Changes of lake organic carbon sinks from closed basins since the Last Glacial Maximum and quantitative evaluation of human impacts

Background Closed basins occupy 21% of the world's land area and can substantially affect global carbon budgets. Conventional understanding suggests that the terminal areas of closed basins collect water and carbon from throughout the entire basin, and changes in lake organic carbon sinks are indicative of basin-wide organic carbon storages. However, this hypothesis lacks regional and global validation. Here, we first validate the depositional process of organic carbon in a typical closed-basin region of northwest China using organic geochemical proxies of both soil and lake sediments. Then we estimate the organic carbon sinks and human impacts in extant closed-basin lakes since the Last Glacial Maximum (LGM). Results Results show that 80.56 Pg organic carbon is stored in extant closed-basin lakes mainly found in the northern mid-latitudes. Carbon accumulation rates vary from 17.54 g C m(-2) yr(-1) during modern times, 6.36 g C m(-2) yr(-1) during the mid-Holocene and 2.25 g C m(-2) yr(-1) during the LGM. Then, we evaluated the influence by human activities during the late Holocene (in the past three thousand years). The ratio of human impacts on lake organic carbon storage in above closed basins is estimated to be 22.79%, and human-induced soil organic carbon emissions in the past three thousand years amounted to 207 Pg. Conclusions While the magnitude of carbon storage is not comparable to those in peatland, vegetation and soil, lake organic carbon sinks from closed basins are significant to long-term terrestrial carbon budget and contain information of climate change and human impact from the whole basins. These observations improve our understanding of carbon sinks in closed basins at various time scales, and provide a basis for the future mitigation policies to global climate change.Peer reviewe

The Kobresia pygmaea ecosystem of the Tibetan highlands – Origin, functioning and degradation of the world's largest pastoral alpine ecosystem: Kobresia pastures of Tibet

With 450,000 km2 Kobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000–6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf. © 201

Evaluation of ERA-Interim Air Temperature Data over the Qilian Mountains of China

In this study, 2 m air temperature data from 24 meteorological stations in the Qilian Mountains (QLM) are examined to evaluate ERA-Interim reanalysis temperature data derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period of 1979–2017. ERA-Interim generally captures the monthly, seasonal, and annual variation very well. High daily correlations ranging from 0.956 to 0.996 indicate that ERA-Interim captures the daily temperature observations very well. However, an average root-mean-square error (RMSE) of ±2.7°C of all stations reveals that ERA-Interim should not be directly applied at individual sites. The biases are mainly attributed to the altitude differences between ERA-Interim grid points and stations. The positive trend (0.457°C/decade) is significant over the Qilian Mountains based on the 1979–2017 observations. ERA-Interim captures the warming trend very well with an increase rate of 0.384°C/decade. The observations and ERA-Interim both show the largest positive trends in summer with the values of 0.552°C/decade and 0.481°C/decade, respectively. We conclude that in general ERA-Interim captures the trend very well for observed 2 m air temperatures and ERA-Interim is generally reliable for climate change research over the Qilian Mountains

Remote Sensing of Land Surface Phenology

Land surface phenology (LSP) uses remote sensing to monitor seasonal dynamics in vegetated land surfaces and retrieve phenological metrics (transition dates, rate of change, annual integrals, etc.). LSP has developed rapidly in the last few decades. Both regional and global LSP products have been routinely generated and play prominent roles in modeling crop yield, ecological surveillance, identifying invasive species, modeling the terrestrial biosphere, and assessing impacts on urban and natural ecosystems. Recent advances in field and spaceborne sensor technologies, as well as data fusion techniques, have enabled novel LSP retrieval algorithms that refine retrievals at even higher spatiotemporal resolutions, providing new insights into ecosystem dynamics. Meanwhile, rigorous assessment of the uncertainties in LSP retrievals is ongoing, and efforts to reduce these uncertainties represent an active research area. Open source software and hardware are in development, and have greatly facilitated the use of LSP metrics by scientists outside the remote sensing community. This reprint covers the latest developments in sensor technologies, LSP retrieval algorithms and validation strategies, and the use of LSP products in a variety of fields. It aims to summarize the ongoing diverse LSP developments and boost discussions on future research prospects

Dynamics of water, carbon, and nitrogen in forest and alpine tundra ecosystems in the Pacific Northwest and the Rocky Mountains of the U.S. under future climate change

