The tundra biome: climate and soils

Overview

Temperature regime of the tundra, active layer and characteristics of a typical tundra soil.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe the temperature regime of the tundra.

   Model answer

   Temperatures are below freezing for most of the year. The mean temperature of the coldest month is typically between minus twenty and minus thirty degrees Celsius, and extremes well below minus forty are recorded in continental interiors. The mean temperature of the warmest month is only around five to ten degrees Celsius, and by definition it does not exceed about ten degrees, which is why trees cannot grow. Only two to four months of the year have mean temperatures above freezing, and because a growing season is reckoned above about five degrees rather than zero, it lasts roughly fifty to ninety days. The annual range is large, and frost can occur in any month.

2. 2

   How much precipitation does the tundra receive, and in what form?

   Model answer

   Very little - typically between about one hundred and two hundred and fifty millimetres a year, which is comparable to a hot desert, so the tundra is often described as a cold desert. Most of it falls as snow, because temperatures are below freezing for most of the year, and the very cold air is able to hold almost no water vapour. There is a slight summer maximum when the air is warmest. Snow is redistributed by strong winds so that some ground is scoured bare and hollows are deeply drifted, which strongly affects where plants can survive.

3. 3

   Apart from temperature and precipitation, what other climatic conditions shape the tundra?

   Model answer

   Daylight is extreme and highly seasonal: within the Arctic Circle there is continuous daylight around midsummer and continuous darkness around midwinter, and even in summer the sun stays low in the sky so the angle of incidence is oblique and insolation is weak. Snow and ice give a very high albedo, reflecting much of the radiation that does arrive back to space. Winds are persistent and strong because there are no trees or relief to slow them, producing severe wind chill, drifting snow and physical abrasion of plants by ice crystals. Humidity is low, and evaporation is limited by cold, which is why the surface can be waterlogged despite tiny precipitation totals.

4. 4

   What is permafrost?

   Model answer

   Permafrost is ground - soil, sediment or rock - that remains at or below zero degrees Celsius continuously for at least two consecutive years, so it is permanently frozen. In the high Arctic it is continuous and may be hundreds of metres thick; towards the southern margin of the tundra it becomes discontinuous and then sporadic, occurring only in patches. It is effectively impermeable, so it prevents water from percolating downwards, and it is the single most important control on tundra soils, drainage, vegetation and landforms.

5. 5

   What is the active layer, and why does it matter?

   Model answer

   The active layer is the shallow surface zone above the permafrost that thaws each summer and refreezes each winter. It is usually only a few centimetres to about a metre deep. It matters because it is the only part of the ground in which roots can grow, water can move and biological activity can take place, so it sets a strict physical limit on the depth of rooting and therefore on the size of plant that the tundra can support. Its shallowness, combined with the impermeable permafrost beneath, also means that summer meltwater cannot drain away.

6. 6

   Why are tundra soils waterlogged in summer when the biome receives so little precipitation?

   Model answer

   Because the water has nowhere to go. The permafrost beneath the active layer is impermeable, so meltwater from the thawing surface and from snowmelt cannot percolate downwards. The relief is generally flat, so there is little runoff. Evaporation is small because temperatures are low, the sun is weak and the air is cool. The result is that even a very small quantity of water saturates the shallow active layer and stands at the surface as pools and marsh, so the same soil that is frozen solid in winter is boggy and anaerobic in summer.

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