On the tundra's southern fringes, winter is significantly less severe.
The slight increase in annual sunlight has enabled a stunted forest to develop.
It's the northern edge of the world's largest woodland.
The boreal forest. 750 billion trees.
A quarter of all the trees on earth. And it encircles the globe.
These forests store twice as much carbon as a tropical forest,
and they play a crucial part in regulating the climate of the entire planet.
One of the most remote tracts stands in the far east of Siberia.
It's cut off from the rest of the world by towering peaks... and precipitous valleys.
And it is the last stronghold of the world's rarest big cat. The amur leopard.
Hunting has brought it close to extinction. Only 120 are left in the wild.
And this is one of the very few ever to be caught on film.
Each must patrol a territory of over 100 square miles to find enough to eat.
In the cold of march, prey is at its most scarce.