Environmental Controls on Multi-Scale Dynamics of Net Carbon Dioxide Exchange From an Alpine Peatland on the Eastern Qinghai-Tibet Plateau

Peatlands are characterized by their large carbon storage capacity and play an essential role in the global carbon cycle. However, the future of the carbon stored in peatland ecosystems under a changing climate remains unclear. In this study, based on the eddy covariance technique, we investigated the net ecosystem CO2 exchange (NEE) and its controlling factors of the Hongyuan peatland, which is a part of the Ruoergai peatland on the eastern Qinghai-Tibet Plateau (QTP). Our results show that the Hongyuan alpine peatland was a CO2 sink with an annual NEE of -226.61 and -185.35 g C m(-2) in 2014 and 2015, respectively. While, the non-growing season NEE was 53.35 and 75.08 g C m(-2) in 2014 and 2015, suggesting that non-growing seasons carbon emissions should not be neglected. Clear diurnal variation in NEE was observed during the observation period, with the maximum CO2 uptake appearing at 12:30 (Beijing time, UTC+8). The Q(10) value of the non-growing season in 2014 and 2015 was significantly higher than that in the growing season, which suggested that the CO2 flux in the non-growing season was more sensitive to warming than that in the growing season. We investigated the multi-scale temporal variations in NEE during the growing season using wavelet analysis. On daily timescales, photosynthetically active radiation was the primary driver of NEE. Seasonal variation in NEE was mainly driven by soil temperature. The amount of precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event was associated with increasing annual carbon uptake. This study highlights the need for continuous eddy covariance measurements and time series analysis approaches to deepen our understanding of the temporal variability in NEE and multi-scale correlation between NEE and environmental factors

