Mapping of Kobresia pygmaea Community Based on Umanned Aerial Vehicle Technology and Gaofen Remote Sensing Data in Alpine Meadow Grassland: A Case Study in Eastern of Qinghai–Tibetan Plateau

The Kobresia pygmaea (KP) community is a key succession stage of alpine meadow degradation on the Qinghai–Tibet Plateau (QTP). However, most of the grassland classification and mapping studies have been performed at the grassland type level. The spatial distribution and impact factors of KP on the QTP are still unclear. In this study, field measurements of the grassland vegetation community in the eastern part of the QTP (Counties of Zeku, Henan and Maqu) from 2015 to 2019 were acquired using unmanned aerial vehicle (UAV) technology. The machine learning algorithms for grassland vegetation community classification were constructed by combining Gaofen satellite images and topographic indices. Then, the spatial distribution of KP community was mapped. The results showed that: (1) For all field observed sites, the alpine meadow vegetation communities demonstrated a considerable spatial heterogeneity. The traditional classification methods can hardly distinguish those communities due to the high similarity of their spectral characteristics. (2) The random forest method based on the combination of satellite vegetation indices, texture feature and topographic indices exhibited the best performance in three counties, with overall accuracy and Kappa coefficient ranged from 74.06% to 83.92% and 0.65 to 0.80, respectively. (3) As a whole, the area of KP community reached 1434.07 km2, and accounted for 7.20% of the study area. We concluded that the combination of satellite remote sensing, UAV surveying and machine learning can be used for KP classification and mapping at community level.

VARIATION IN GREEN-UP DATE OF <i>KOBRESIA PYGMAEA</i> ALPINE MEADOW IN QINGHAI-TIBETAN PLATEAU DURING 1961–2016

Vegetation phenology in Qinghai-Tibetan Plateau (QTP) has been proved to be more sensitive to climate change. The previous researches have reported divergent phonological responses of alpine vegetation in QTP from the analyses of remote sensing data, thus the demand for further analysis based on long-term observed vegetation data becomes more urgent. In the lack of long-term remote sensing monitoring data and ground observation data, phenology model simulation can be used as an effective remedy. In this study, we used a phenology model (unified model, UM) to simulate the green-up date for Kobresia pygmaea alpine meadow, and model evaluation shows that the unified model is able to make reasonable estimation for green-up date derived from satellite observations. Based on the phenology model and its parameters, spatial and temporal changes in green-up dates for Kobresia pygmaea alpine meadow in QTP from 1961 to 2016 were simulated, the results indicated that, the variation in green-up date presented an overall insignificant temporal trend with obvious spatial differences; furthermore, complex stage characteristics about the variation in green-up date were also clearly revealed, the period from 1991 to 2005 could be considered as a turning point, before which the trend was mainly in advan1ce and after which the trend was mainly in delay.

Grazing promotes plant functional diversity in alpine meadows on the Qinghai-Tibetan Plateau

Grazing exclosures and rotational grazing have been extensively applied to prevent grassland degradation and to restore grassland ecosystem function and services. The mechanisms associated with changes in alpine plant traits, and functional diversity under different grazing regimes have not been deeply explored. We examined the variations of plant leaf traits and functional diversity of an alpine meadow under different grazing regimes in a 3-year experiment. The results showed, after 3 years of yak grazing, that the coverage of Stipa capillata increased, whereas that of Kobresia pygmaea decreased under grazing exclosure. Stipa capillata had a lower ratio of leaf nitrogen content to phosphorus content (N:P) under grazing exclosure than under rotational grazing and continuous grazing, whereas Kobresia pygmaea showed no significant differences among grazing treatments. Among grazing regimes, the specific leaf area (SLA) of Stipa capillata was similar, whereas that of Kobresia pygmaea was higher under grazing exclosure. At the interspecific level, leaf area and weight were negatively correlated with SLA, whereas leaf carbon (C) content, leaf N content, leaf C:P and leaf N:P were negatively related to leaf P content and leaf C:N. These findings indicated that growth-defence trade-off strategies might lead to variations in plant traits and coverage. Large-leaved species, due to high maintenance costs, were less commonly distributed in the community, and they were better defended and unpalatable to yaks due to lower SLA, this formed the species coverage distribution pattern of the community. Various N and P utilisation efficiency of different species indicated diverse economic resources utilisation strategies might be due to niche differentiation in the community. Plots that had been excluded from grazing had the lowest functional richness, evenness, and divergence. Rotational and continuous grazing were equivalent in promoting alpine plant functional diversity.