Projection of ecosystem functions and biogeochemical cycling of elements under future climate change requires a quantitative understanding of both ecosystem processes and site-specific climate change scenarios. Biogeochemical and ecological studies over the last decades have provided the intellectual basis for these projections, especially at the small watershed scale. Recent developments in biophysical sciences and computationally based meteorology coupled with advanced downscaling techniques have made it possible to project future climate change scenarios at the small watershed scale. Using a biogeochemical model, PnET-BGC, which has been extensively applied to the forest ecosystems in the northeastern United States, the interactive effects of multiple environmental factors on ecosystem function and element dynamics can be investigated. In this dissertation, I applied PnET-BGC to three ecosystems, including one in Oregon (The H. J. Andrews Experimental Forest) and two in Colorado (Niwot Ridge and Loch Vale Watershed) to evaluate the effects of climate change at the intensively studied watersheds with distinct climate and vegetation type. Results from these three sites were compared and contrasted with projections conducted in the northeastern U.S. using PnET-BGC to identify which sites are vulnerable to future climate change and what factors contribute to this vulnerability. Future climate considered in this study was developed from two radiative forcing scenarios under the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathways (RCPs). The site specific climate inputs are statistically downscaled from outputs of four general circulation models (GCMs) to drive PnET-BGC. To more accurately depict different types of ecosystems in this study, updated parameters and improved algorithms were incorporated into PnET-BGC, taking advantage of findings from recent studies. This study expands the type of ecosystem from which PnET-BGC is applied. It also provides a basis for future studies on these ecosystems to examine the interactive effects of climate change with other disturbances, such as changes in atmospheric deposition or land disturbance. In this research, I tested the hypotheses that 1) climate change at high elevation watersheds in the western U.S. will result in physiological stress on vegetation that is adapted to its native climate, and alter future dynamics of water, carbon, and nitrogen in these ecosystems; 2) other aspects of global change such as elevated atmospheric CO2 concentrations and an extended growing season will alleviate the impacts of physiological stress on ecosystem function and element dynamics; and 3) ecosystem responses to climate change will vary among the three sites in the western U.S. and are distinct from patterns in the northeastern U.S. due to the differences in vegetation type and site specific current and future climate conditions. This work improved understanding of the effects of climate change on element dynamics and the function of different types of ecosystems. It complements existing literature on response of ecosystem structure and function to future climate change scenarios. I conducted my research in this dissertation in four phases. In phase one, I applied the model at Watershed 2 in H. J. Andrews Experimental Forest, an old-growth Douglas-fir forest located in the western Cascade Range of Oregon. The model algorithm on calculation of vapor pressure deficit was improved for the Pacific Northwest. Parameters on plant functional traits and soil characteristics were also updated using local observations. Simulation outputs were validated against local observations. Seasonal and long-term projections show large increases in stomatal conductance throughout the year from 1986-2010 to 2076-2100 and increases in leaf carbon assimilation between October and June over the same period, but future dynamics of water and carbon under the RCP scenarios are largely affected by a reduction in foliar biomass resulting from severe air temperature and humidity stress to the forest in summer. Projected future decreases in foliar biomass in the old-growth Douglas-fir forest results in 1) decreases in transpiration and increases in summer and fall soil moisture; 2) decreases in photosynthesis, plant biomass, and soil organic matter under the high radiative forcing scenario; and 3) altered foliar and soil stoichiometry of carbon to nitrogen. In phase two, I developed the first alpine tundra version of PnET-BGC and applied the model at the Saddle of Niwot Ridge in Colorado. Projections indicate that in the future this watershed will become more energy-limited on an annual basis, and the seasonal distribution of the water supply will become decoupled from energy inputs due to advanced snowmelt, causing soil moisture stress to plants during the growing season. The model simulations suggest that future shortened snow-covered periods may cause decreases in winter soil decomposition by 9% to 16% due to limitations in subnivean microbial activity; while the associated extended growing season is projected to result in only slight decreases in carbon sequestration of 8% under the high radiative forcing scenario, despite a 33% reduction in leaf production due to the soil moisture stress. The analyses demonstrate that future nitrogen uptake by alpine plants is regulated by nitrogen supply from mineralization, but plant nitrogen demand may also affect plant uptake under the aggressive RCP8.5 scenario. In addition, PnET-BGC simulations suggest that potential CO2 fertilization effects on alpine plants are projected to cause larger increases in concentrations of non-structural carbohydrates and lipids than leaf and root production. In phase three, PnET-BGC model was applied at Loch Vale watershed, a subalpine forest near Niwot Ridge in the southern Rocky Mountains of Colorado. Necessary improvements of the model were made on processes that are important in subalpine forests but negligible in other ecosystems such as soil evaporation. The analyses using the Budyko curve suggest that future evapotranspiration may become more water-limited in the subalpine forest. From 1986-2010 to 2076-2100, evapotranspiration increases at the start and end of the growing season. Recurring plant soil moisture stress is projected between July and September which reduces foliar biomass by 5% to 16%. However, the annual rate of photosynthesis and wood biomass are projected to increase by up to 29% and 76%, respectively, due to the increasing temperature under the RCP8.5 scenario. Unexpectedly, an extended growing season had little contribution to the dynamics of water and carbon. Fertilization by elevated atmospheric CO2 concentrations is projected to result in 16% to 27% higher rates of annual photosynthesis under RCP4.5 and RCP8.5 scenarios, respectively, and increasing carbon accumulation in wood biomass. In the fourth phase, I conducted a cross-site analysis of the three western sites in Oregon and Colorado with Hubbard Brook Experimental Forest in New Hampshire which was simulated in a previous study. Various ecosystem responses from the four sites under the RCP scenarios were attributed to the differences in vegetation type and site specific current and future climate conditions. Projections in the western and northeastern U.S. suggest water-use efficiency and soil water holding capacity may largely determine the type of physiological stress that plants experience in the future, while foliar retention time and wood turnover rate may largely affect the storage and decomposition of soil organic matter in forest ecosystems. Although foliar nitrogen contents have large variation among the four sites, their future changes were not projected to be large in any sites, therefore having little impact on carbon or water dynamics of the watersheds. Projections also suggest future increases in temperature may impact ecosystem and biogeochemical processes to a smaller extent during the winters of alpine tundra ecosystems than other seasons and sites in which the temperature is above or close to freezing. An extended growing season was projected in all sites under the RCP scenarios, but showed distinct impacts on ecosystem functions at different sites. Potential CO2 fertilization effects on carbon dynamics were mainly manifested in enhanced wood growth from forest ecosystems but result in large increases in non-structural carbohydrates in the alpine tundra ecosystem

