Angel RA, Conrad R. 2009. In situ measurement of methane fluxes and analysis of transcribed particulate methane monooxygenase in desert soils. Environmental Microbiology 11: 2598-2610.
These authors examined the methanotroph communities of sub-tropical desert soils in Israel, using both field and lab measurements of methane fluxes and molecular investigation of sampled microbes. Negative surface flux, indicating consumption of atmospheric methane by soil, was found only at an undisturbed site in this study; the 4 other sites were varying degrees of agricultural and did not show clear patterns of consumption of methane at low concentrations. Addition of water, simulating a typical local rainfall event, eliminated methanotroph activity for about 12 hours, then this activity rebounded to well above background for up to 48 hours. This process of apparent short-term community dynamics was not investigated or discussed in much detail by these authors, though I found it one of the most interesting observations.
Methanotrophs were identified in soil samples by the usual suite of molecular biology methods. One set of primers used here is described as also targeting certain clades of amoA, a gene prominent in ammonia oxidation. These primers were successful at amplifying sequences even from soils in which methanotrophic activity had not been detected, suggesting that many or all of the sequences amplified by these primers were not actually methanotroph sequences, but rather sequences from apparently ubiquitous ammonia oxidizing bacteria.
The target gene for methanotrophs encodes a membrane-bound protein involved in transporting methane into the cytoplasm. From the way some primers also targeted amoA, I think perhaps there is a shared ancestry among the pathways for scavenging environmental ammonia and for scavenging environmental methane, though these authors do not delve into that discussion.
This paper was apparently instrumental in structuring the thoughts of my co-author (Dr. Siciliano) regarding how we should structure the manuscripts we are preparing based on the 2009 Alexandra Fjord field season. Up to this paper, we had been considering including both molecular analysis (based mainly on qPCR) and soil-properties (nutrients, root exudates, moisture, trace gases, etc.) in our nascent “Pits & Probes” manuscript. However, this paper demonstrates the considerable volume of work required to achieve a useful molecular dataset, suggesting that we would be better off saving these DNA data for a subsequent study, where they can be described and analyzed at the appropriate level of detail.
Thursday, November 26, 2009
Thursday, November 12, 2009
Wrage et al. 2004
Wrage N, Lauf J, del Prado A, Pinto M, Pietrzak S, Yamulki S, Oenema O, Gebauer G. 2004. Distinguishing sources of N2O in European grasslands by stable isotope analysis. Rapid Communications in Mass Spectrometry 18: 1201-1207.
These authors used the signature ratios of stable isotopes of oxygen and nitrogen in a range of soil chemicals and microbial metabolic pathways to identify the source of N2O produced in grasslands monitored as part of a long-term greenhouse gas international experiment. There are 3 known pathways to N2O production: nitrification, in which N2O is produced as a by-product of ammonium oxidation to nitrate, nitrifier denitrification, in which nitrifying organisms reduce nitrite to dinitrogen gas via N2O, especially under anaerobic conditions, and denitrification, in which nitrate is reduced to dinitrogen gas via N2O by denitrifying organisms.
Previous studies had often used acetylene to inhibit N2O production, but this has been found to be unreliable. In contrast, the stable isotope approach used here was able to detect both N2O production and consumption even when reservoirs of N2O were very low. Nitrification was the most important N-transforming process found in these systems, with most N2O produced probably from reduction pathways.
These authors used the signature ratios of stable isotopes of oxygen and nitrogen in a range of soil chemicals and microbial metabolic pathways to identify the source of N2O produced in grasslands monitored as part of a long-term greenhouse gas international experiment. There are 3 known pathways to N2O production: nitrification, in which N2O is produced as a by-product of ammonium oxidation to nitrate, nitrifier denitrification, in which nitrifying organisms reduce nitrite to dinitrogen gas via N2O, especially under anaerobic conditions, and denitrification, in which nitrate is reduced to dinitrogen gas via N2O by denitrifying organisms.
