David Holl
A5035424108- Climate change and permafrost
- Cryospheric studies and observations
- Peatlands and Wetlands Ecology
- Atmospheric and Environmental Gas Dynamics
- Geology and Paleoclimatology Research
- Methane Hydrates and Related Phenomena
- Plant Water Relations and Carbon Dynamics
- Case Reports on Hematomas
- Fire effects on ecosystems
- Trauma and Emergency Care Studies
- Composite Structure Analysis and Optimization
- Geological Studies and Exploration
- Elasticity and Wave Propagation
- Arctic and Antarctic ice dynamics
- Soil Carbon and Nitrogen Dynamics
- Abdominal Trauma and Injuries
- Coastal wetland ecosystem dynamics
- Ecology and biodiversity studies
- Rice Cultivation and Yield Improvement
- Climate variability and models
- Spectroscopy and Laser Applications
- Science and Climate Studies
- Structural Analysis and Optimization
- Coal Properties and Utilization
- Structural Behavior of Reinforced Concrete
Universität Hamburg
2017-2025
Institute of Soil Science
2016-2025
Hamburg Institut (Germany)
2024
Atac (United States)
2009
James S. McDonnell Foundation
1992
Iowa State University
1936-1953
Ames National Laboratory
1939
Abstract The regional variability in tundra and boreal carbon dioxide (CO 2 ) fluxes can be high, complicating efforts to quantify sink‐source patterns across the entire region. Statistical models are increasingly used predict (i.e., upscale) CO large spatial domains, but reliability of different modeling techniques, each with specifications assumptions, has not been assessed detail. Here, we compile eddy covariance chamber measurements annual growing season gross primary productivity (GPP),...
Abstract Tundra and boreal ecosystems encompass the northern circumpolar permafrost region are experiencing rapid environmental change with important implications for global carbon (C) budget. We analysed multi-decadal time series containing 302 annual estimates of dioxide (CO 2 ) flux across 70 non-permafrost ecosystems, 672 summer CO 181 ecosystems. find an increase in sink but not despite similar increases uptake. Thus, recent non-growing-season losses have substantially impacted balance...
Abstract The Arctic–Boreal Zone is rapidly warming, impacting its large soil carbon stocks. Here we use a new compilation of terrestrial ecosystem CO 2 fluxes, geospatial datasets and random forest models to show that although the was overall an increasing sink from 2001 2020 (mean ± standard deviation in net exchange, −548 140 Tg C yr −1 ; trend, −14 P < 0.001), more than 30% region source. Tundra regions may have already started function on average as sources, demonstrating shift...
Methane emissions from natural wetlands tend to increase with temperature and therefore may lead a positive feedback under future climate change. However, their response includes confounding factors appears differ on different time scales. Observed methane depend strongly seasonal basis, but if the annual mean are compared between sites, there is only small effect. We hypothesize that microbial dynamics major driver of cycle they can explain this apparent discrepancy. introduce relatively...
Abstract. Past efforts to synthesize and quantify the magnitude change in carbon dioxide (CO2) fluxes terrestrial ecosystems across rapidly warming Arctic–boreal zone (ABZ) have provided valuable information but were limited their geographical temporal coverage. Furthermore, these been based on data aggregated over varying time periods, often with only minimal site ancillary data, thus limiting potential be used large-scale budget assessments. To bridge gaps, we developed a standardized...
Abstract Despite the importance of high-latitude surface energy budgets (SEBs) for land-climate interactions in rapidly changing Arctic, uncertainties their prediction persist. Here, we harmonize SEB observations across a network vegetated and glaciated sites at circumpolar scale (1994–2021). Our variance-partitioning analysis identifies vegetation type as an important predictor SEB-components during Arctic summer (June-August), compared to other SEB-drivers including climate, latitude...
Abstract. Peatlands store substantial amounts of carbon and are vulnerable to climate change. We present a modified version the Organising Carbon Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model for simulating hydrology, energy, CO2 fluxes peatlands on daily annual timescales. The includes separate soil tile in each 0.5° grid cell, defined from global peatland map identified with peat-specific hydraulic properties. Runoff non-peat vegetation within cell containing fraction peat...
Abstract Many remote sensing‐based evapotranspiration (RSBET) algorithms have been proposed in the past decades and evaluated using flux tower data, mainly over North America Europe. Model evaluation across South has done locally or only a single algorithm at time. Here, we provide first of multiple RSBET models, daily scale, wide variety biomes, climate zones, land uses America. We used meteorological data from 25 towers to force four models: Priestley–Taylor Jet Propulsion Laboratory...
Abstract. In the global methane budget, largest natural source is attributed to wetlands, which encompass all ecosystems composed of waterlogged or inundated ground, capable production. Among them, northern peatlands that store large amounts soil organic carbon have been functioning, since end last glaciation period, as long-term sources (CH4) and are one most significant among wetlands. To reduce uncertainty quantifying flux in it significance understand underlying processes for production...