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

Responses and adaptation strategies of terrestrial ecosystems to climate change

Terrestrial ecosystems are likely to be affected by climate change, as climate change-induced shift of water and heat stresses patterns will have significant impacts on species composition, habitat distribution, and ecosystem functions, and thereby weaken the terrestrial carbon (C) sink and threaten global food security and biofuel production. This thesis investigates the responses of terrestrial ecosystems to climate change and is structured in four main chapters.;The first chapter of the thesis is directed towards the impacts of snow variation on ecosystem phenology. Variations in seasonal snowfall regulate regional and global climatic systems and vegetation growth by changing energy budgets of the lower atmosphere and land surface. We investigated the effects of snow on the start of growing season (SGS) of temperate vegetation in China. Across the entire temperate region in China, the winter snow depth increased at a rate of 0.15 cm•yr-1 (p=0.07) during the period 1982-1998, and decreased at a rate of 0.36 cm•yr-1 (p=0.09) during the period 1998-2005. Correspondingly, the SGS advanced at a rate of 0.68 d•yr-1 (p\u3c0.01) during 1982 to 1998, and delayed at a rate of 2.13 d•yr-1 (p=0.07) during 1998 to 2005, against a warming trend throughout the entire study period of 1982-2005. Spring air temperature strongly regulated the SGS of both deciduous broad-leaf and coniferous forests; whilst the winter snow had a greater impact on the SGS of grassland and shrubs. Snow depth variation combined with air temperature contributed to the variability in the SGS of grassland and shrubs, as snow acted as an insulator and modulated the underground thermal conditions. Additionally, differences were seen between the impacts of winter snow depth and spring snow depth on the SGS; as snow depths increased, the effect associated went from delaying SGS to advancing SGS. The observed thresholds for these effects were snow depths of 6.8 cm (winter) and 4.0 cm (spring). The results of this study suggest that the response of the vegetation\u27s SGS to seasonal snow change may be attributed to the coupling effects of air temperature and snow depth associated with the soil thermal conditions.;The second chapter further addresses snow impacts on terrestrial ecosystem with focus on regional carbon exchange between atmosphere and biosphere. Winter snow has been suggested to regulate terrestrial carbon (C) cycling by modifying micro-climate, but the impacts of snow cover change on the annual C budget at the large scale are poorly understood. Our aim is to quantify the C balance under changing snow depth. Here, we used site-based eddy covariance flux data to investigate the relationship between snow cover depth and ecosystem respiration (Reco) during winter. We then used the Biome-BGC model to estimate the effect of reductions in winter snow cover on C balance of Northern forests in non-permafrost region. According to site observations, winter net ecosystem C exchange (NEE) ranged from 0.028-1.53 gC•m-2•day-1, accounting for 44 +/- 123% of the annual C budget. Model simulation showed that over the past 30 years, snow driven change in winter C fluxes reduced non-growing season CO2 emissions, enhancing the annual C sink of northern forests. Over the entire study area, simulated winter ecosystem respiration (Reco) significantly decreased by 0.33 gC•m-2•day -1•yr-1 in response to decreasing snow cover depth, which accounts for approximately 25% of the simulated annual C sink trend from 1982 to 2009. Soil temperature was primarily controlled by snow cover rather than by air temperature as snow served as an insulator to prevent chilling impacts. A shallow snow cover has less insulation potential, causing colder soil temperatures and potentially lower respiration rates. Both eddy covariance analysis and model-simulated results showed that both Reco and NEE were significantly and positively correlated with variation in soil temperature controlled by variation in snow depth. Overall, our results highlight that a decrease in winter snow cover restrains global warming through emitting less C to the atmosphere.;The third chapter focused on assessing drought\u27s impact on global terrestrial ecosystems. Drought can affect the structure, composition and function of terrestrial ecosystems, yet the drought impacts and post-drought recovery potential of different land cover types have not been extensively studied at a global scale. Here, we evaluated drought impacts on gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) of different global terrestrial ecosystems, as well as the drought-resilience of each ecosystem type during the period of 2000 to 2011. We found the rainfall and soil moisture during drought period were dramatically lower than these in non-drought period, while air temperatures were higher than normal during drought period with amplitudes varied by land cover types. The length of recovery days (LRD) presented an evident gradient of high (\u3e 60 days) in mid- latitude region and low (\u3c 60 days) in low (tropical area) and high (boreal area) latitude regions. As average GPP increased, the LRD showed a significantly decreasing trend among different land covers (R 2=0.53, p\u3c0.0001). Moreover, the most dramatic reduction of the drought-induced GPP was found in the mid-latitude region of north Hemisphere (48% reduction), followed by the low-latitude region of south Hemisphere (13% reduction). In contrast, a slightly enhanced GPP (10%) was showed in the tropical region under drought impact. Additionally, the highest drought-induced reduction of ET was found in the Mediterranean area, followed by Africa. The water use efficiency, however, showed a pattern of decreasing in the north Hemisphere and increasing in the south Hemisphere.;The last chapter compared the differences of performance in trading water for carbon in planted forest and natural forest, with specific focus on China. Planted forests have been widely established in China as an essential approach to improving the ecological environment and mitigating climate change. Large-scale forest planting programs, however, are rarely examined in the context of tradeoffs between carbon sequestration and water yield between planted and natural forests. We reconstructed evapotranspiration (ET) and gross primary production (GPP) data based on remote-sensing and ground observational data, and investigated the differences between natural and planted forests, in order to evaluate the suitability of tree-planting activity in different climate regions where the afforestation and reforestation programs have been extensively implemented during the past three decades in China. While the differences changed with latitude (and region), we found that, on average, planted forests consumed 5.79% (29.13mm) more water but sequestered 1.05% (-12.02 gC m-2 yr -1) less carbon than naturally generated forests, while the amplitudes of discrepancies varied with latitude. It is suggested that the most suitable lands in China for afforestation should be located in the moist south subtropical region (SSTP), followed by the mid-subtropical region (MSTP), to attain a high carbon sequestration potential while maintain a relatively low impact on regional water balance. The high hydrological impact zone, including the north subtropical region (NSTP), warm temperate region (WTEM), and temperate region (TEM) should be cautiously evaluated for future afforestation due to water yield reductions associated with plantations