Previous studies had often used acetylene to inhibit N2O production, but this has been found to be unreliable. In contrast, the stable isotope approach used here was able to detect both N2O production and consumption even when reservoirs of N2O were very low. Nitrification was the most important N-transforming process found in these systems, with most N2O produced probably from reduction pathways.
Tack et al. 2006
Tack FMG, Van Ranst E, Lievens C, Vandeberghe RE. 2006. Soil solution Cd, Cu and Zn concentrations as affected by short-time drying or wetting: the role of hydrous oxides of Fe and Mn. Geoderma 137: 83-89.
These authors examined the effects of short-term changes in redox conditions on the interaction between trace heavy metals and oxides of iron and manganese in agricultural soils of Flanders. The expectation was that two weeks of water-saturated and hence reducing conditions would allow these iron and manganese oxides to dissolve, and then re-precipitate when soils were returned to oxidizing conditions at field capacity or dry conditions. This process of iron and manganese chemistry would dominate redox conditions in soil solution, and dominate the chemistry of trace metals, controlling the solubilities of the trace metals.
Contrary to expectations, short-term wetting and drying cycles did not push the chemistry of trace metals around to a significant degree. The authors state that they cannot distinguish between the competing explanatory hypotheses of very slow transformations of the expected type or that these processes occurred at the expected rates but reverted during the later stages of the incubations of soils at particular moisture levels.
In the conclusion, these authors describe the importance of periods of drying, where soil moisture levels fall far below field capacity. Unlike cycles of saturation and field capacity, dried soil has much higher oxygen levels, and the moisture shortage has important effects on soil microorganisms, and the local redox conditions in microhabitats. Occasional very dry periods have a disproportionate effect on heavy-metal chemistry in soils.
For my purposes, this paper was most useful for the clear description of the methods used to measure iron oxides in soils, from a distinctly geochemistry perspective. These authors clearly state that measurement of soil levels of “amorphous” and “crystalline” Fe(III)-oxides are based on operational definitions, as the measurements are based on not-entirely-specific dissolutions of particular fractions of soil iron with particular solutions. These dissolution methods are widely used, and it was useful to see the limitations of these methods clearly described.
These authors examined the effects of short-term changes in redox conditions on the interaction between trace heavy metals and oxides of iron and manganese in agricultural soils of Flanders. The expectation was that two weeks of water-saturated and hence reducing conditions would allow these iron and manganese oxides to dissolve, and then re-precipitate when soils were returned to oxidizing conditions at field capacity or dry conditions. This process of iron and manganese chemistry would dominate redox conditions in soil solution, and dominate the chemistry of trace metals, controlling the solubilities of the trace metals.
Contrary to expectations, short-term wetting and drying cycles did not push the chemistry of trace metals around to a significant degree. The authors state that they cannot distinguish between the competing explanatory hypotheses of very slow transformations of the expected type or that these processes occurred at the expected rates but reverted during the later stages of the incubations of soils at particular moisture levels.
In the conclusion, these authors describe the importance of periods of drying, where soil moisture levels fall far below field capacity. Unlike cycles of saturation and field capacity, dried soil has much higher oxygen levels, and the moisture shortage has important effects on soil microorganisms, and the local redox conditions in microhabitats. Occasional very dry periods have a disproportionate effect on heavy-metal chemistry in soils.
For my purposes, this paper was most useful for the clear description of the methods used to measure iron oxides in soils, from a distinctly geochemistry perspective. These authors clearly state that measurement of soil levels of “amorphous” and “crystalline” Fe(III)-oxides are based on operational definitions, as the measurements are based on not-entirely-specific dissolutions of particular fractions of soil iron with particular solutions. These dissolution methods are widely used, and it was useful to see the limitations of these methods clearly described.
Nemergut et al. 2007
Nemergut DR, Anderson SP, Cleveland CC, Martin AP, Miller AE, Seimon A, Schmidt SK. 2007. Microbial community succession in an unvegetated, recently deglaciated soil. Microbial Ecology 53: 110-122.
