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John D. Aber - One of the best experts on this subject based on the ideXlab platform.
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modeling nitrogen saturation in Forest Ecosystems in response to land use and atmospheric deposition
Ecological Modelling, 1997Co-Authors: John D. Aber, Scott V Ollinger, Charles T DriscollAbstract:Abstract A generalized, lumped-parameter model of carbon (C), water, and nitrogen (N) interactions in Forest Ecosystems (PnET-CN) is presented. The model operates at a monthly time step and at the stand-to-watershed scale, and is validated against data on annual net primary productivity, monthly carbon and water balances, annual net N mineralization, nitrification, foliar N concentration and annual and monthly N leaching losses for two sites, Hubbard Brook (West Thornton, NH) and Harvard Forest (Petersham, MA). It is then used to predict transient responses in function resulting from changes in land use and N deposition, as well as the maximum rate of N cycling which can be sustained for any given combination of site, climate and species. Model predictions suggest a very long legacy effect of land use history on N cycling. Even with only one ‘active’ soil organic matter pool, complete recovery from three modest harvests at Hubbard Brook is predicted to require more than two centuries at current N deposition rates. Complete recovery is predicted to take even longer at the Harvard Forest where biomass removals have been more intense. PnET-CN is used to predict maximum sustainable rates of N cycling for 14 sites throughout the northeastern USA. Predicted maximum values were higher, as expected, than measured N mineralization rates for all but one site. The measured fraction of N mineralization nitrified at these 14 sites showed a general relationship with the ratio of measured to maximum net N mineralization. This latter ratio is discussed as a potentially useful indicator of the degree of nitrogen saturation in Forest Ecosystems. A regional map of predicted maximum N cycling rates is presented based on regressions between model predictions and summary climatic variables.
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Nitrogen cycling and nitrogen saturation in temperate Forest Ecosystems.
Trends in ecology & evolution, 1992Co-Authors: John D. AberAbstract:Abstract The last decade has seen a dramatic shift in the focus of nitrogen cycling research in Forest Ecosystems. Concerns over nitrogen deficiencies and effects of removal in harvest have given way to concerns over excess nitrogen availability and the potential for Forest decline and surface water pollution. Driving this paradigm shift is the increase in atmospheric deposition of nitrogen to Forests due to industrial and agricultural activity. At the core of the new paradigm is the concept of ‘nitrogen saturation' of Forest Ecosystems. The purpose of this review is to synthesize recent advances in research relating to nitrogen deposition effects on temperate zone Forest Ecosystems, and the further effects of nitrogen saturation on environmental quality.
Mia J Tegner - One of the best experts on this subject based on the ideXlab platform.
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Kelp Forest Ecosystems: Biodiversity, stability, resilience and future
Environmental Conservation, 2002Co-Authors: Robert S Steneck, Bruce J. Bourque, Debbie Corbett, Jon M. Erlandson, Michael H. Graham, James A. Estes, Mia J TegnerAbstract:Kelp Forests are phyletically diverse, structurally complex and highly productive components of coldwater rocky marine coastlines. This paper reviews the conditions in which kelp Forests develop globally and where, why and at what rate they become deForested. The ecology and long archaeological history of kelp Forests are examined through case studies from southern California, the Aleutian Islands and the western North Atlantic, well-studied locations that represent the widest possible range in kelp Forest biodiversity. Global distribution of kelp Forests is physiologically constrained by light at high latitudes and by nutrients, warm temperatures and other macrophytes at low latitudes. Within mid-latitude belts (roughly 40–60° latitude in both hemispheres) well-developed kelp Forests are most threatened by herbivory, usually from sea urchins. Overfishing and extirpation of highly valued vertebrate apex predators often triggered herbivore population increases, leading to widespread kelp deForestation. Such deForestations have the most profound and lasting impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Globally urchin-induced deForestation has been increasing over the past 2–3 decades. Continued fishing down of coastal food webs has resulted in shifting harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp Forests have returned in some locations but, for the first time, these Forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and they have become the new apex predator in this system. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. Three North American case studies of kelp Forests were examined to determine their long history with humans and project the status of future kelp Forests to the year 2025. Fishing impacts on kelp Forest systems have been both profound and much longer in duration than previously thought. Archaeological data suggest that coastal peoples exploited kelp Forest organisms for thousands of years, occasionally resulting in localized losses of apex predators, outbreaks of sea urchin populations and probably small-scale deForestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The large-scale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp Forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp Forests, widespread deForestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp Forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impacts on kelp Forest Ecosystems and efforts to control the emission of greenhouse gasses should be a global priority. However, overfishing appears to be the greatest manageable threat to kelp Forest Ecosystems over the 2025 time horizon. Management should focus on minimizing fishing impacts and restoring populations of functionally important species in these systems.
Per Gundersen - One of the best experts on this subject based on the ideXlab platform.
