Answers to Analyzing Data 17.1 Questions
1. Aspen suckers colonize all the successional stages but are most abundant in aspen stands and least abundant in meadow and fir stands. In contrast, subalpine fir seedlings are most abundant in the mixed aspen-fir and fir stands and least abundant in the meadow and aspen stands.
The data show that aspen are the first to colonize meadows, establishing aspen stands that are then colonized by firs. As firs increase and form mixed and fir-dominated stands, aspen decline and fir seedlings increase. This pattern supports the successional sequence described in the introductory paragraph of this Analyzing Data exercise.2. The most consistent hypothesis is that fir seedlings are facilitated by aspen, because their densities are highest in aspen stands but lowest in meadows. However, competition seems to drive aspen out in later stages, because they decline in mixed aspen-fir and fir-dominated stages.
3. Fir trees have lower mortality when they live close to aspen than when they live farther away, suggesting that they are facilitated by aspen. However, aspen trees show greater mortality close to firs than farther away, suggesting that they compete with firs and are eventually excluded from the community. These results support the previous hypotheses from Question 2.
4. This study best fits the facilitation model inspired by Frederick Clements and later described by Connell and Slatyer. In this model, only certain species, such as aspen, can establish themselves in early successional habitats. This might be a consequence of clonal growth. In time, species such as aspen modify the habitat in such a way that they facilitate later successional species such as firs, which may be intolerant of full sun. As firs grow and mature, they continue to be facilitated by aspen but eventually displace them through competition for resources. In this system, firs are expected to dominate and exclude aspen until they are disturbed by fire or humans, resetting the system back to meadow.