These authors describe the partly-predictable patterns of succession among the soil microbes of a glacial foreland in Peru. Primary succession on new terrain, as found in front of a receding glacier, has been studied to some extent, especially regarding the vegetation. Studies of the microbial communities have been rarer, but the few that have been conducted have suggested that these communities also show predictable patterns of community assembly and turnover associated with soil age. Basic ecological theory has led to the nitrogen paradigm of primary succession in soils: nitrogen is absent from new mineral substrate, thus nitrogen fixing organisms have a competitive advantage and are therefore abundant. The presence of nitrogen fixers is tightly linked to the accumulation of soil nitrogen; hence these organisms may facilitate later successional stages.
The study site in this paper is in Peru, at a glacier that seems to be receding quickly. These authors sampled from 3 transects arranged parallel to the front of the glacier, located adjacent to the glacier on soil less than a year old, 100m away on soil about 4 years old, and 500m away on soil about 20 years old. This area receives very high inputs of pollen, leading to the hypothesis that heterotrophic, nitrogen-fixing organisms may be present, using the pollen as a carbon source but drawing nitrogen from the atmosphere because the C:N ratio of pollen is higher than that of microbial biomass. Surface soil samples were collected, kept at 0C, and analyzed in Colorado. Much of the analyses were detailed phylogenetic examination, including the P-test of Martin (2002); note that he is one of the authors of this paper. OTU and a range of sequence-data fine-tuning techniques were also employed.
Over the study area, autotrophic nitrogen fixers were abundant. The bacteria found were extremely diverse at the highest taxonomic levels, and many sequences identified were not closely related to existing sequences in public databases. Diversity increased rapidly from the youngest soils to the 4-year-old, then plateaued.
One very interesting group of bacteria found are the Comamodaceae; sequences reported in other studies were in many cases derived from glacial or ice-sheet ice. The Comamodaceae found in the youngest soils here may have persisted as viable populations in the glacier; differences between the two closely-examined youngest communities suggest physical and genetic isolation for hundreds to thousands of years, allowing speciation events to accumulate differences.
Other patterns among the sequences identified suggest that the earliest colonizers of new terrain may be cosmopolitan – some of the Comamodaceae sequences, for example, are similar to those derived from a glacier in Nunavut. Later colonizers may be more endemic, and displace the earliest colonizers as soils age. The trophic status of the first colonizers is not clear; these authors did not have a test for definite autotrophs or heterotrophs, as Comamodaceae are known to include both modes. At the macrobiological level, the earliest arrivals on new terrain are typically heterotrophs, insects that feed on deposited organic matter such as wind-blown pollen.
This paper is very useful to me, describing as it does a complete set of analytical procedures for my planned biogeographic / phylogenetic studies, as well as providing data in the form of publicly-accessible sequences and analyzed information on patterns of soil microbial community assembly.
These authors describe the partly-predictable patterns of succession among the soil microbes of a glacial foreland in Peru. Primary succession on new terrain, as found in front of a receding glacier, has been studied to some extent, especially regarding the vegetation. Studies of the microbial communities have been rarer, but the few that have been conducted have suggested that these communities also show predictable patterns of community assembly and turnover associated with soil age. Basic ecological theory has led to the nitrogen paradigm of primary succession in soils: nitrogen is absent from new mineral substrate, thus nitrogen fixing organisms have a competitive advantage and are therefore abundant. The presence of nitrogen fixers is tightly linked to the accumulation of soil nitrogen; hence these organisms may facilitate later successional stages.