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influence of different tree harvesting intensities on Forest soil carbon stocks in boreal and northern temperate Forest Ecosystems
Forest Ecology and Management, 2015Co-Authors: Nicholas Clarke, Per Gundersen, Ulrika Jonssonbelyazid, Janne O Kjonaas, Tryggve Persson, Bjarni D Sigurdsson, Inge Stupak, Lars VesterdalAbstract:Effective Forest governance measures are crucial to ensure sustainable management of Forests, but so far there has been little specific focus in boreal and northern temperate Forests on governance measures in relation to management effects, including harvesting effects, on soil organic carbon (SOC) stocks. This paper reviews the findings in the scientific literature concerning the effects of harvesting of different intensities on SOC stocks and fluxes in boreal and northern temperate Forest Ecosystems to evaluate the evidence for significant SOC losses following biomass removal. An overview of existing governance measures related to SOC is given, followed by a discussion on how scientific findings could be incorporated in guidelines and other governance measures. The currently available information does not support firm conclusions about the long-term impact of intensified Forest harvesting on SOC stocks in boreal and northern temperate Forest Ecosystems, which is in any case species-, site- and practice-specific. Properly conducted long-term experiments are therefore necessary to enable us to clarify the relative importance of different harvesting practices on the SOC stores, the key processes involved, and under which conditions the size of the removals becomes critical. At present, the uncertainty gap between the scientific results and the need for practically useable management guidelines and other governance measures might be bridged by expert opinions given to authorities and certification bodies.
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atmospheric deposition and leaching of nitrogen in chinese Forest Ecosystems
Journal of Forest Research, 2011Co-Authors: Per Gundersen, Yunting Fang, Rolf D Vogt, Keisuke Koba, Fusheng Chen, Xi Yun ChenAbstract:Data have been compiled from published sources on nitrogen (N) fluxes in precipitation, throughfall, and leaching from 69 Forest Ecosystems at 50 sites throughout China, to examine at a national level: (1) N input in precipitation and throughfall, (2) how precipitation N changes after the interaction with canopy, and (3) whether N leaching increases with increasing N deposition and, if so, to what extent. The deposition of dissolved inorganic N (DIN) in precipitation ranged from 2.6 to 48.2 kg N ha−1 year−1, with an average of 16.6 kg N ha−1 year−1. Ammonium was the dominant form of N at most sites, accounting for, on average, 63% of total inorganic N deposition. Nitrate accounted for the remaining 37%. On average, DIN fluxes increased through Forest canopies, by 40% and 34% in broad-leaved and coniferous Forests, respectively. No significant difference in throughfall DIN inputs was found between the two Forest types. Overall, 22% of the throughfall DIN input was leached from Forest Ecosystems in China, which is lower than the 50–59% observed for European Forests. Simple calculations indicate that Chinese Forests have great potential to absorb carbon dioxide from the atmosphere, because of the large Forest area and high N deposition.
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nitrate leaching in Forest Ecosystems is related to Forest floor c n ratios
Environmental Pollution, 1998Co-Authors: Per Gundersen, Ingeborg Callesen, W De VriesAbstract:Abstract Relationships between nitrogen (N) output with seepage water and Forest floor C N ratios were analysed by use of three independent datasets: (i) a compilation of input-output studies in temperate Forest Ecosystems in Europe; (ii) a seven-year national Danish survey of nitrate concentrations in Forest soils; and (iii) a similar one year Dutch survey. Nitrate leaching and nitrate concentrations were negatively correlated with Forest floor C N ratios in all three datasets, though the correlation was weak in the Dutch dataset. Sites with a C N ratio below 25 leached nitrate or had elevated nitrate concentration in the three datasets. Nitrate was not present in the subsoil at sites with C N ratios above 30 in the European and Danish data. In the less intensively monitored Dutch Forest soils nitrate concentrations at C N ratios above 30 were variable. Forest floor C N ratios may be used to assess risk for nitrate leaching in conifer stands using >30, 25 to 30, and
Charles T Driscoll - One of the best experts on this subject based on the ideXlab platform.
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Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory
Ecosystems, 2003Co-Authors: Benjamin Z. Houlton, Charles T Driscoll, Timothy J Fahey, Peter M Groffman, Gene E. Likens, Emily S. Bernhardt, Donald C. BusoAbstract:Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in Forest Ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone Forest Ecosystems. Prior to the ice storm, fluxes of nitrate (NO_3 ^−) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO_3 ^− concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO_3 ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO_3 ^-. At the regional scale, we observed high variability in the magnitude of NO_3 ^- losses: while six of the surveyed watersheds showed accelerated rates of NO_3 ^− loss, three did not. Moreover, in contrast to the strong linear relationship between NO_3 ^− loss and crown damage within HBEF watersheds [ r ^2: (W1 = 0.91, W6 = 0.85)], stream water NO_3 ^− concentrations were weakly related to crown damage ( r ^2 = 0.17) across our regional sites. The efflux of NO_3 ^− associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO_3 ^− fluxes associated with Forest harvesting. Because over one half of the entire year’s worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone Forest Ecosystems.