Response of leaf traits of common plants in alpine meadow to plateau pika disturbance

Leaf traits have been proven to reflect the adaptation of individual plants to disturbance environments in a grassland ecosystem. A field survey was conducted to investigate the effects of the disturbance intensity of plateau pika on the leaf traits of a dominant (Kobresia pygmaea) and two common plants (Elymus nutans and Anemone rivularis var. flore-minore) in an alpine meadow. This study indicated that the plateau pika disturbance enables the individuals of three plants to exhibit respective plasticity because the three plants had different leaf indices (LI) as the disturbance intensity increased. K. pygmaea, E. nutans and A. rivularis var. flore-minore had high specific leaf area (SLA), leaf dry mass content (LDMC), and leaf nitrogen content (LNC) at relatively low, moderate, and high disturbance intensities of plateau pika, respectively. K. pygmaea, E. nutans and A. rivularis var. flore-minore suffered low nutrient stress at low, moderate and high disturbance intensities due to high N : P at corresponding disturbance intensities. These results indicated that K. pygmaea, E. nutans and A. rivularis var. flore-minore grew well at relatively low, moderate, and high disturbance intensity conditions, respectively, which contributed to the improvement of alpine meadows with a higher proportion of E. nutans at a moderate disturbance intensity or the deterioration of alpine meadows with a higher proportion of A. rivularis var. flore-minore at a high disturbance intensity. Our findings suggest that leaf traits are effective tools to explain how small burrowing herbivore disturbances often lead to the improvement or deterioration of alpine meadows under different disturbance intensities.

The Kobresia pygmaea ecosystem of the Tibetan highlands – origin, functioning and degradation of the world’s largest pastoral alpine ecosystem

Kobresia pastures in the eastern Tibetan highlands occupy 450000 km2 and form the world’s largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresia pygmaea, which forms a lawn with a durable turf cover anchored by a felty root mat, and occurs from 3000 m to nearly 6000 m a.s.l. The existence and functioning of this unique ecosystem and its turf cover have not yet been explained against a backdrop of natural and anthropogenic factors, and thus its origin, drivers, vulnerability or resilience remain largely unknown. Here we present a review on ecosystem diversity, reproduction and ecology of the key species, pasture health, cycles of carbon (C), water and nutrients, and on the paleo-environment. The methods employed include molecular analysis, grazing exclusion, measurements with micro-lysimeters and gas exchange chambers, 13C and 15N labelling, eddy-covariance flux measurements, remote sensing and atmospheric modelling.The following combination of traits makes Kobresia pygmaea resilient and highly competitive: dwarf habit, predominantly below-ground allocation of photo assimilates, mixed reproduction strategy with both seed production and clonal growth, and high genetic diversity. Growth of Kobresia pastures is co-limited by low rainfall during the short growing season and livestock-mediated nutrient withdrawal. Overstocking has caused pasture degradation and soil deterioration, yet the extent remains debated. In addition, we newly describe natural autocyclic processes of turf erosion initiated through polygonal cracking of the turf cover, and accelerated by soil-dwelling endemic small mammals. The major consequences of the deterioration of the vegetation cover and its turf include: (1) the release of large amounts of C and nutrients and (2) earlier diurnal formation of clouds resulting in (3) decreased surface temperatures with (4) likely consequences for atmospheric circulation on large regional and, possibly global, scales.Paleo-environmental reconstruction, in conjunction with grazing experiments, suggests that the present grazing lawns of Kobresia pygmaea are synanthropic and may have existed since the onset of pastoralism. The traditional migratory rangeland management was sustainable over millennia and possibly still offers the best strategy to conserve, and possibly increase, the C stocks in the Kobresia turf, as well as its importance for climate regulation.