Planet Earth2006

Narrator:
As we travel south, so the Sun's influence grows stronger, and at 50 degrees of latitude a radical transformation begins. Summers here are long enough for broad leaved trees to replace conifers. Broad leaves are much easier to digest, so now animals can collect their share of the energy that has come from the Sun. It's summer and these forests are bustling with life. [birds, butterflied, wrestling antelope]

Narrator:
But the good times will not last. [green transforms to white] Broad leaves must be shed in winter, for they are damaged by frost. As they disappear, so the land become barren, with little for animals to eat. The inhabitants must migrate, hibernate, or face months of near-starvation.