Spatial and temporal drivers of soil respiration in a tundra environment

Arktisten alueiden maaperään on varastoitunut merkittäviä määriä hiiltä, joka on peräisin kasvillisuudesta. Hiiltä vapautuu maaperästä respiraatiossa kasvien juurten ja maaperän mikrobien hajotustoiminnan seurauksena. Pohjoisen pallonpuoliskon tundra-alueilla ekosysteemiin fotosynteesin kautta sitoutunut hiilen määrä on ollut suurempi kuin respiraatiossa ilmakehään vapautuvan hiilen määrä. Kylmien ja kosteiden olosuhteiden ansiosta arktiset maaperät ovat olleet merkittävä hiilen nielu. Lämpötilojen globaali nousu ja muutokset hydrologiassa ovat kuitenkin aiheuttaneet muutoksia hiilen kierrossa maaperän ja ilmakehän välillä. Arktiset alueet ovat erityisen herkkiämuutoksille, ja ne saattavat synnyttää voimakkaita takaisinkytkentöjä ilmaston lämpenemiseen. Tekijät, jotka säätelevät maaperän respiraatiovuota, tunnetaan heikosti, erityisesti niiden keskinäiset suhteet erilaisissa ympäristöissä ja niiden ajallinen dynamiikka. Maaperän respiraation ymmärtäminen prosessina on erittäin tärkeää, jotta voitaisiin paremmin ymmärtää ja ennustaa tulevia muutoksissa hiilen kierrossa globaalisti.Tämän tutkimuksen tavoitteena olitunnistaa maaperän respiraatiota sääteleviä ympäristötekijöitä tundralla maisemamittakaavalla ja niiden merkitystä kasvukauden eri vaiheissa. Tutkimusalue olikahden tunturin välinen laakso Kilpisjärvellä, käsivarren Lapissa. Maaperän respiraatiota mitattiin käyttäen kammiomenetelmää sadalla tutkimuspisteellä 3 x 2 km alueella kolmesti kesän 2018 aikana. Alueelta kerättiin myösympäristöaineistoa maaperän mikroilmastosta ja kasvillisuuden ominaisuuksista. Ympäristötekijöiden vaikutusta respiraatioon tarkasteltiin yleistetyillä lineaarisilla malleilla käyttäen erilaisia selittävien muuttujien yhdistelmiä. Tulokset osoittivat, että runsas kasvillisuus indikoi korkeaa respiraatiota, koska kasvien karike ja juuristo tuottavat maaperän mikrobeille hajotettavaa ainesta ja muita resursseja, kuten ravinteita. Suurin respiraatio mitattiin kasvukauden huippuna, jolloin korkealämpötila aiheutti respiraation kasvua tutkimuspisteillä, jotka sijaitsivat tundraniityillä. Kasvillisuus ja maaperän laatu säätelevät siis myös respiraation lämpötilavastetta. Respiraatiovuo kasvoi lämpimässä vain paikoilla, joilla oletetaan olevan riittävästi ravinteita ja hajotettavaa aineista, jotta korkeampi respiraatio on mahdollinen. Tämän tutkimuksen mukaan maisemamittakaavalla kasvillisuuden tarjoamat resurssit ovat respiraatiolle tärkeämpiä kuin ilmastolliset tekijät sekä alueellisesti että ajallisesti.Jatkossa respiraatiotutkimus vaatii lisää empiiristä aineistoa, jotta tulevaisuuden muutoksia voidaan tarkemmin mallintaa. Etenkin respiraation valtavaan alueelliseen ja ajalliseen vaihteluun tulee kiinnittää huomiota arktisten alueiden tutkimuksessa.Arctic soils store significant amounts of carbon deposited by plants and litter. Carbon is released from the soil in respiration due to plant roots and decomposition by microbes. In the northern hemisphere, carbon inputs from photosynthesis have exceeded releases of carbon to atmosphere via respiration. Arctic soils have been a globally remarkable carbon sink due to cold and waterlogged conditions. However, rising global temperatures and changes in hydrology have caused the carbon fluxes in soil-atmosphere interface to alter. Arctic areas are considered especially vulnerable to climate change and alterations in the arctic soil carbon pools could create powerful feedbacks to warming. Furthermore, drivers controlling soil respiration flux remain poorly known, especially their contributions in different environments and their dynamics in time. Thus, understanding soil respiration as a process is vital in understanding future changes in the global carbon cycle.The aim of this study was to identify environmental drivers of soil respiration in tundra at landscape-scale and their relative importance in different stages of growing season. The study area was a valley between two fells at Kilpisjärvi, Finland. Soil respiration was measured using the chamber method in 100 study sites on the 3 x 2 km landscape three times during the summer of 2018. Environmental data on soil microclimate and vegetation properties was gathered fromthe area as well. The impact of environmental conditions to respiration flux was studied using multiple generalized linear models with different explanatory variable combinations.Results suggest that abundant vegetation causes high respiration by providing resources for belowground microbes and creating extensive root network. Highest respiration was measured in peak growing season, when elevated temperatures stimulated respiration exclusively in tundra meadows. It seems that vegetation and soil parameters also define the temperature response of respiration. The flux increased with elevated temperatures only on soils that are assumed to have adequate nutrient and carbon composition to support higher respiration. This study suggests that onlandscape-scale, the resources provided by vegetation are of bigger importance to respiration than climatic changes both spatially and temporally.Moving forward, more empiricaldata is needed in order to accurately model future changes in respiration. Intense sampling efforts from the Arctic tundra areasthatcover the large spatial and temporal variabilityof respiration are necessary

