments. The results showed that with increased cadmium stress, the fruit germination rate, final germination rate, fruit-vigor, average germination time, and germination-speed index for the two grass species gradually decreased, and the 50% germination time for the seed gradually increased. Root length, biomass, and the number of leaves decreased in both species. We quantified the fruit germination and growth of plants in the cadmium environment and found that E. sinosubmuticus S.L. Chen had better fruit germination and fruit growth, and it had the development potential of cadmium pollution control.

Detecting the Turning Points of Grassland Autumn Phenology on the Qinghai-Tibetan Plateau: Spatial Heterogeneity and Controls

Autumn phenology, commonly represented by the end of season (EOS), is considered to be the most sensitive and crucial productivity indicator of alpine and cold grassland in the Qinghai-Tibetan Plateau. Previous studies typically assumed that the rates of EOS changes remain unchanged over long time periods. However, pixel-scale analysis indicates the existence of turning points and differing EOS change rates before and after these points. The spatial heterogeneity and controls of these turning points remain unclear. In this study, the EOS turning point changes are extracted and their controls are explored by integrating long time-series remote sensing images and piecewise regression methods. The results indicate that the EOS changed over time with a delay rate of 0.08 days/year during 1982–2015. The rates of change are not consistent over different time periods, which clearly highlights the existence of turning points. The results show that temperature contributed most strongly to the EOS changes, followed by precipitation and insolation. Furthermore, the turning points of climate, human activities (e.g., grazing, economic development), and their intersections are found to jointly control the EOS turning points. This study is the first quantitative investigation into the spatial heterogeneity and controls of the EOS turning points on the Qinghai-Tibetan Plateau, and provides important insight into the growth mechanism of alpine and cold grassland.

How Does Spring Phenology Respond to Climate Change in Ecologically Fragile Grassland? A Case Study from the Northeast Qinghai-Tibet Plateau

Vegetation phenology is an important indicator of global climate change, and the response of grassland phenology to climate change is particularly sensitive in ecologically fragile areas. To enhance the ecological security of the Tibetan Plateau, it is crucial to determine the relationship between fluctuations in the start of the growing season (SOS) and the response to environmental factors. We investigated the trends of the intra-annual (ten-day) and interannual spatiotemporal dynamics of the SOS on the Northeast Qinghai-Tibet Plateau (NQTP) from 2000–2020 with MOD09GA data. We identified the response relationships with environmental factors (climate, terrain) using the maximum value composite method and the Savitzky–Golay filtering and dynamic threshold method. The SOS was concentrated from the 110th to 150th days; the average annual SOS was on the 128th day, with a spatial pattern of “early in the east and late in the west”. The overall trend of the SOS was advanced (45.48%); the regions with the advanced trend were mainly distributed in the eastern part of the NQTP. The regions with a delayed SOS were mainly concentrated in the higher-altitude regions in the southwest (38.31%). The temperature, precipitation and SOS exhibited a reverse fluctuation trend around the midpoint of 2010. Precipitation affected the SOS earlier than temperature. When temperature became a limitation of the SOS, precipitation had a more significant regulatory effect on the SOS. The SOS and aspect, slope and altitude were distributed in axisymmetric, pyramidal and inverted pyramidal shapes, respectively. The SOS on shaded slopes was earlier and more intensive than that on sunny slopes. With increasing slope, the area of the SOS decreased, and it occurred later. The SOS area was largest at 4500–5000 m and decreased at lower and higher altitude intervals. The SOS occurred later as altitude increased.

Soil degradation influences soil bacterial and fungal community diversity in overgrazed alpine meadows of the Qinghai-Tibet Plateau

Over half of the alpine meadows in the Qinghai-Tibet Plateau (QTP) are degraded due to human activities. Soil degradation from overgrazing is the most direct cause of grassland degradation. It is thus important to synthesize the effects of multiple soil degradation indicators on the belowground biomass of plants and soil microorganisms in the degraded QTP. We studied the diversities and structures of soil bacterial and fungal communities using soil bacterial 16S rRNA and the fungal ITS gene under four degradation gradients, D1: lightly degraded, D2: moderately degraded, D3: highly degraded, and a non-degraded control site (CK). The bacterial Shannon diversity in D3 was significantly lower than that in D1 (p < 0.001), and the bacterial richness index in D3 was significantly lower than that in D1 (p < 0.001). There was no difference in soil fungal diversity among the different degradation levels; however, soil fungal richness decreased significantly from CK to D3. The phyla Actinobacteria, Acidobacteria and the genus Mortierella were differed significantly under the four degradation gradients. Plant litter mass and root C/N ratio were important factors associated with bacterial and fungal diversity and richness. These results indicated that alpine meadow degradation can lead to variations in both microbial diversity and the potential functioning of micro-organisms in the QTP.

