How is life and death related to the four seasons or the tree of life?

It is called spiritual relation.

According to Christian orthodox the Garden of Eden had 2 (specific) trees the first was the tree of life which Adam and Eve ate from on a regular basis to insure their immortality. The second was the tree of the knowledge of good and evil (which we all know god said not to eat from and the snake, and all that jazz). When Adam and eve ate from the second tree they were expelled from the garden, thus they were then unable to eat from the first tree anymore. Therefore they became mortal, they were able to die.

The seasons then become important because Adam is damned to plow fields and face hardships to gain food and provide for his family. Thus he must pay attention to the seasons to understand when to plant his crops, harvest, expect rain, etc…

Furthermore, many non Christian faiths view all time as cylindrical rather than linier. When they look at winter as a death, spring a rebirth, etc… this is what gives some faiths foundations for the idea of an afterlife or reincarnation. Because just as a tree will die in winter and be reborn in spring so will the human body die and the spirit be reborn. They do not feel that there is a beginning and an end but rather an ongoing cycle, like the seasons.