disc four



"Seasonal Forests"

The penultimate episode surveys the coniferous and deciduous seasonal woodland habitats — the most extensive forests on Earth. Conifers begin sparsely in the Arctic but soon dominate the land, and the taiga circles the globe, containing a third of all the Earth's trees. Few creatures can survive the Arctic climate all year round, but the moose and wolverine are exceptions. 1,600 kilometres (990 mi) to the south, on the Pacific coast of North America, conifers have reached their full potential. These include some of the world's tallest trees: the redwoods. Here, a pine marten is shown stalking a squirrel, and great grey owl chicks take their first flight. Further south still, in the Valdivian forests of Chile, a population of smaller animals exist, including the pudú and the kodkod. During spring in a European broad-leafed forest, a mandarin duck leads its day-old family to leap from its tree trunk nest to the leaf litter below. Bialowieza Forest typifies the habitat that characterised Europe around 6000 years ago: only a fragment remains in Poland and Belarus. On a summer night on North America's east coast, periodical cicadas emerge en masse to mate — an event that occurs every seventeen years. After revisiting Russia's Amur leopards in winter, a timelapse sequence illustrates the effect of the ensuing spring on the deciduous forest floor. In India's teak forests, a langur monkey strays too far from the chital that act as its sentinels and falls prey to a tiger. Planet Earth Diaries explains how aerial shots of the baobab were achieved by the use of a cinebulle, an adapted hot air balloon.

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"Ocean Deep"

The final instalment concentrates on the most unexplored area of the planet: the deep ocean. It begins with a whale shark used as a shield by a shoal of bait fish to protect themselves from yellowfin tuna. Also shown is an oceanic whitetip shark trailing rainbow runners. Meanwhile, a 500-strong school of dolphins head for the Azores, where they work together to feast on scad mackerel. Down in the ocean's furthest reaches, some creatures defy classification. On the sea floor, scavengers such as the spider crab bide their time, awaiting carrion from above. The volcanic mountain chain at the bottom of the Atlantic Ocean also sustains life through the bacteria that surround its sulphide vents. There are thought to be around 30,000 undersea volcanoes, some of them taller than Mount Everest. Their sheer cliffs provide anchorage for several corals and sponges. Nearer the surface, the currents that surround these seamounts force nutrients up from below and thus marine life around them is abundant. Off the Mexican coast, a large group of sailfish encircle another shoal of bait fish. The hunters change colour as a message of their intentions, since an attack could also be fatal to others of their number. The last sequence depicts the largest animal on Earth: the blue whale, of which 300,000 once roamed the world's oceans. Now fewer than 3% remain. Planet Earth Diaries shows the search in the Bahamas for oceanic whitetip sharks.