OK, so the normal annual cycle contains only four seasons. Why then are there five trees in this series?
The answer lies in the nature of a cycle - with its beginning and its end. Two points that are adjacent; so close that they may as well be considered coincident. The cycle of nature that begins with the unborn state and concludes with death. The time before birth is one of potential, of the summoning of the forces of creation. A time of fire and the first emergence of light onto the landscape.
And so the cycle begins with Winter Dawn.
And is followed by Spring Morning, Summer Afternoon and Autumn Evening.
Before ending in the cold dark death of Winter Night.
Ergo, Five Seasons.