Long, cold winters, the phenomenon of the polar night, and relatively short (up to 60 days) growing season creates in environment in which short, scrubby plants and lichens grow. Thin soils poor in minerals do not allow lush growth of plants, and the presence of permafrost is a natural barrier, impossible for plants' root systems (which are quite simple and poorly developed) to penetrate. This is why there are no trees on the tundra, and most of the plants do not grow higher than a few centimeters.
Tundra is one of the biggest carbon dioxide sinks on Earth. In other words, the biomass (organic matter, both plant and animal), which is biodegradable, takes more carbon dioxide than it gives back to the atmosphere when it decomposes.
What is the tundra using CO2 for? Plants absorb it along with with sunlight and water when they carry out photosynthesis. They give it back to the atmosphere when their life cycle is over, and their remains decompose. But in the Arctic, these natural processes are significantly slowed down because of the cold climate.
Short summers and long, cold winters stunt the decomposition of organic matter. Plant remains, which as a result of continuous freezing and thawing of the soil, can remain trapped in a layer of permafrost for several thousand years. This is why tundra is a carbon sink, trapping carbon dioxide and and keeping it from returining to the atmosphere.
As a result of climate change, tundra soils are thawing deeper and deeper and for a longer periods of time in the upper layers of permafrost. This causes organic matter to decompose faster, releasing larger amounts of carbon dioxide into the atmosphere.
The impact on the environment is still not clear. According to research conducted within the Arctic Long-Term Ecological Research (LTER) project around Toolik Lake in Alaska, carbon compounds may remain in the soil, and some of them can be used by plants in the growth phase.
An example of decomposing organic matter (reindeer remains), southern Spitsbergen (author: Piotr Andryszczak)