Jones GA, Henry GHR. 2003. Primary plant succession on recently deglaciated terrain in the Canadian High Arctic. Journal of Biogeography 30: 277-296.
These authors examined five glacial foregrounds on Ellesmere Island, one intensively and the other 4 “extensively”, to determine patterns of succession among plant communities on sterile ground. The ecological literature recognizes several different modes of succession, including a categorization by Henry and Svoboda (1987) based on the relative strengths of biotic and abiotic factors. This model of succession recognizes 3 modes; directional succession with species replacement, directional succession without species replacement, and non-directional succession without replacement. They are arranged in increasing importance of abiotic factors, referred to here as “environmental resistance”, which operates in opposition to “biological driving forces”.
In temperate regions, where much of the relevant ecological theory has been developed, biotic factors are mainly competition. In the High Arctic, a successional pattern consistent with directional-with-replacement was found, demonstrating that this can occur even in environments with obviously severe abiotic factors. However, these authors argue that the biotic factor driving this succession was probably not competition, because total plant cover remains below 10% by area even at the fourth stage recognized here, and species richness is always very low. The polar oasis landscape with 80-100% plant cover was never reached within the approximately 50-year old glacial forelands examined by these authors, though it is likely that competition is important in that “stage 5” level of High Arctic succession.
Other biotic variables suggested playing a role in successional dynamics in the High Arctic included facilitation and life-history characteristics. These factors are not independent; later successional species such as Salix arctica appear not to be able to establish until soil fertility has been improved by mats of very-early-colonizing mosses, and are long-lived, slow-growing species that contribute little to the early seed bank and seed rain. Thus, multiple plant and environmental characteristics appear to interact when structuring early communities.
I read this paper to try to gain some understanding of ecological succession and the role of time-since-deglaciation among the ecosystems of Alexandra Fjord. Rather than being distinct successional stages in sequence as I had previously supposed, it appears the various lowland ecosystems are all of a similar age, and have different vegetation communities as a result of other factors besides simply relative proximity to the Twin Glacier. Dryas integrefolia and Cassiope tetragona were important parts of this study, and both appear in stage 4, after primary-colonizing mosses, and early-colonizing forbs such as Papaver radicatum and early-colonizing deciduous shrubs like Saxifraga spp. Both Dryas and Cassiope form associations with mycorrhyzal fungi, a requirement that may slow their colonization of novel habitats; earlier-spreading plants do not form these associations, and instead may be limited by seed dispersal.
This was helpful in organizing the structure of the manuscript I am currently working on, which will describe some of the soil biotic communities both in the Alexandra Fjord lowlands and in the adjacent polar desert. It is not a simple story of succession from one ecosystem to the next, but succession does play a role.
Showing posts with label Alexandra Fjord. Show all posts
Showing posts with label Alexandra Fjord. Show all posts
Monday, November 2, 2009
Tuesday, March 17, 2009
Staal et al. 2001
Staal M, te Lintel-Hekkert S, Harren F, Stal L. 2001. Nitrogenase activity in cyanobacteria measured by the acetylene reduction assay: a comparison between batch incubation and on-line monitoring. Environmental Microbiology 3(5): 343-351.
These authors present two methods for measuring nitrogenase activity in cyanobacteria, both based on continuous on-line measurement of ethylene produced by the reduction of acetylene by nitrogenase. One method relies on a gas chromatograph to detect ethylene, the other on a not-yet-commercially available laser system. Nitrogenase normally reduces N2 to NH3, but will also reduce other triple bonds such as that between the carbon atoms in acetylene, hence this measurement assay was developed in the late 1960s. The nitrogenase enzyme is inhibited by oxygen, but is very energy-intensive when reducing N2, thus cyanobacteria may fix Nitrogen in a manner dependent upon but separated from photosynthesis in either (at night) time or space (specialized cells).
Previous ethylene-based methods were based on incubations of cells in air-tight containers, for incubation periods sufficient to saturate nitrogenase with acetylene and accumulate sufficient ethylene for detection. Changes in O2 and CO2 concentrations during these hours-long incubations introduce conflating variables; O2 is depleted and CO2 accumulates in the dark, vice-versa in the light. CO2 concentration affects pH, while O2 inhibits nitrogenase and indirectly relates to available energy. In addition, long incubations will fail to detect any event occuring on a frequency shorter than the incubation time, such that processes occurring on time scales of seconds to minutes will not be registered. Finally, saturation of nitrogenase with acetylene eventually leads to nitrogen starvation and the synthesis of more nitrogenase.
