<P> There are two main seasons, winter and summer, in the polar tundra areas . During the winter it is very cold and dark, with the average temperature around − 28 ° C (− 18 ° F), sometimes dipping as low as − 50 ° C (− 58 ° F). However, extreme cold temperatures on the tundra do not drop as low as those experienced in taiga areas further south (for example, Russia's and Canada's lowest temperatures were recorded in locations south of the tree line). During the summer, temperatures rise somewhat, and the top layer of seasonally - frozen soil melts, leaving the ground very soggy . The tundra is covered in marshes, lakes, bogs and streams during the warm months . Generally daytime temperatures during the summer rise to about 12 ° C (54 ° F) but can often drop to 3 ° C (37 ° F) or even below freezing . Arctic tundras are sometimes the subject of habitat conservation programs . In Canada and Russia, many of these areas are protected through a national Biodiversity Action Plan . </P> <P> Tundra tends to be windy, with winds often blowing upwards of 50--100 km / h (30--60 mph). However, in terms of precipitation, it is desert - like, with only about 15--25 cm (6--10 in) falling per year (the summer is typically the season of maximum precipitation). Although precipitation is light, evaporation is also relatively minimal . During the summer, the permafrost thaws just enough to let plants grow and reproduce, but because the ground below this is frozen, the water cannot sink any lower, and so the water forms the lakes and marshes found during the summer months . There is a natural pattern of accumulation of fuel and wildfire which varies depending on the nature of vegetation and terrain . Research in Alaska has shown fire - event return intervals (FRIs) that typically vary from 150 to 200 years, with dryer lowland areas burning more frequently than wetter highland areas . </P> <P> The biodiversity of tundra is low: 1,700 species of vascular plants and only 48 species of land mammals can be found, although millions of birds migrate there each year for the marshes . There are also a few fish species . There are few species with large populations . Notable animals in the Arctic tundra include reindeer (caribou), musk ox, Arctic hare, Arctic fox, snowy owl, lemmings, and even polar bears near the ocean . Tundra is largely devoid of poikilotherms such as frogs or lizards . </P> <P> Due to the harsh climate of Arctic tundra, regions of this kind have seen little human activity, even though they are sometimes rich in natural resources such as petroleum, natural gas and uranium . In recent times this has begun to change in Alaska, Russia, and some other parts of the world: for example, the Yamalo - Nenets Autonomous Okrug produces 90% of Russia's natural gas . </P>

What is the main reason that the tundra does not have large plants