Refining the role of phenology in regulating gross ecosystem productivity across European peatlands

Abstract The role of plant phenology as regulator for gross ecosystem productivity (GEP) in peatlands is empirically not well constrained. This is because proxies to track vegetation development with daily coverage at the ecosystem scale have only recently become available and the lack of such data has hampered the disentangling of biotic and abiotic effects. This study aimed at unraveling the mechanisms that regulate the seasonal variation in GEP across a network of eight European peatlands. Therefore, we described phenology with canopy greenness derived from digital repeat photography and disentangled the effects of radiation, temperature and phenology on GEP with commonality analysis and structural equation modeling. The resulting relational network could not only delineate direct effects but also accounted for possible effect combinations such as interdependencies (mediation) and interactions (moderation). We found that peatland GEP was controlled by the same mechanisms across all sites: phenology constituted a key predictor for the seasonal variation in GEP and further acted as distinct mediator for temperature and radiation effects on GEP. In particular, the effect of air temperature on GEP was fully mediated through phenology, implying that direct temperature effects representing the thermoregulation of photosynthesis were negligible. The tight coupling between temperature, phenology and GEP applied especially to high latitude and high altitude peatlands and during phenological transition phases. Our study highlights the importance of phenological effects when evaluating the future response of peatland GEP to climate change. Climate change will affect peatland GEP especially through changing temperature patterns during plant-phenologically sensitive phases in high latitude and high altitude regions.Peer reviewe

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

Nutrients and water availability constrain the seasonality of vegetation activity in a Mediterranean ecosystem

Anthropogenic nitrogen (N) deposition and resulting differences in ecosystem N and phosphorus (P) ratios are expected to impact photosynthetic capacity, that is, maximum gross primary productivity (GPP). However, the interplay between N and P availability with other critical resources on seasonal dynamics of ecosystem productivity remains largely unknown. In a Mediterranean tree–grass ecosystem, we established three landscape-level (24 ha) nutrient addition treatments: N addition (NT), N and P addition (NPT), and a control site (CT). We analyzed the response of ecosystem to altered nutrient stoichiometry using eddy covariance fluxes measurements, satellite observations, and digital repeat photography. A set of metrics, including phenological transition dates (PTDs; timing of green-up and dry-down), slopes during green-up and dry-down period, and seasonal amplitude, were extracted from time series of GPP and used to represent the seasonality of vegetation activity. The seasonal amplitude of GPP was higher for NT and NPT than CT, which was attributed to changes in structure and physiology induced by fertilization. PTDs were mainly driven by rainfall and exhibited no significant differences among treatments during the green-up period. Yet, both fertilized sites senesced earlier during the dry-down period (17–19 days), which was more pronounced in the NT due to larger evapotranspiration and water usage. Fertilization also resulted in a faster increase in GPP during the green-up period and a sharper decline in GPP during the dry-down period, with less prominent decline response in NPT. Overall, we demonstrated seasonality of vegetation activity was altered after fertilization and the importance of nutrient–water interaction in such water-limited ecosystems. With the projected warming-drying trend, the positive effects of N fertilization induced by N deposition on GPP may be counteracted by an earlier and faster dry-down in particular in areas where the N:P ratio increases, with potential impact on the carbon cycle of water-limited ecosystems.The authors acknowledge the Alexander von Humboldt Foundation for supporting this research with the Max-Planck Prize to Markus Reichstein. Yunpeng Luo and Mirco Migliavacca gratefully acknowledge financial support from the China Scholarship Council. Gerardo Moreno acknowledges financial support from the grant agreement IB16185 of the Regional Government of Extremadura

On Interpreting Eddy Covariance In Small Area Agricultural Situations With Contrasting Site Management.

