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Institution

University of New Hampshire

EducationDurham, New Hampshire, United States
About: University of New Hampshire is a education organization based out in Durham, New Hampshire, United States. It is known for research contribution in the topics: Population & Solar wind. The organization has 9379 authors who have published 24025 publications receiving 1020112 citations. The organization is also known as: UNH.


Papers
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Journal ArticleDOI
TL;DR: OCO-2 SIF generally had a better performance for predicting GPP than satellite-derived vegetation indices and a light use efficiency model, and the generally consistent slope of the relationship among biomes suggests a nearly universal rather than biome-specific SIF-GPP relationship.
Abstract: Solar-induced chlorophyll fluorescence (SIF) has been increasingly used as a proxy for terrestrial gross primary productivity (GPP). Previous work mainly evaluated the relationship between satellite-observed SIF and gridded GPP products both based on coarse spatial resolutions. Finer resolution SIF (1.3 km × 2.25 km) measured from the Orbiting Carbon Observatory-2 (OCO-2) provides the first opportunity to examine the SIF-GPP relationship at the ecosystem scale using flux tower GPP data. However, it remains unclear how strong the relationship is for each biome and whether a robust, universal relationship exists across a variety of biomes. Here we conducted the first global analysis of the relationship between OCO-2 SIF and tower GPP for a total of 64 flux sites across the globe encompassing eight major biomes. OCO-2 SIF showed strong correlations with tower GPP at both midday and daily timescales, with the strongest relationship observed for daily SIF at the 757 nm (R2 = 0.72, p < 0.0001). Strong linear relationships between SIF and GPP were consistently found for all biomes (R2 = 0.57-0.79, p < 0.0001) except evergreen broadleaf forests (R2 = 0.16, p < 0.05) at the daily timescale. A higher slope was found for C4 grasslands and croplands than for C3 ecosystems. The generally consistent slope of the relationship among biomes suggests a nearly universal rather than biome-specific SIF-GPP relationship, and this finding is an important distinction and simplification compared to previous results. SIF was mainly driven by absorbed photosynthetically active radiation and was also influenced by environmental stresses (temperature and water stresses) that determine photosynthetic light use efficiency. OCO-2 SIF generally had a better performance for predicting GPP than satellite-derived vegetation indices and a light use efficiency model. The universal SIF-GPP relationship can potentially lead to more accurate GPP estimates regionally or globally. Our findings revealed the remarkable ability of finer resolution SIF observations from OCO-2 and other new or future missions (e.g., TROPOMI, FLEX) for estimating terrestrial photosynthesis across a wide variety of biomes and identified their potential and limitations for ecosystem functioning and carbon cycle studies.

233 citations

Journal ArticleDOI
TL;DR: This finding replicates previous results showing that people without drug abuse problems show steep discounting of alcohol and suggests that alcohol may be steeply discounted as part of a general process involving primary/consumable reinforcers, not necessarily because it is a drug.

232 citations

Journal ArticleDOI
TL;DR: Wollheim et al. as discussed by the authors presented a conceptual approach for evaluating the biological and hydrological controls of nutrient removal in different sized rivers within an entire river network, finding that larger rivers within a basin potentially exert considerable influence over nutrient exports.
Abstract: [1] We present a conceptual approach for evaluating the biological and hydrological controls of nutrient removal in different sized rivers within an entire river network. We emphasize a per unit area biological parameter, the nutrient uptake velocity (nf), which is mathematically independent of river size in benthic dominated systems. Standardization of biological parameters from previous river network models to nf reveals the nature of river size dependant biological activity in these models. We explore how geomorphic, hydraulic, and biological factors control the distribution of nutrient removal in an idealized river network, finding that larger rivers within a basin potentially exert considerable influence over nutrient exports. Citation: Wollheim, W. M.,

232 citations

Journal ArticleDOI
TL;DR: In this article, the authors calculated DON and DIN budgets using data on precipitation and streamwater chemistry collected from 9 forested watersheds at 4 sites in New England, and found that DON made up the majority of total dissolved nitrogen (TDN) in stream exports.
Abstract: Relatively high deposition of nitrogen (N) in the northeastern United States has caused concern because sites could become N saturated. In the past, mass-balance studies have been used to monitor the N status of sites and to investigate the impact of increased N deposition. Typically, these efforts have focused on dissolved inorganic forms of N (DIN = NH4-N + NO3-N) and have largely ignored dissolved organic nitrogen (DON) due to diffi- culties in its analysis. Recent advances in the measurement of total dissolved nitrogen (TDN) have facilitated measurement of DON as the residual of TDN DIN. We calculated DON and DIN budgets using data on precipitation and streamwater chemistry collected from 9 forested watersheds at 4 sites in New England. TDN in precipitation was composed primarily of DIN. Net retention of TDN ranged from 62 to 89% (4.7 to 10 kg ha 1 yr 1 ) of annual inputs. DON made up the majority of TDN in stream exports, suggesting that inclusion of DON is critical to assessing N dynamics even in areas with large anthropogenic inputs of DIN. Despite the dominance of DON in streamwater, precipitation inputs of DON were approximately equal to outputs. DON concentrations in streamwater did not appear significantly influenced by seasonal biological controls, but did increase with discharge on some watersheds. Stream- water NO3-N was the only fraction of N that exhibited a seasonal pattern, with concentrations increasing during the winter months and peaking during snowmelt runoff. Concentrations of NO3-N varied considerably among watersheds and are related to DOC:DON ratios in stream- water. Annual DIN exports were negatively correlated with streamwater DOC:DON ratios, indicating that these ratios might be a useful index of N status of upland forests.