The study site in this paper is in Peru, at a glacier that seems to be receding quickly. These authors sampled from 3 transects arranged parallel to the front of the glacier, located adjacent to the glacier on soil less than a year old, 100m away on soil about 4 years old, and 500m away on soil about 20 years old. This area receives very high inputs of pollen, leading to the hypothesis that heterotrophic, nitrogen-fixing organisms may be present, using the pollen as a carbon source but drawing nitrogen from the atmosphere because the C:N ratio of pollen is higher than that of microbial biomass. Surface soil samples were collected, kept at 0C, and analyzed in Colorado. Much of the analyses were detailed phylogenetic examination, including the P-test of Martin (2002); note that he is one of the authors of this paper. OTU and a range of sequence-data fine-tuning techniques were also employed.
Over the study area, autotrophic nitrogen fixers were abundant. The bacteria found were extremely diverse at the highest taxonomic levels, and many sequences identified were not closely related to existing sequences in public databases. Diversity increased rapidly from the youngest soils to the 4-year-old, then plateaued.
One very interesting group of bacteria found are the Comamodaceae; sequences reported in other studies were in many cases derived from glacial or ice-sheet ice. The Comamodaceae found in the youngest soils here may have persisted as viable populations in the glacier; differences between the two closely-examined youngest communities suggest physical and genetic isolation for hundreds to thousands of years, allowing speciation events to accumulate differences.
Other patterns among the sequences identified suggest that the earliest colonizers of new terrain may be cosmopolitan – some of the Comamodaceae sequences, for example, are similar to those derived from a glacier in Nunavut. Later colonizers may be more endemic, and displace the earliest colonizers as soils age. The trophic status of the first colonizers is not clear; these authors did not have a test for definite autotrophs or heterotrophs, as Comamodaceae are known to include both modes. At the macrobiological level, the earliest arrivals on new terrain are typically heterotrophs, insects that feed on deposited organic matter such as wind-blown pollen.
This paper is very useful to me, describing as it does a complete set of analytical procedures for my planned biogeographic / phylogenetic studies, as well as providing data in the form of publicly-accessible sequences and analyzed information on patterns of soil microbial community assembly.
Monday, November 9, 2009
van Bodegom et al. 2004
van Bodegom PM, Scholten JCM, Stams AJM. 2004. Direct inhibition of methanogenesis by ferric iron. FEMS Microbiology Ecology 49: 261-268.
These authors present the novel finding that inhibition of methanogenesis in anaerobic sediments by Fe(III) is a direct effect, and not the result of competition for resources between different species of microbes. They used three pure strains of methanogenic archaea under controlled-atmosphere conditions, and found a clear signal of methanogenesis inhibition related to the amount of Fe(III) added to liquid culture.
This paper raises some issues I need to consider in the methanogenic, anaerobic soils at some of the systems of Alexandra Fjord, especially regarding redox conditions and available electron acceptors. Additionally, this paper provides references for widely-accepted and apparently highly effective methods for measuring Fe(II) and Fe(III) in soil solutions.
These authors present the novel finding that inhibition of methanogenesis in anaerobic sediments by Fe(III) is a direct effect, and not the result of competition for resources between different species of microbes. They used three pure strains of methanogenic archaea under controlled-atmosphere conditions, and found a clear signal of methanogenesis inhibition related to the amount of Fe(III) added to liquid culture.
This paper raises some issues I need to consider in the methanogenic, anaerobic soils at some of the systems of Alexandra Fjord, especially regarding redox conditions and available electron acceptors. Additionally, this paper provides references for widely-accepted and apparently highly effective methods for measuring Fe(II) and Fe(III) in soil solutions.
Lindsay 1991
Lindsay WL. 1991. Iron oxide solubilization by organic matter and its effect on iron availability. Plant and Soil 130: 27-34.
This author reviews the chemistry of bioavailable iron in soil solutions. The major controls on the availability of iron in soil solution, which is normally very low, are pH, redox status, and the presence of organic matter and microsites where organic matter is decomposed by microorganisms under oxygen-limited conditions.