In contrast, on-line methods involve the continuous flow of gas over the sample. This can be used to measure gas flux only when the system reaches a steady state (however, see the discussion of steady-state and non-steady-state modes in gas sampling chambers in Davidson et al., 2002). For nitrogenase-ethylene, this steady state may be reached as quickly as 1 minute under ideal, high-surface-area conditions. In addition, while O2 and CO2 concentrations are controlled during on-line measurement, ethylene cannot accumulate, thus only very low concentrations will be present.
Nitrogenase activity was higher under light-saturation conditions than in the dark, but became inhibitory with longer incubation times. Changes in nitrogenase activity with time and light level probably relate to both energy limitation in the dark and oxygen inhibition in the light. Changes relating to growth, internal rythms, or energy depletion only occurred after very long incubations, such as 24 hours. I am not certain how long-term energy depletion is distinct from short term energy limitation in the dark.
This paper suggests it may be possible for us to measure nitrogenase activity with acetylene and ethylene using the Gasmet FTIR system and its chambers.
These authors present two methods for measuring nitrogenase activity in cyanobacteria, both based on continuous on-line measurement of ethylene produced by the reduction of acetylene by nitrogenase. One method relies on a gas chromatograph to detect ethylene, the other on a not-yet-commercially available laser system. Nitrogenase normally reduces N2 to NH3, but will also reduce other triple bonds such as that between the carbon atoms in acetylene, hence this measurement assay was developed in the late 1960s. The nitrogenase enzyme is inhibited by oxygen, but is very energy-intensive when reducing N2, thus cyanobacteria may fix Nitrogen in a manner dependent upon but separated from photosynthesis in either (at night) time or space (specialized cells).
Previous ethylene-based methods were based on incubations of cells in air-tight containers, for incubation periods sufficient to saturate nitrogenase with acetylene and accumulate sufficient ethylene for detection. Changes in O2 and CO2 concentrations during these hours-long incubations introduce conflating variables; O2 is depleted and CO2 accumulates in the dark, vice-versa in the light. CO2 concentration affects pH, while O2 inhibits nitrogenase and indirectly relates to available energy. In addition, long incubations will fail to detect any event occuring on a frequency shorter than the incubation time, such that processes occurring on time scales of seconds to minutes will not be registered. Finally, saturation of nitrogenase with acetylene eventually leads to nitrogen starvation and the synthesis of more nitrogenase.
In contrast, on-line methods involve the continuous flow of gas over the sample. This can be used to measure gas flux only when the system reaches a steady state (however, see the discussion of steady-state and non-steady-state modes in gas sampling chambers in Davidson et al., 2002). For nitrogenase-ethylene, this steady state may be reached as quickly as 1 minute under ideal, high-surface-area conditions. In addition, while O2 and CO2 concentrations are controlled during on-line measurement, ethylene cannot accumulate, thus only very low concentrations will be present.
Nitrogenase activity was higher under light-saturation conditions than in the dark, but became inhibitory with longer incubation times. Changes in nitrogenase activity with time and light level probably relate to both energy limitation in the dark and oxygen inhibition in the light. Changes relating to growth, internal rythms, or energy depletion only occurred after very long incubations, such as 24 hours. I am not certain how long-term energy depletion is distinct from short term energy limitation in the dark.
This paper suggests it may be possible for us to measure nitrogenase activity with acetylene and ethylene using the Gasmet FTIR system and its chambers.
Marion et al. 1997
Marion GM, Henry GHR, Freckman DW, Johnstone J, Jones G, Jones MH, Levesque E, Molau U, Molgaard P, Parsons AN, Svoboda J, Virginia RA. 1997. Open-top designs for mainpulating field temperature in high-latitude ecosystems. Global Change Biology 3 (suppl. 1): 20-32.
These authors evaluated 4 different chamber designs under field conditions, examining many variables but focusing on temperature differences between the inside and outside of the chambers, and unintended ecological effects. The four designs were termed “ITEX corners”, “cone chambers”, “hexagon chambers”, and “plastic tent”. There were 6 field sites, 5 in the Arctic from Sweden to Canada, and 1 in Antarctica.
This paper represents one of the outcomes of a meeting that established the International Tundra Experiment (ITEX); at this meeting a list of requirements for long-term temperature manipulation devices was constructed, leading to these 4 designs and a requirement to measure ecological effects such as changes in snow accumulation or melting.
The results were fairly consistent across chamber designs. In general, open-top chambers cause fewer and less severe ecological side-effects than closed designs, but warm the surface of the soil by 1-2 degrees compared with up to 10 or 15 degrees for some closed designs. Side-effects of the open-top chambers included some shading and interception of PAR by the chamber materials, changes in moisture concentrations in the air immediately above the soil surface (though these may have been driven by changes in temperature), and the possibility of interference from animals, such as nutrient addition by birds perching on the chambers. However, CO2 concentrations were not affected by chambers.