232 citations

Journal ArticleDOI
24 Jul 2014-Nature
TL;DR: The estimate of about 160 petagrams of Holocene organic carbon in deep lake basins of Siberia and Alaska increases the circumpolar peat carbon pool estimate for permafrost regions by over 50 per cent and potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene.
Abstract: Observations and modelling show that the deep thermokarst lakes that formed in Siberia and Alaska when the permafrost warmed in the Holocene epoch changed from climate-warming methane sources to climate-cooling carbon sinks about 5,000 years ago. With the onset of a warmer Holocene climate, permafrost degradation gave rise to numerous thermokarst lakes — lakes formed when meltwater accumulates in surface depressions over thawing permafrost — across large regions of Siberia, Alaska and northern Canada. These lakes are commonly regarded as a net source of atmospheric methane and carbon dioxide due to organic matter degradation. But the question arises, can carbon taken up by these lakes in the form of organic matter accumulation offset their greenhouse gas emissions? This study finds that carbon accumulation in deep thermokarst lake sediments increases the circumpolar peat carbon pool estimate for permafrost regions by more than half, and is larger than the mass of Pleistocene-aged permafrost carbon released as greenhouses gases when the lakes first formed. The authors suggest that thermokarst basins switched from a net radiative warming to a net cooling climate effect about 5,000 years ago. Thermokarst lakes formed across vast regions of Siberia and Alaska during the last deglaciation and are thought to be a net source of atmospheric methane and carbon dioxide during the Holocene epoch1,2,3,4. However, the same thermokarst lakes can also sequester carbon5, and it remains uncertain whether carbon uptake by thermokarst lakes can offset their greenhouse gas emissions. Here we use field observations of Siberian permafrost exposures, radiocarbon dating and spatial analyses to quantify Holocene carbon stocks and fluxes in lake sediments overlying thawed Pleistocene-aged permafrost. We find that carbon accumulation in deep thermokarst-lake sediments since the last deglaciation is about 1.6 times larger than the mass of Pleistocene-aged permafrost carbon released as greenhouse gases when the lakes first formed. Although methane and carbon dioxide emissions following thaw lead to immediate radiative warming, carbon uptake in peat-rich sediments occurs over millennial timescales. We assess thermokarst-lake carbon feedbacks to climate with an atmospheric perturbation model and find that thermokarst basins switched from a net radiative warming to a net cooling climate effect about 5,000 years ago. High rates of Holocene carbon accumulation in 20 lake sediments (47 ± 10 grams of carbon per square metre per year; mean ± standard error) were driven by thermokarst erosion and deposition of terrestrial organic matter, by nutrient release from thawing permafrost that stimulated lake productivity and by slow decomposition in cold, anoxic lake bottoms. When lakes eventually drained, permafrost formation rapidly sequestered sediment carbon. Our estimate of about 160 petagrams of Holocene organic carbon in deep lake basins of Siberia and Alaska increases the circumpolar peat carbon pool estimate for permafrost regions by over 50 per cent (ref. 6). The carbon in perennially frozen drained lake sediments may become vulnerable to mineralization as permafrost disappears7,8,9, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene.

232 citations


Authors

Showing all 9489 results

NameH-indexPapersCitations
Derek R. Lovley16858295315
Peter B. Reich159790110377
Jerry M. Melillo13438368894
Katja Klein129149987817
David Finkelhor11738258094
Howard A. Stone114103364855
James O. Hill11353269636
Tadayuki Takahashi11293257501
Howard Eichenbaum10827944172
John D. Aber10720448500
Andrew W. Strong9956342475
Charles T. Driscoll9755437355
Andrew D. Richardson9428232850
Colin A. Chapman9249128217
Nicholas W. Lukacs9136734057
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Performance
Metrics
No. of papers from the Institution in previous years
YearPapers
202351
2022183
20211,148
20201,128
20191,140
20181,089