Sentence:  We examined one of several <WSD>seres</WSD> found in the middle Rocky Mountains that progress from a subalpine or montane forb-dominated meadow to a climax forest dominated by Engelmann spruce ("Picea engelmannii"). 
Meaning of the word marked with <WSD> tags:  A natural succession of animal or plant communities in an ecosystem, especially a series of communities succeeding one another from the time a habitat is unoccupied to the point when a climax community is achieved. (from early 20th c.)