Alaskan tundra

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Introduction

Even though the relative productivity across the tundra biome may be small, it still plays a large role in the carbon cycle. The tundra biome consists of a twenty percent of the Earth’s terrestrial surface (Kaplan 1996), and contains approximately 14% of the global carbon stored within soils (Billings 1987); making it a significant contributor to widespread global processes.

Physical and Chemical Environment

Temperature

During the warmest summer months, the tundra biome typically has an average temperature nearly 10 degrees Celsius. During the coldest months, the average temperature is found to be around 30 degrees Celsius (Remer 2009).

Drought

The tundra also exhibits extremely low amounts of precipitation. The typical amount of precipitation received throughout the tundra biome is only approximately 15-20 cm a year (Remer 2009).

Nutrient Limitations

Because of lower temperatures and high levels of moisture (due mostly to permafrost), decomposition is limited. This leads to nutrients existing in a form that is not readily available to many organisms. Other than this, unfrozen water content of soils below 0 degrees Celsius limit the diffusion of nutrients that plant and microorganisms can uptake, leading to further nutrient limitation (Ostroumov and Siegert 1996).

Heavy Winds

The tundra biome typically exhibits periods of high winds.

High Levels of Solar Radiation

The tundra biome has a relatively higher level of solar radiation compared to other biomes.

Biological interactions

Reduced Decomposition

Lower temperatures result in lower decomposition rates. Even though the tundra exhibits low annual precipitation, much of the ground remains saturated through much of the year. This water is typically frozen and inaccessible to plants. This high moisture content limits oxygen availability, which is needed to decompose the dead organic matter. Despite the low productivity of the tundra climate, organic matter accumulates because the decomposition of plant litter is limited by low soil temperatures and often wet, anaerobic conditions (Heal et al. 1981; Graglir et al. 2001). This leaves much of the nutrients that plants and microorganisms need for growth and development in a form that remains inaccessible to them (Jonasson and Shaver 1999).

Enhanced Mycorrhizal Relationships

Mycorrhizas are the mutualistic symbiosis between plant roots and fungi. Mycorrhizal associations are believed to be most beneficial in habitats where plants face strong nutrient limitations. Although the strong mycorrhizal dependence of the majority of plants across the globe is well known, it is even more important for tundra floral species because of a limiting nutrient supply caused by lower relative decomposition rates. When the plants lose their tissues, the nitrogen contained within them becomes reaccessible slowly relative to most other plants existing elsewhere. In these habitats, mycorrhizal plants tend to be strongly dependent on their fungal relationships for nutrient acquisition (Allen and Allen 1991).

Enhanced Vegetative Defenses Against Herbivory

Herbivory also poses a greater detrimental effect to these plant species. Tundra plant species cannot easily reacquire nutrients lost to herbivores. Because of this, they need to allocate more resources to chemical defenses. A study performed by Cates and Orians (1975) found that evergreen shrubs had stronger chemical defenses than their deciduous relatives. The same study also recognized that Graminoids of the region retained a relatively high content of nutrients stored in rhizome reserves as well as overall belowground biomass.

The Tundra and Climate Change

Positive Feedback

As mentioned before, the decomposition of plant litter is limited by low soil temperatures and often wet, anaerobic conditions (Heal et al. 1981; Graglia et al. 2001). Even though productivity in the tundra biome is relatively small, there are large accumulations of undecomposed organic matter. It is estimated that the tundra contains 14% of the global carbon stored within soils (Billings 1987). Since the end of the last glacial maximum to the present, the tundra has constituted as a large carbon sink, and may have been a contributing factor in the pre-industrial decrease of atmospheric carbon dioxide (Adams et al. 1990; Gorham 1991). Because of recent warming of the tundra due to climate change, the inevitable release of carbon from this carbon pool could pose a serious threat. This could cause possibly lead to a vicious positive feedback cycle that would contribute greatly to higher levels of carbon dioxide.

Negative Feedback

Specialized tundra plant species remain successful in harsh environmental conditions because they are able to maintain photosynthetic rates yielding higher energy values than they consume, as is necessary for the existence of all primary producers. For these plant species, the tundra setting acts as prolific scenario largely because of the lack of competition, parasites, and diseases (Callaghan et al. 2004). As temperatures begin to increase, it is plausible that these plant communities will be replaced by plants less suited for extreme conditions and with higher photosynthetic capabilities. The degree to which plant species can tolerate or take advantage of changing climate conditions depends on characteristics such as growth form, phenology, and allocation and storage patterns of carbon and nutrients (Shaver & Kummerow 1992). If a plant species exchanges defenses to an extreme climate with photosynthetic capability in non-extreme climate, it typically cannot contend well with other species with an opposite mechanism. With increased temperatures, one would expect a negative feedback mechanism due to an increase in the vegetative carbon pool of the tundra.

Prevailing Feedback Cycle

One could speculate which feedback mechanism would have a greater impact on the global carbon cycle. Again, a positive feedback mechanism would be induced by increasing temperatures accelerating decomposition and reducing the carbon sink within the soil. A negative feedback cycle could occur due to a higher rate of photosynthesis and thus a greater amount of carbon in the vegetative pool. Already, tundra ecosystems have changed substantially in terms of shrub abundance, primary production, and carbon exchange (Walker et al. 2004).

Microbial processes

What microbial processes define this environment? Describe microbial processes that are important in this habitat, adding sections/subsections as needed. Look at other topics in MicrobeWiki. Are some of these processes already described? Create links where relevant.

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Key Microorganisms

What kind of microbes do we typically find in this environment? Or associated with important processes in this environment? Describe key groups of microbes that we find in this environment, and any special adaptations they may have evolved to survive in this environment. Add sections/subsections as needed. Look at other microbe listings in MicrobeWiki. Are some of the groups of microbes from your environment already described? Create links to those pages. Specific microbial populations will be included in the next section.

Subsection 1

Subsection 1a

Subsection 1b

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Examples of organisms within the group

List examples of specific microbes that represent key groups or are associated with important processes found in this environment. Link to other MicrobeWiki pages where possible.

Current Research

Enter summaries of recent research here--at least three required

References

Billings, W. D. 1987. Carbon balance of Alaskan tundra and taiga ecosystems – past, present, and future. Quaternary Science Reviews 6:165-177.

Graglia, E., S. Jonasson, A. Michelsen, I. K. Schmidt, M. Havstrom, and L. Gustavsson. 2001. Effects of environmental perturbations on abundance of subarctic plants after three, seven and ten years of treatments. Ecography 24:5-12.

Heal, O. W., P. W. Flanagan, D. D. French, and S. F. MacLean, Jr. 1981. Decomposition and accumulation of organic matter in tundra. Tundra ecosystems: a comparative analysis.:587-633.

Jonasson, S. and G. R. Shaver. 1999. Within-stand nutrient cycling in arctic and boreal wetlands. Ecology 80:2139-2150.

Remer, L. 2009. Temperature and Precipitation Graphs. NASA Earth Observatory. http://earthobservatory.nasa.gov/Experiments/Biome/graphs.php. November 12, 2009.

Edited by Eric Johnston, a student of Angela Kent at the University of Illinois at Urbana-Champaign.