The usual concentration of iron in solution in soils is extremely low, and its exact value suggests contributions from multiple solid iron species, including amorphous and a range of crystalline forms of Fe(III) oxides. Organic matter produces transient small organic acids as it is decomposed, which complex with iron and help to bring it into solution. However, these organic acids are in the middle of the soil organic matter decomposition pathway, and do not persist for long in soils. Bringing more iron into solution relies on a combination of local reducing conditions around respiring roots and among microbe-and-SOM microsites, and local pH. Plants and other organisms secrete compounds that are effective at solubilizing iron and making it available to plants. The other major source of iron for organisms is local fluctuations of reducing and oxidizing conditions, which bring iron into solution and precipitate it as metastable, mixed-valence ferrosic hydroxide, which provides iron to solution at much higher concentrations than most other solid forms of iron.
This paper provides useful background information about the chemistry of iron in soils, but it is not clear to me which forms of iron should be targeted and measured in soils if one wishes to learn something about local aerobic and redox conditions.
This author reviews the chemistry of bioavailable iron in soil solutions. The major controls on the availability of iron in soil solution, which is normally very low, are pH, redox status, and the presence of organic matter and microsites where organic matter is decomposed by microorganisms under oxygen-limited conditions.
The usual concentration of iron in solution in soils is extremely low, and its exact value suggests contributions from multiple solid iron species, including amorphous and a range of crystalline forms of Fe(III) oxides. Organic matter produces transient small organic acids as it is decomposed, which complex with iron and help to bring it into solution. However, these organic acids are in the middle of the soil organic matter decomposition pathway, and do not persist for long in soils. Bringing more iron into solution relies on a combination of local reducing conditions around respiring roots and among microbe-and-SOM microsites, and local pH. Plants and other organisms secrete compounds that are effective at solubilizing iron and making it available to plants. The other major source of iron for organisms is local fluctuations of reducing and oxidizing conditions, which bring iron into solution and precipitate it as metastable, mixed-valence ferrosic hydroxide, which provides iron to solution at much higher concentrations than most other solid forms of iron.
This paper provides useful background information about the chemistry of iron in soils, but it is not clear to me which forms of iron should be targeted and measured in soils if one wishes to learn something about local aerobic and redox conditions.
Friday, November 6, 2009
Elberling et al. 2004
Elberling B, Jakobsen BH, Berg P, Sondergaard J, Sigsgaard C. 2004. Influence of vegetation, temperature, and water content on soil carbon distribution and mineralization in four High Arctic soils. Arctic, Antarctic, and Alpine Research, 36: 528-538.
These authors examined the carbon pools and carbon dioxide effluxes from four ecosystems at Zackenberg, in north-east Greenland. Their four ecosystems are very similar to the Alexandra Fjord systems of Dryas (CAVM P1), Cassiope (P2), Salix (G3), and Wet Sedge Meadow (W1).
There are two major sources of soil CO2: respiration by plant roots, and microbial respiration. Which of these two processes dominates CO2 production is a matter of some debate, with studies in the 1990s and 2000s indicating either when measuring similar arctic ecosystems. This study does not settle that debate, with estimated ratios of the two processes ranging from 9:1 to 1:9. What is clear is that plants and microbes compete for resources in arctic soils, with considerable variation in both time and space, even among vegetation communities in one valley with a consistent above-ground climate.
These soils include a buried A-horizon, with birch leaves present in pockets of organic-rich former topsoil indicative of surface conditions during the previous climatic mild period approximately 5000 years ago. These pockets of “Ab” create something of a wildcard situation for CO2 evolution, being responsible for a considerable fraction of the measured CO2 efflux at all ecosystems to varying extent.
To measure subsurface concentrations of CO2, these authors extended a probe to a range of depths and connected it to their gas analyzer also used for measuring surface fluxes, much as we have done at Alexandra Fjord. However, their gas analyzer only measures CO2, and they did not allow their probes to equilibrate to subsurface conditions for very long; a few minutes seems to be the usual protocol in this case.