Relationship between ecosystem productivity and photosynthetically-active radiation for northern peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils

Climate Change and Environmental Sustainability- Volume 5

This volume of Climate Change and Environmental Sustainability covers topics on greenhouse gas emissions, climatic impacts, climate models and prediction, and analytical methods. Issues related to two major greenhouse gas emissions, namely of carbon dioxide and methane, particularly in wetlands and agriculture sector, and radiative energy flux variations along with cloudiness are explored in this volume. Further, climate change impacts such as rainfall, heavy lake-effect snowfall, extreme temperature, impacts on grassland phenology, impacts on wind and wave energy, and heat island effects are explored. A major focus of this volume is on climate models that are of significance to projection and to visualise future climate pathways and possible impacts and vulnerabilities. Such models are widely used by scientists and for the generation of mitigation and adaptation scenarios. However, dealing with uncertainties has always been a critical issue in climate modelling. Therefore, methods are explored for improving climate projection accuracy through addressing the stochastic properties of the distributions of climate variables, addressing variational problems with unknown weights, and improving grid resolution in climatic models. Results reported in this book are conducive to a better understanding of global warming mechanisms, climate-induced impacts, and forecasting models. We expect the book to benefit decision makers, practitioners, and researchers in different fields and contribute to climate change adaptation and mitigation

Direct and indirect effects of climatic variations on the interannual variability in net ecosystem exchange across terrestrial ecosystems

Climatic variables not only directly affect the interannual variability (IAV) in net ecosystem exchange of CO2 (NEE) but also indirectly drive it by changing the physiological parameters. Identifying these direct and indirect paths can reveal the underlying mechanisms of carbon (C) dynamics. In this study, we applied a path analysis using flux data from 65 sites to quantify the direct and indirect climatic effects on IAV in NEE and to evaluate the potential relationships among the climatic variables and physiological parameters that represent physiology and phenology of ecosystems. We found that the maximum photosynthetic rate was the most important factor for the IAV in gross primary productivity (GPP), which was mainly induced by the variation in vapour pressure deficit. For ecosystem respiration (RE), the most important drivers were GPP and the reference respiratory rate. The biome type regulated the direct and indirect paths, with distinctive differences between forests and non-forests, evergreen needleleaf forests and deciduous broadleaf forests, and between grasslands and croplands. Different paths were also found among wet, moist and dry ecosystems. However, the climatic variables can only partly explain the IAV in physiological parameters, suggesting that the latter may also result from other biotic and disturbance factors. In addition, the climatic variables related to NEE were not necessarily the same as those related to GPP and RE, indicating the emerging difficulty encountered when studying the IAV in NEE. Overall, our results highlight the contribution of certain physiological parameters to the IAV in C fluxes and the importance of biome type and multi-year water conditions, which should receive more attention in future experimental and modelling research