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mycorrhizal weathering of apatite as an important calcium source in base poor Forest Ecosystems
Nature, 2002Co-Authors: Joel D Blum, Charles T Driscoll, Timothy J Fahey, Andrea Klaue, Carmen A Nezat, Chris E Johnson, Thomas G Siccama, Christopher Eagar, Gene E. LikensAbstract:The depletion of calcium in Forest Ecosystems of the northeastern USA is thought to be a consequence of acidic deposition and to be at present restricting the recovery of Forest and aquatic systems now that acidic deposition itself is declining. This depletion of calcium has been inferred from studies showing that sources of calcium in Forest Ecosystems namely, atmospheric deposition and mineral weathering of silicate rocks such as plagioclase, a calcium-sodium silicate do not match calcium outputs observed in Forest streams. It is therefore thought that calcium is being lost from exchangeable and organically bound calcium in Forest soils. Here we investigate the sources of calcium in the Hubbard Brook experimental Forest, through analysis of calcium and strontium abundances and strontium isotope ratios within various soil, vegetation and hydrological pools. We show that the dissolution of apatite (calcium phosphate) represents a source of calcium that is comparable in size to known inputs from atmospheric sources and silicate weathering. Moreover, apatite-derived calcium was utilized largely by ectomycorrhizal tree species, suggesting that mycorrhizae may weather apatite and absorb the released ions directly, without the ions entering the exchangeable soil pool. Therefore, it seems that apatite weathering can compensate for some of the calcium lost from base-poor Ecosystems, and should be considered when estimating soil acidification impacts and calcium cycling.
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modeling nitrogen saturation in Forest Ecosystems in response to land use and atmospheric deposition
Ecological Modelling, 1997Co-Authors: John D. Aber, Scott V Ollinger, Charles T DriscollAbstract:Abstract A generalized, lumped-parameter model of carbon (C), water, and nitrogen (N) interactions in Forest Ecosystems (PnET-CN) is presented. The model operates at a monthly time step and at the stand-to-watershed scale, and is validated against data on annual net primary productivity, monthly carbon and water balances, annual net N mineralization, nitrification, foliar N concentration and annual and monthly N leaching losses for two sites, Hubbard Brook (West Thornton, NH) and Harvard Forest (Petersham, MA). It is then used to predict transient responses in function resulting from changes in land use and N deposition, as well as the maximum rate of N cycling which can be sustained for any given combination of site, climate and species. Model predictions suggest a very long legacy effect of land use history on N cycling. Even with only one ‘active’ soil organic matter pool, complete recovery from three modest harvests at Hubbard Brook is predicted to require more than two centuries at current N deposition rates. Complete recovery is predicted to take even longer at the Harvard Forest where biomass removals have been more intense. PnET-CN is used to predict maximum sustainable rates of N cycling for 14 sites throughout the northeastern USA. Predicted maximum values were higher, as expected, than measured N mineralization rates for all but one site. The measured fraction of N mineralization nitrified at these 14 sites showed a general relationship with the ratio of measured to maximum net N mineralization. This latter ratio is discussed as a potentially useful indicator of the degree of nitrogen saturation in Forest Ecosystems. A regional map of predicted maximum N cycling rates is presented based on regressions between model predictions and summary climatic variables.
Robert S Steneck - One of the best experts on this subject based on the ideXlab platform.
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Kelp Forest Ecosystems: Biodiversity, stability, resilience and future
Environmental Conservation, 2002Co-Authors: Robert S Steneck, Bruce J. Bourque, Debbie Corbett, Jon M. Erlandson, Michael H. Graham, James A. Estes, Mia J TegnerAbstract:Kelp Forests are phyletically diverse, structurally complex and highly productive components of coldwater rocky marine coastlines. This paper reviews the conditions in which kelp Forests develop globally and where, why and at what rate they become deForested. The ecology and long archaeological history of kelp Forests are examined through case studies from southern California, the Aleutian Islands and the western North Atlantic, well-studied locations that represent the widest possible range in kelp Forest biodiversity. Global distribution of kelp Forests is physiologically constrained by light at high latitudes and by nutrients, warm temperatures and other macrophytes at low latitudes. Within mid-latitude belts (roughly 40–60° latitude in both hemispheres) well-developed kelp Forests are most threatened by herbivory, usually from sea urchins. Overfishing and extirpation of highly valued vertebrate apex predators often triggered herbivore population increases, leading to widespread kelp deForestation. Such deForestations have the most profound and lasting impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Globally urchin-induced deForestation has been increasing over the past 2–3 decades. Continued fishing down of coastal food webs has resulted in shifting harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp Forests have returned in some locations but, for the first time, these Forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and they have become the new apex predator in this system. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. Three North American case studies of kelp Forests were examined to determine their long history with humans and project the status of future kelp Forests to the year 2025. Fishing impacts on kelp Forest systems have been both profound and much longer in duration than previously thought. Archaeological data suggest that coastal peoples exploited kelp Forest organisms for thousands of years, occasionally resulting in localized losses of apex predators, outbreaks of sea urchin populations and probably small-scale deForestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The large-scale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp Forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp Forests, widespread deForestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp Forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impacts on kelp Forest Ecosystems and efforts to control the emission of greenhouse gasses should be a global priority. However, overfishing appears to be the greatest manageable threat to kelp Forest Ecosystems over the 2025 time horizon. Management should focus on minimizing fishing impacts and restoring populations of functionally important species in these systems.