A range of soil parameters were measured, and in general variation between vegetation types in these parameters exceeded variation within. The Cassiope system was the most variable, but also had the most variable Ab layers, which probably accounted for most of the variation. These patterns of variation at all ecosystems, however, suggest that the effects of climate change will not be uniform across the High Arctic, with increased temperatures leading to perhaps increases or decreases in the decomposition of buried organic matter and CO2 effluxes.
Like our own results from Alexandra Fjord, the Salix ecosystem at Zackenberg showed the highest below-ground CO2 concentrations. At Zackenberg, CO2 concentrations in Salix were mostly related to microbial decomposition of organic matter, with reduced soil water content leading to more oxygenation and higher temperatures, both increasing the rate of decomposition. In contrast, the water-saturated Eriophorum system had very high carbon stores and the highest CO2 efflux, but a decrease in water levels here leads to a shift from methane production to CO2 production, rather than a simple increase in one rate.
Overall, this paper provides some results and considerations of high relevance to my own work, especially given the high overlap in the range of ecosystems under consideration and the range of methods employed in their analysis. Their systems are not identical to those I studied; for example, the Dryas system at Zackenberg appears to be considerably drier and with less vegetation cover compared to the system with the same name at Alexandra Fjord.
These authors examined the carbon pools and carbon dioxide effluxes from four ecosystems at Zackenberg, in north-east Greenland. Their four ecosystems are very similar to the Alexandra Fjord systems of Dryas (CAVM P1), Cassiope (P2), Salix (G3), and Wet Sedge Meadow (W1).
There are two major sources of soil CO2: respiration by plant roots, and microbial respiration. Which of these two processes dominates CO2 production is a matter of some debate, with studies in the 1990s and 2000s indicating either when measuring similar arctic ecosystems. This study does not settle that debate, with estimated ratios of the two processes ranging from 9:1 to 1:9. What is clear is that plants and microbes compete for resources in arctic soils, with considerable variation in both time and space, even among vegetation communities in one valley with a consistent above-ground climate.
These soils include a buried A-horizon, with birch leaves present in pockets of organic-rich former topsoil indicative of surface conditions during the previous climatic mild period approximately 5000 years ago. These pockets of “Ab” create something of a wildcard situation for CO2 evolution, being responsible for a considerable fraction of the measured CO2 efflux at all ecosystems to varying extent.
To measure subsurface concentrations of CO2, these authors extended a probe to a range of depths and connected it to their gas analyzer also used for measuring surface fluxes, much as we have done at Alexandra Fjord. However, their gas analyzer only measures CO2, and they did not allow their probes to equilibrate to subsurface conditions for very long; a few minutes seems to be the usual protocol in this case.
A range of soil parameters were measured, and in general variation between vegetation types in these parameters exceeded variation within. The Cassiope system was the most variable, but also had the most variable Ab layers, which probably accounted for most of the variation. These patterns of variation at all ecosystems, however, suggest that the effects of climate change will not be uniform across the High Arctic, with increased temperatures leading to perhaps increases or decreases in the decomposition of buried organic matter and CO2 effluxes.
Like our own results from Alexandra Fjord, the Salix ecosystem at Zackenberg showed the highest below-ground CO2 concentrations. At Zackenberg, CO2 concentrations in Salix were mostly related to microbial decomposition of organic matter, with reduced soil water content leading to more oxygenation and higher temperatures, both increasing the rate of decomposition. In contrast, the water-saturated Eriophorum system had very high carbon stores and the highest CO2 efflux, but a decrease in water levels here leads to a shift from methane production to CO2 production, rather than a simple increase in one rate.
Overall, this paper provides some results and considerations of high relevance to my own work, especially given the high overlap in the range of ecosystems under consideration and the range of methods employed in their analysis. Their systems are not identical to those I studied; for example, the Dryas system at Zackenberg appears to be considerably drier and with less vegetation cover compared to the system with the same name at Alexandra Fjord.
Monday, November 2, 2009
Jones and Henry 2003
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.
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.
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