One of these authors, GHR Henry, will be working with me this summer at Alexandra Fjord; this was also one of the study sites in this paper.
These authors evaluated 4 different chamber designs under field conditions, examining many variables but focusing on temperature differences between the inside and outside of the chambers, and unintended ecological effects. The four designs were termed “ITEX corners”, “cone chambers”, “hexagon chambers”, and “plastic tent”. There were 6 field sites, 5 in the Arctic from Sweden to Canada, and 1 in Antarctica.
This paper represents one of the outcomes of a meeting that established the International Tundra Experiment (ITEX); at this meeting a list of requirements for long-term temperature manipulation devices was constructed, leading to these 4 designs and a requirement to measure ecological effects such as changes in snow accumulation or melting.
The results were fairly consistent across chamber designs. In general, open-top chambers cause fewer and less severe ecological side-effects than closed designs, but warm the surface of the soil by 1-2 degrees compared with up to 10 or 15 degrees for some closed designs. Side-effects of the open-top chambers included some shading and interception of PAR by the chamber materials, changes in moisture concentrations in the air immediately above the soil surface (though these may have been driven by changes in temperature), and the possibility of interference from animals, such as nutrient addition by birds perching on the chambers. However, CO2 concentrations were not affected by chambers.
One of these authors, GHR Henry, will be working with me this summer at Alexandra Fjord; this was also one of the study sites in this paper.
Mastepanov et al. 2008
Mastepanov M, Sigsgaard C, Dlugokencky EJ, Houweling S, Strom L, Tamstorf MP, Christensen TR. 2008. Large tundra methane burst during onset of freezing. Nature 456: 628-631.
These authors describe a large emission of methane from a wet tundra site in the Greenland High Arctic, which occurred in late autumn and early winter as the ground froze. This burst of methane emission is of a similar magnitude to the total methane emission from this site during the growing season, and accounts for the previously observed “shoulder” of methane in autumn at high altitudes.
The site is Zackenberg Valley, in the north-east of Greenland at about 74ºN latitude. This site appears to be broadly similar to other High Arctic tundra meadows such as Truelove Lowlands (Devon Island) and Alexandra Fjord (Ellesmere Island) and large parts of northern Russia, with an active layer 20 to 100 cm thick. The growing season measurements here were similar to previous years, and similar to another study in Siberia.
These measurements were made using an automated methane-only laser-based system, that took readings of methane flux every hour, with a data-gathering time of 1 second. Late-season pulses of methane were not observed at lower-latitude sites, possibly because a deeper permafrost “floor” allows methane to diffuse down to deeper soil layers rather than being forced upwards. Spatial and temporal variablity of the freezing methane emissions were very high, suggesting the diffusion paths of methane squeezed out of the soil were dependent upon plant root systems and similar structures.
This paper is the reason my field season at Alexandra Fjord in 2009 may extend as late as August 25 (planned) or September 10 (worst-case scenario). The figures in this paper imply the difference between measured and modeled methane emissions became apparent approximately in late August.
These authors describe a large emission of methane from a wet tundra site in the Greenland High Arctic, which occurred in late autumn and early winter as the ground froze. This burst of methane emission is of a similar magnitude to the total methane emission from this site during the growing season, and accounts for the previously observed “shoulder” of methane in autumn at high altitudes.
The site is Zackenberg Valley, in the north-east of Greenland at about 74ºN latitude. This site appears to be broadly similar to other High Arctic tundra meadows such as Truelove Lowlands (Devon Island) and Alexandra Fjord (Ellesmere Island) and large parts of northern Russia, with an active layer 20 to 100 cm thick. The growing season measurements here were similar to previous years, and similar to another study in Siberia.
These measurements were made using an automated methane-only laser-based system, that took readings of methane flux every hour, with a data-gathering time of 1 second. Late-season pulses of methane were not observed at lower-latitude sites, possibly because a deeper permafrost “floor” allows methane to diffuse down to deeper soil layers rather than being forced upwards. Spatial and temporal variablity of the freezing methane emissions were very high, suggesting the diffusion paths of methane squeezed out of the soil were dependent upon plant root systems and similar structures.
This paper is the reason my field season at Alexandra Fjord in 2009 may extend as late as August 25 (planned) or September 10 (worst-case scenario). The figures in this paper imply the difference between measured and modeled methane emissions became apparent approximately in late August.
Holtan-Hartwig et al. 2002
Holtan-Hartwig L, Dörsch P, Bakken LR. 2002. Low temperature control of soil denitrifying communities: kinetics of N2O production and reduction. Soil Biology & Biochemistry 34: 1797-1806.