Quantifying the role of shade on microclimate conditions and water use efficiency of a subalpine wetland in the Canadian Rocky Mountains, Kananaskis, Alberta

Alpine regions contribute 60 % of annual surface runoff, playing an important role in regulating the global water balance. Many of the world’s major river networks originate from alpine headwater basins, popularizing mountains as the “Water Towers of the World”. The Rocky Mountains represent Western Canada’s “Water Tower” since they store and distribute water resources to over 13 million people across Western Canada and the Pacific Northwest USA. At the headwater, topography causes land surfaces to cycle in and out of shadows, creating distinct microclimates that strongly influence evapotranspiration (ET) and carbon fluxes. Yet, relatively few studies have observed the relationship between the energy, water, and carbon fluxes of mountain catchments; and have rather focused on periods of snow and ice cover. Therefore, understanding the contribution of subalpine wetlands to the water budget remains a leading hydrological need in mountain areas worldwide. This thesis attempts to address these knowledge gaps by investigating the influence of complex terrain on the spatial and temporal variability of shade across a subalpine wetland (2,083 m a.s.l.) in the Canadian Rocky Mountains and the effect of shade on seasonal flux dynamics. Meteorological and eddy covariance equipment was installed from June 7th to September 10th to establish baseline environment conditions and to monitor the turbulent and radiative fluxes over the 2018 snow free period. Hill shade and solar radiation models for clear-sky days were compared to field observations to understand how shade impacted the energy, water, and carbon fluxes. Water Use Efficiency (WUE) was used as a metric to understand the relationship between water and carbon cycling. Overall, shade shortened the growing season and prolonged snowmelt. Shade was greatest near the headwall and reduced cumulative solar radiation by 86.4 MJ over the study period. When shade was low and constant during the period of Stable Shade (June 7th – July 30th), it had a non-significant relationship with incoming solar radiation (K↓) and net radiation (Q*); however, when shade rapidly increased during the period of Dynamic Shade (July 31st – September 10th) it strongly influenced K↓ and Q*. On average, during Dynamic Shade, each hourly increase of shade per day, reduced K↓ and Q* by 32 W/m2 and 28 W/m2, equivalent to 13 % and 16 %, respectively. Water and carbon fluxes had a similar response to shade as the energy fluxes. Each hourly increase of shade reduced ET and Gross Primary Production (GPP) by similar margins: 17 % and 15 %, respectively. Therefore, WUE remained relatively unaffected by horizon shade, because shade equally reduced ET and GPP. These findings indicate that under uncertain future climate scenarios (i.e. increased risk of flood, drought, and forest fires), shade may be an important mechanism for moisture conservation in a variety of subalpine ecosystems that are at risk of late season water stress

The current state of CO2 flux chamber studies in the Arctic tundra : A review

The Arctic tundra plays an important role in the carbon cycle as it stores 50% of global soil organic carbon reservoirs. The processes (fluxes) regulating these stocks are predicted to change due to direct and indirect effects of climate change. Understanding the current and future carbon balance calls for a summary of the level of knowledge regarding chamber-derived carbon dioxide (CO2) flux studies. Here, we describe progress from recently (2000-2016) published studies of growing-season CO2 flux chamber measurements, namely GPP (gross primary production), ER (ecosystem respiration), and NEE (net ecosystem exchange), in the tundra region. We review the study areas and designs along with the explanatory environmental drivers used. Most of the studies were conducted in Alaska and Fennoscandia, and we stress the need for measuring fluxes in other tundra regions, particularly in more extreme climatic, productivity, and soil conditions. Soil respiration and other greenhouse gas measurements were seldom included in the studies. Although most of the environmental drivers of CO2 fluxes have been relatively well investigated (such as the effect of vegetation type and soil microclimate on fluxes), soil nutrients, other greenhouse gases and disturbance regimes require more research as they might define the future carbon balance. Particular attention should be paid to the effects of shrubification, geomorphology, and other disturbance effects such as fire events, and disease and herbivore outbreaks. An improved conceptual framework and understanding of underlying processes of biosphere-atmosphere CO2 exchange will provide more information on carbon cycling in the tundra.Peer reviewe