In the given case, the biosphere is limited by the atmospheric temperature.
The tundra soils are produced at high altitudes. It is generally very cold in the tundra, the tundra soils are classified as Gelisols and are usually frozen. In tundra, the top of the mixture of gravel, soil, and finer substances are frozen for the maximum of the year as the temperature do not increase much for the soil to get thaw.
In tundra, the soil is also very much deficient in nutrients, thus, it lacks phosphorus and nitrogen, that is, the two essential elements needed by the plant to grow. This makes the plants small and slow in growth. In this region, the permafrost produces when plants die and the organic matter accumulates. With time, it is compacted and frozen but never gets dissociated.