Dynamic Threshold of Carbon Phenology in Two Cold Temperate Grasslands in China

Plant phenology, especially the timing of the start and the end of the vegetation growing season (SOS and EOS), plays a major role in grassland ecosystem carbon cycles. As the second-largest grassland country in the world, China’s grasslands are mainly distributed in the northern cold temperate climate zone. The accuracies and relations of plant phenology estimations from multialgorithms and data resources are poorly understood. Here, we investigated vegetation phenology in two typical cold temperate grasslands, Haibei (HB) and Inner Mongolia (NM) grasslands, in China from 2001 to 2017. Compared to ground vegetation phenology observations, we analyzed the performance of the moderate resolution imaging spectroradiometer MODIS phenology products (MCD12Q2) and two remote sensing-based vegetation phenology algorithms from the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) time series (five satellite-based phenology algorithms). The optimal algorithm was used to compare with eddy covariance (EC)-based carbon phenology, and to calculate the thresholds of carbon phenology periods (SOSt and EOSt) in each site. Results showed that satellite-based phenology estimations (all five algorithms in this study) were strongly coupled with the temporal variation of the observed phenological period but significantly overestimated the SOS, predicting it to be over 21 days later than the field data. The carbon phenology thresholds of HB grassland (HB_SOSt and HB_EOSt) had a significant upward trend, with the multiyear average values being 0.14 and 0.29, respectively. In contrast, the thresholds of NM grasslands (NM_SOSt and NM_EOSt) also showed a certain upward trend, but it was not significant (p > 0.05), with the multiyear average values being 0.17 and 0.2, respectively. Our study suggested the thresholds of carbon phenology periods (SOSt and EOSt, %) could be simply and effectively estimated based on their significant relationship with the EC-based maximum of gross primary productivity observations (GPPmax) at a specific site and time. Therefore, this study suggested the thresholds of carbon phenology were not fixed even in a specific ecosystem, which also provided simple bridges between satellite-based vegetation phenology and EC-based carbon phenology in similar grasslands.

Status and Challenges of Qinghai–Tibet Plateau’s Grasslands: An Analysis of Causes, Mitigation Measures, and Way Forward

Grassland ecosystems on the Qinghai–Tibet Plateau (QTP) provide numerous ecosystem services and functions to both local communities and the populations living downstream through the provision of water, habitat, food, herbal medicines, and shelter. This review examined the current ecological status, degradation causes, and impacts of the various grassland degradation mitigation measures employed and their effects on grassland health and growth in the QTP. Our findings revealed that QTP grasslands are continually being degraded as a result of complex biotic and abiotic drivers and processes. The biotic and abiotic actions have resulted in soil erosion, plant biomass loss, soil organic carbon loss, a reduction in grazing and carrying capacity, the emergence of pioneer plant species, loss of soil nutrients, and an increase in soil pH. A combination of factors such as overgrazing, land-use changes, invasive species encroachment, mining activities, rodent burrowing activities, road and dam constructions, tourism, migration, urbanization, and climate change have caused the degradation of grasslands on the QTP. A conceptual framework on the way forward in tackling grassland degradation on the QTP is presented together with other appropriate measures needed to amicably combat grassland degradation on the QTP. It is recommended that a comprehensive and detailed survey be carried out across the QTP to determine the percentage of degraded grasslands and hence, support a sound policy intervention.

Vegetation dynamic analysis based on multisource remote sensing data in the east margin of the Qinghai-Tibet Plateau, China

This study focuses on the vegetation dynamic caused by global environmental change in the eastern margin of the Qinghai-Tibet Plateau (EMQTP). The Qinghai–Tibet Plateau (QTP) is one of the most sensitive areas responding to global environmental change, particularly global climate change, and has been recognized as a hotspot for coupled studies on changes in global terrestrial ecosystems and global climates. An important component of terrestrial ecosystems, vegetation dynamic has become a key issue in global environmental change, and numerous case studies have been conducted on vegetation dynamic trends using multi-source data and multi-scale methods across different study periods. The EMQTP is regarded as a transitional area located between the QTP and the Sichuan basin, and has special geographical and climatic conditions. Although this area is ecologically fragile and sensitive to climate change, few studies about vegetation dynamics have been carried out in this area. Thus, in this study, we used long-term series datasets of GIMMS 3g NDVI and VGT/PROBA-V NDVI to analyze the vegetation dynamics and phenological changes from 1982 to 2018. Validation was performed based on Landsat NDVI and Vegetation Index & Phenology (VIP) data. The results reveal that the year 1998 was a vital turning point in the start of growing season (SGS) in vegetation ecosystems. Before this turning point, the SGS had an average slope of 9.2 days/decade, and after, the average slope was 3.9 days/decade. The length of growing season (LGS) was slightly prolonged between 1982 to 2015. Additionally, the largest national alpine wetland grassland experienced significant vegetation degradation; in autumn, the degraded area accounted for 63.4%. Vegetation degradation had also appeared in the arid valleys of the Yalong River and the Jinsha River. Through validation analysis, we found that the main causes of vegetation degradation are the natural degradation of wetland grassland and human activities, specifically agricultural development and residential area expansion.