Long-term atmospheric CO2 concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since 1990s. A variety mechanisms been proposed to explain reduced net sink northern ecosystems with increased air temperature, including water stress vegetation and respiration over recent decades. However, lack consistent long-term flux situ soil moisture data has severely limited our ability identify responsible for strength. In this study, we used...
Abstract Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations the has limited our understanding impact covarying environmental drivers on balance In this study, we address some these uncertainties through a novel record 119 site-years summer data from eddy covariance towers representing dominant vegetation types located continuous permafrost Arctic. Here found that earlier snowmelt was...
Biogeochemical cycling in permafrost-affected ecosystems remains associated with large uncertainties, which could impact the Earth's greenhouse gas budget and future climate policies. In particular, increased nutrient availability following permafrost thaw perturb exchange these systems, an effect largely unexplored until now. Here, we enhance terrestrial ecosystem model QUINCY (QUantifying Interactions between Nutrient CYcles system), simulates fully coupled carbon (C), nitrogen (N)...
Abstract. Ground-based observations of land–atmosphere fluxes are necessary to progressively improve global climate models. Observed data can be used for model evaluation and develop or tune process In arctic permafrost regions, climate–carbon feedbacks amplified. Therefore, increased efforts better represent these regions in models have been made recent years. We present a multi-annual time series carbon dioxide measured situ with the eddy covariance technique Siberian Arctic (72∘22′ N,...
Abstract The author gives, by the method of finite differences, an approximate solution problem a length cantilever plate which bears concentrated load at longitudinal free edge. All boundary conditions are taken into account, and action is determined approximately all points plate. out that secondary maximum transverse stress occurs clamped edge nearest loading point, greatest directly under point.
Abstract. Arctic permafrost landscapes have functioned as a global carbon sink for millennia. These are very heterogeneous, and the omnipresent water bodies within them act source. Yet, few studies focused on impact of these landscape budget. We deepen our understanding emissions from thermokarst ponds constrain their by comparing dioxide methane fluxes to surrounding tundra. use eddy covariance measurements tower located at border between large pond semi-terrestrial When we take open-water...
Abstract A long‐standing challenge in studying the global carbon cycle has been understanding factors controlling inter–annual variation (IAV) of fluxes, and improving their representations existing biogeochemical models. Here, we compared an optimality‐based model a semi‐empirical light use efficiency to understand how current models can be improved simulate IAV gross primary production (GPP). Both simulated hourly GPP were parameterized for (a) each site–year, (b) site with additional...
Abstract. Arctic tundra ecosystems are currently facing amplified rates of climate warming. Since these store significant amounts soil organic carbon, which can be mineralized to carbon dioxide (CO2) and methane (CH4), rising temperatures may cause increasing greenhouse gas fluxes the atmosphere. To understand how net ecosystem exchange (NEE) CO2 will respond changing climatic environmental conditions, it is necessary individual responses processes contributing NEE. Therefore, this study...
Abstract. Permafrost-affected soils contain large quantities of soil organic carbon (SOC). Changes in the SOC pool a particular ecosystem can be related to its net balance (NECB) which (C) influxes and effluxes is expressed. For polygonal tundra landscapes, accounts balances literature are often solely based on estimates vertical fluxes. To fill this gap, we present data regarding lateral export rates dissolved inorganic (DIC) (DOC) from site north Siberian Lena River delta, Russia. We use...
Abstract. The near-pristine bog ecosystems of Tierra del Fuego in southernmost Patagonia have so far not been studied terms their current carbon dioxide (CO2) sink strength. CO2 flux data from Southern Hemisphere peatlands are scarce general. In this study, we present net ecosystem exchange (NEE) fluxes two Fuegian with contrasting vegetation communities. One site is located a glaciogenic valley and developed as peat moss-dominated raised bog, the other vascular plant-dominated cushion at...
Abstract. With respect to their role in the global carbon cycle, natural peatlands are characterized by ability sequester atmospheric carbon. This trait is strongly connected water regime of these ecosystems. Large parts soil profile saturated, leading anoxic conditions and a diminished decomposition plant litter. In functioning peatlands, rate fixation photosynthesis larger than dead organic material. Over time, amount that remains not converted back dioxide grows. Land use often goes along...
Abstract. The current assessments of the carbon turnover in Arctic tundra are subject to large uncertainties. This problem can (inter alia) be ascribed both general shortage flux data from vast and sparsely inhabited region, as well typically high spatiotemporal variability fluxes ecosystems. Addressing these challenges, dioxide on an active flood plain situated Siberian Lena River Delta were studied during two growing seasons with eddy covariance method. footprint exhibited a heterogeneous...