Yes, SUCCESSIONS is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, C 3, C 3, E 1, S 1, S 1, I 1, O 1, N 1, S 1.

Is it a valid word?

SUCCESSIONS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a following of one thing after another in time
a group of people or things arranged or following in order
the action of following in order
(ecology) the gradual and orderly process of change in an ecosystem brought about by the progressive replacement of one community by another until a stable climax is established
acquisition of property by descent or by will

Also: chronological sequence, chronological succession, ecological succession, taking over.