The Season attribute in the Clarisse plugin exactly mirrors the season slider in Cinema. In Clarisse, it runs from 0% to 100% with 0% representing spring, 33% summer, 66% autumn, and 100% winter.
It is possible to copy a SpeedTree object and vary the season attribute to get a scene like the one shown above, all from a single STE file.
Show Dropped Leaves also mirrors the Cinema attribute by the same name.
For season materials to work properly in Clarisse, you'll need to use STE files, not SPM. The reasons are detailed here.
The SpeedTree seasonal states are not smoothly animatable. That is, you cannot animate Season from 0% to 100% and expect a smooth animation. The Season attribute controls the state of the tree model at a given point in time. Attempting to animate the attribute will result in leaves changing position or popping in and out over time.