Actual and standard crop coefficients for semi‑natural and planted grasslands and grasses: a review aimed at supporting water management to improve production and ecosystem services

Natural and planted grasslands play a very important role in agriculture as source of various ecosystem services, including carbon sequestration and biodiversity, and are responsible for a large fraction of agricultural water use in rainfed and irrigated fields. It is, therefore, relevant to precisely know their water use and vegetation requirements with consideration of relevant climate, from extremely cold, dry, with long winter seasons, to tropical humid and hot climates, thus with a large variability of vegetation. Semi-natural grasslands are basically used for grazing and mainly refer to highland pastures and meadows, steppes, savannas, pampas, and mixed forest systems. The FAO method to compute crop (vegetation) evapotranspiration (ETc) through the product of a crop coefficient (K c ) by the reference evapotranspiration (ETo ) is adopted. The selected papers were those where actual ETc (ETc act ) was derived from field observations and ETo was computed with the FAO56 definition, or with another method that could be referred to the former. Field derived ETc act methods included soil water balance, Bowen ratio and eddy covariance measurements, as well as remote sensing vegetation indices or surface energy balance models, thus reviewed Kc act (ETc act/ETo) values were obtained from field data. These Kc act refer to initial, mid-season and end season (K c act ini , K c act mid , K c act end ) when reported values were daily or monthly; otherwise, only average values (K c act avg ) were collected. For cases relative to cold or freezing winters, data refer to the warm season only. For grasses cut for hay, K c act ini , Kc act mid , and Kc act end refer to a cut cycle. Kc act values rarely exceeded 1.25, thus indicating that field measurements reported did respect the available energy for evaporation. Overall, K c act mid for semi-natural grasslands in cold climates were lower than those in hot climates except when available water was high, with K c act mid for meadows and mountain pastures gener- ally high. Steppes have K c act mid values lower than savannas. Grasses commonly planted for hay and for landscape generally showed high K c act mid values, while a larger variability was observed with grasses for grazing. The collected K c act values were used to define standard Kc values for all grassland and grasses. Nevertheless, the tabulated Kc act are indicative values of K c to be used for actual water management purposes and/or irrigation scheduling of planted grasslands. It is expected that a better knowledge of the standard and/or indicative K c values for a wide variety of grasslands and grasses will support better management aimed to improve grass productivity and ecosystem services, including biodiversity and carbon sequestrationinfo:eu-repo/semantics/publishedVersio

Vegetation Dynamics Revealed by Remote Sensing and Its Feedback to Regional and Global Climate

This book focuses on some significant progress in vegetation dynamics and their response to climate change revealed by remote sensing data. The development of satellite remote sensing and its derived products offer fantastic opportunities to investigate vegetation changes and their feedback to regional and global climate systems. Special attention is given in the book to vegetation changes and their drivers, the effects of extreme climate events on vegetation, land surface albedo associated with vegetation changes, plant fingerprints, and vegetation dynamics in climate modeling

Energy and Water Cycles in the Third Pole

As the most prominent and complicated terrain on the globe, the Tibetan Plateau (TP) is often called the “Roof of the World”, “Third Pole” or “Asian Water Tower”. The energy and water cycles in the Third Pole have great impacts on the atmospheric circulation, Asian monsoon system and global climate change. On the other hand, the TP and the surrounding higher elevation area are also experiencing evident and rapid environmental changes under the background of global warming. As the headwater area of major rivers in Asia, the TP’s environmental changes—such as glacial retreat, snow melting, lake expanding and permafrost degradation—pose potential long-term threats to water resources of the local and surrounding regions. To promote quantitative understanding of energy and water cycles of the TP, several field campaigns, including GAME/Tibet, CAMP/Tibet and TORP, have been carried out. A large amount of data have been collected to gain a better understanding of the atmospheric boundary layer structure, turbulent heat fluxes and their coupling with atmospheric circulation and hydrological processes. The focus of this reprint is to present recent advances in quantifying land–atmosphere interactions, the water cycle and its components, energy balance components, climate change and hydrological feedbacks by in situ measurements, remote sensing or numerical modelling approaches in the “Third Pole” region