These authors measured the activation energies of N2O production and reduction in soils taken from agricultural settings in Finland, Sweden, and Germany. The underlying observation is that temperate soils show an unexpectedly large emission profile of N2O in late winter and early spring. Other authors have attributed this release to freeze-thaw effects, such as release of N2O trapped in frozen soils. Differences in activation energies could also explain these observations if these activation energies are asymmetrical at low temperatures, such that the activation energy of N2O reduction is much higher than that for N2O production.
N2O is both a greenhouse gas and an ozone-layer depleter. From soils, it is produced in the penultimate step in a series of reactions known collectively as the denitrification pathway: these reactions when run to completion convert nitrate (NO3-) to N2. N2 of course represents a net loss of nitrogen from an ecosystem, since it is no longer available to organisms. However, N2 is utterly harmless, while N2O has important physical effects on the atmosphere. The basic biochemistry and temperature response of this pathway is described in Firestone (1982).
Another underlying observation for this study is that the product ratio of N2O/N2 increases with decreasing temperature; in other words, proportionally more N2O is released from the system compared to N2. If N2O reduction (to N2) has a higher activation energy than N2O production (from NO), this would explain this observation. Another possibility is that the enzyme responsible for N2O reduction is strongly inhibited at some critical low temperature threshold.
The laboratory analyses carried out for this paper are strongly divergent from field conditions, involving anaerobic slurries with an excess of electron acceptors and the removal of NO3-. However, the differences observed between the different soils support other conclusions that N2O emissions from soils varies strongly with soil types and soil sources.
Previously reported activation energies for NO3- loss by denitrification range between about 41 and 89 kJ/mol; a similar range of activation energies was found here for both N2O production and reduction. This suggests that asymmetrical activation energies are not driving the observed changes in N2O flux by season. It seems temperatures close to 0ºC represent a particular challenge to the microbial communities of these soils, but the nature of this challenge remains unclear – this study did not examine community dynamics in any detail. Some details of the methods of preparing soils used here may be important in this regard.
Another possibility, not mutually exclusive with this threshold effect, is that strong decreases in metabolic rates at low temperatures (60-70% per 10ºC) combined with weak decreases in N2O diffusion rates (20-25% per 10ºC in water) allow N2O to escape the biological pathway as temperatures approach zero.
This paper was recommended to me by Dr. Steven Siciliano, as a guide to some of the calculations and comparisons we are doing with soils and greenhouse gases (including N2O) from Alexandra Fjord.
These authors measured the activation energies of N2O production and reduction in soils taken from agricultural settings in Finland, Sweden, and Germany. The underlying observation is that temperate soils show an unexpectedly large emission profile of N2O in late winter and early spring. Other authors have attributed this release to freeze-thaw effects, such as release of N2O trapped in frozen soils. Differences in activation energies could also explain these observations if these activation energies are asymmetrical at low temperatures, such that the activation energy of N2O reduction is much higher than that for N2O production.
N2O is both a greenhouse gas and an ozone-layer depleter. From soils, it is produced in the penultimate step in a series of reactions known collectively as the denitrification pathway: these reactions when run to completion convert nitrate (NO3-) to N2. N2 of course represents a net loss of nitrogen from an ecosystem, since it is no longer available to organisms. However, N2 is utterly harmless, while N2O has important physical effects on the atmosphere. The basic biochemistry and temperature response of this pathway is described in Firestone (1982).
Another underlying observation for this study is that the product ratio of N2O/N2 increases with decreasing temperature; in other words, proportionally more N2O is released from the system compared to N2. If N2O reduction (to N2) has a higher activation energy than N2O production (from NO), this would explain this observation. Another possibility is that the enzyme responsible for N2O reduction is strongly inhibited at some critical low temperature threshold.
The laboratory analyses carried out for this paper are strongly divergent from field conditions, involving anaerobic slurries with an excess of electron acceptors and the removal of NO3-. However, the differences observed between the different soils support other conclusions that N2O emissions from soils varies strongly with soil types and soil sources.
Previously reported activation energies for NO3- loss by denitrification range between about 41 and 89 kJ/mol; a similar range of activation energies was found here for both N2O production and reduction. This suggests that asymmetrical activation energies are not driving the observed changes in N2O flux by season. It seems temperatures close to 0ºC represent a particular challenge to the microbial communities of these soils, but the nature of this challenge remains unclear – this study did not examine community dynamics in any detail. Some details of the methods of preparing soils used here may be important in this regard.
Another possibility, not mutually exclusive with this threshold effect, is that strong decreases in metabolic rates at low temperatures (60-70% per 10ºC) combined with weak decreases in N2O diffusion rates (20-25% per 10ºC in water) allow N2O to escape the biological pathway as temperatures approach zero.
This paper was recommended to me by Dr. Steven Siciliano, as a guide to some of the calculations and comparisons we are doing with soils and greenhouse gases (including N2O) from Alexandra Fjord.
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