This dissertation examined the carbon sequestration potential of a low C:N soil amendment and its incorporation into the soil over a rolling agricultural field. A segmented planar fit was developed to assess and correct the systematic errors the topography introduces on the carbon dioxide fluxes. The carbon dioxide fluxes were then be partitioned into gross primary productivity and soil respiration to understand the influence of the contrasting management practices, using flux variance partitioning. Concomitant with the partitioning, high resolution temporal and spatial scale remote sensing images were interpolated and standardized to conduct hypothesis testing for treatment effects

Land- atmosphere exchange of carbon dioxide in a high Arctic fen : importance of wintertime fluxes

Global warming is predicted to have a major impact on the ecosystems over the polar latitudes including the Arctic region which is thought to be especially sensitive to changes in climate. So far, the research studying greenhouse gases in the Arctic has primarily been focused on the short and intense growing season when carbon flux is mostly driven by plants and soil microorganisms. Regarding winter time little is known about what factors that influence the carbon flux between the land and the atmosphere (Net Ecosystem Exchange, NEE) and how big impact it has on the annual carbon budget. This study investigated the importance of wintertime CO2 fluxes (Net ecosystem exchange; NEE) on a net annual exchange basis and which environmental variables that affected CO2 flux during wintertime. If seasonality (i.e. early winter, dark winter, late winter) affected relationships between carbon flux and the driving variables was also examined. The study was based on two years of data (August 2012- October 2014) from an eddy covariance tower on the fen in Zackenberg, Greenland. It was found that winter time flux in the year 2012/2013 was 67.6 g C m-2 (emission of CO2 to the atmosphere) and for the year 2013/2014 the winter time flux was 31.4 g C m-2. The early winter time (September -7th of November) was the winter season where the strongest relationship between environmental variables and NEE was seen for both years. Here NEE increased exponentially with air temperature and soil temperature (-10 cm) but the relationship was strongest with air temperature. Air temperature, PAR, soil temperature and snow depth were factors that affected CO2 flux during wintertime but no clear relationship could be seen with snow temperature. Seasonality clearly had an impact on the relationship between carbon flux and the driving variables. In this study only a few environmental variables were tested and to be able to cover the complete pictures of what factors that affect NEE more studies have to be done, for example of soil and snow temperatures at more depth, water table depth, thaw depth, day of snowmelt and air pressure. A longer time series would also have been valuable but it is certain that winter time carbon flux is important when making an annual carbon budget in the Arctic.Utbyte av koldioxid mellan mark och atmosfär i en nordlig arktisk myr: betydelsen av vinterflöden Med ett varmare klimat i och med den globala uppvärmningen hotas permafrosten i Arktis att smälta och stora mängder kol frigöras till atmosfären. Koldioxid är en viktig växthusgas och kolflödet i Arktis är därför mycket aktuellt att studera. Årligen pågår ett utbyte mellan mark och atmosfär då växterna tar upp kol under sommaren genom fotosyntesen samtidigt som det sker ett mindre utsläpp av kol genom respiration. Kolflödet mellan mark och atmosfär under sommaren är väl undersökt, men hur stor del har egentligen vintertiden i det årliga kolflödet? Under vintern sker ett litet utsläpp av kol från respirationen av mikroorganismer som fortsätter under snötäcket. Vilka miljöfaktorer är det egentligen som styr kolflödet under vinterhalvåret? Kan det vara lufttemperatur, marktemperatur, snödjup eller inkommande solstrålning? Det här är spännande och betydelsefulla frågor då balansen i ekosystemet kan komma att rubbas med en stigande temperatur i framtiden. Vinterperioden är lång i Arktis och räknas i den här studien från september till juni, då det ofta tar lång tid för snötäcket att försvinna på våren. Under november till februari är det polarnatt och solen når aldrig över horisonten. I motsats finna också polardagen när solen aldrig försvinner under horisonten under den korta sommaren. Den här studien har använt data från ett eddy covariance-torn över en myr i Zackenberg på Grönland. Data består av koldioxidflödet mellan mark och atmosfär (g C m-2), samt snödjup, inkommande strålning, lufttemperatur, snötemperatur och marktemperatur. Data från två år, 2012/2012 och 2013/2014 har analyserats. Slutsatsen i den här undersökningen var att kolflödet under vintertiden verkligen hade en stor del av den årliga kolbudgeten. Under sommaren skedde ett upptag av kol och under vintern ett utsläpp av kol till atmosfären. Sett över ett helt år skedde dock ett upptag av kol över myren i Zackenberg. Lufttemperatur och marktemperaturen hade den största påverkan på kolflödet och det var ett positivt samband: när temperaturen ökade, ökade också kolflödet. Att den här studien visar att vintertidens utsläpp är större än väntat pekar på betydelsen av att inkludera detta i budgetberäkningar i framtiden. Lufttemperatur och marktemperatur har stor påverkan på mikroorganismerna och med en varmare temperatur är det möjligt att respirationen kan vara större samt fortsätta under en längre tidsperiod

