Summary

The Arctic as a whole is characterized by high content of soil organic matter and low plant biomass. The organic matter accumulation, biomass, and net primary production generally decrease from south to north, but the variability is even greater among neighboring ecosystem types. Local variation is strongly related to topography, which creates gradients of snow depth and water availability that exert landscape-scale controls over ecosystem structure and function. Permanent wet or moist ecosystem types generally have the largest stocks of soil organic matter as a consequence of constrained microbial decomposition.

In spite of large soil stores of organic matter and plant nutrients, net primary production within almost all arctic ecosystem types is limited by low availability of plant-available nutrients, particularly N, and of P in wet ecosystem types. This is due to slow microbial mineralization rates associated with low temperature and often combined with extreme wet or dry conditions in several ecosystem types. Furthermore, soil microorganisms immobilize nutrients and may even act as competitors with the plants for nutrients during the growing season, when the nutrient demand by both microbes and plants is high. Recent research has, however, shown that plants can partially circumvent possible competition from microbes by absorbing low-molecular-weight organic N compounds directly or indirectly through ecto- and ericoid myc-orrhiza. Organic N uptake can probably explain part of the great discrepancies between measured low annual N mineralization and much higher annual plant N uptake.

Experimental manipulations of various vegetation types across the Arctic have given much information on the controls of ecosystem processes. Almost all vegetation types have responded strongly and consistently to fertilizer application by, e.g., increased net primary production and plant N and P mass, and in most cases by increase of standing biomass. The responses to warming have been more variable, ranging from no to pronounced increase in biomass, whereas the response to water addition has been small. Modeling and summer gas exchange studies suggest that fertilizer addition leads to enhanced sequestering of C by the ecosystems and it is likely that the ecosystems that have shown pronounced biomass increase after warming also have increased their C pool sizes. In ecosystems with low response in biomass accumulation, warming generally leads to strongly increased respiration, which results in a short-term carbon loss from the system. However, a model based on the experimental data from one such ecosystem type, the tussock tundra, showed that the respiratory C loss levels off over a longer time period as the N supply rate from the soil to the vegetation increases, resulting in enhanced plant growth and carbon sequestration.

The strength of the environmental response by vegetation is variable among sites but shows consistent patterns within similar vegetation types. This suggests that the responses can be scaled up to the regional scale for realistic future modeling of longer-term responses to environmental change.

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