THE USE OF REMOTE SENSING AND EDDY COVARIANCE TECHNOLOGIES TO CHARACTERIZE CROPLAND, DROUGHT AND LAND MANAGEMENTS AND THEIR IMPACTS ON ECOSYSTEM DYNAMICS

With the increasing population, human needs more food, fresh water, and other ecosystem services, which burdens the agricultural and natural ecosystems. Under the background of climate change, meeting these human needs becomes more challenging because of increasing temperature, climate extremes, etc. and their interaction with human activities. Thus, it is important to understand the impacts of climate change and human activities on ecosystem dynamics. The land-use and land-cover change, one of the most important human activities, greatly affects the function and dynamics of ecosystems. Drought is one of the most costly natural disasters and imposes wide-ranging impacts on the economy, environment, and society. This dissertation aimed to strengthen the usage of remote sensing and eddy covariance techniques in paddy rice mapping, agricultural drought monitoring, land management effects assessment, and evaluating the impacts of drought on cattle production. Chapter 2 identified the different flooding/transplanting periods of paddy rice and natural wetlands. The natural wetlands foods earlier and have a shorter duration than paddy rice in the Panjin Plain, a temperate region in China. Using this asynchronous flooding stages, this chapter extracted the paddy rice planting area from the rice-wetland coexistent area using MODIS and Landsat 8 imagery. The comparison and validation tests indicated high accuracy of our paddy rice map. Chapter 3 quantified the agricultural drought of tallgrass prairie in the SGP using a remotely sensed water-related vegetation index derived from MODIS. The results are comparable to other widely used drought products. The spatial pattern of drought duration was highly correlated with the decreasing precipitation gradient from east to west. LSWI-based drought depictions are sensitive to both precipitation anomalies from the historical mean and abnormal seasonal precipitation distributions. A comparison with other widely used drought products is made. Chapter 4 examined the impacts of burning, baling, and grazing on canopy and carbon fluxes in a pasture through integrating PhenoCam images, satellite remote sensing, and eddy covariance data. Landsat images were used to assess the baling area and the trajectory of vegetation recovery. MODIS vegetation indices (VIs) were used in the Vegetation Photosynthesis Model (VPM) to estimate gross primary production (GPPVPM) at a MODIS pixel for the flux tower (baled) site. Multiple datasets allowed studying intra-annual variations caused by various management practices. The larger increase of GPP after large rain in baled grassland (photosynthetically more active vegetation) compensated the reduction in GPP caused by baling. This result indicated that the interaction of management practices with climate is important when studying their impacts on GPP. Chapter 5 evaluated the impacts of drought on cattle production in the SGP during 2000-2015 use meteorological, remote sensing, and statistical data. The results showed that the consecutive years of drought and high temperatures in 2011 and 2012 dramatically decreased the cattle production in OK and TX. The decrease extent in KS was smaller probably because of the greater accessibility to the groundwater resource. 2011 was a whole year drought in the SGP which decreased the hay production and thus cattle production, while 2012 was a summer drought year in the Corn Belt which increased the corn price and thus cattle production. The Random Forest method performed well and shows the potential in predicting the dynamics of cattle production

Interpreting canopy development and physiology using the EUROPhen camera network at flux sites

Peer reviewe

Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites

Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluate flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. Monthly 80% footprint climatologies vary across sites and through time ranging four orders of magnitude from 103 to 107 m2 due to the measurement heights, underlying vegetation- and ground-surface characteristics, wind directions, and turbulent state of the atmosphere. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. These biases are site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark against models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-periods suitable for specific applications and to provide general guidance for data use