Question

Detailed explanation would be appreciated.

3. Now consider Figure B. What happens to fir trees when they live close ( <0.5 m) to aspen trees? What happens to aspen when they live close ( <0.5 m) to fir trees? Do these data
support your previous hypotheses (from question 2) about the types of species interactions driving succession?

4. Which of Connell and Slatyer's three models (see Figure 17.8) best fits the results of this study? Why?

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Answer to #2 was ===>What type of interspecific interaction would you hypothesize could account for the difference between the number
of fir seedlings in the aspen stage and in the meadow stage in Figure A? What type of interspecific interaction might explain the difference between the number of aspen suckers
in the mixed and the fir stages? In figure B it is shown that the Aspen that are less distant from the FIR grow very significantly at a higher rate in FIR stage as compared to the mixed aspen Fir stage. It means there is commensalism where one species is benefitted while the other remains unaffected. Since the FIR are more prone to fire or deforestation therefore helps to protect the aspen sucker. In this way it can be said that the aspen that are more closer to the fir will be protected and increased in the density will be seen.

We learned in Concept 17.3 that successional patterns are often the result of complex species interactions. Such inter- actions are exemplified in a study investigating the patterns of succession in mountain forests in Utah dominated by quaking aspen (Populus tremuloides) and subalpine fir (Abies lasiocarpa) (Calder and St. Clair 2012).* In some cases, aspen can form stable and self-sustaining populations, but more commonly these trees occur in mixed stands with firs. Observations show that aspen initiate the earliest stage of secondary suc- cession in open meadows created by fire or deforestation, using root suckers (underground shoots that produce clonal plants; see Figure 9.5) to colonize open meadows. Over time, mixed aspen-fir stands are formed as the shade-tolerant firs become established and increase in abundance while aspen decline. The stands are eventually dominated by firs, which are more susceptible to fire than pure stands ofaspen, thus increasing the chance of starting the successional cycle anew. Figure A Aspen suckers Fir seedlings 1.4 1.2 1.0 2 0.8 0.6 0.4 0.2 Meadow As Mixed aspen-fir Fir Successional stage Figure B 80 Aspen>0.5 m from fir Aspen<0.5 m from fir 6Fir> 0.5 m from aspen Fir < 0.5 m from aspen To understand the transition from one successional stage to another, Calder and St. Clair counted the aspen suck- ers and fir seedlings in four successional stages-meadow, aspen, mixed aspen-fir, and fir-with the results shown in Figure A. To test for the type of interaction important in the transition from one stage to another, the researchers then measured the mortality of canopy aspen and subalpine fir trees as a function of their distance from the nearest neigh- boring tree of the other species. These results are given in Figure B. Error bars show one SE of the mean 1. Based on the data in Figure A, what is the pattern of aspen 40 20 abundance over the four successional stages? How does the abundance pattern of subalpine fir differ? Do these patterns ofabundance ofaspen and fir supportthe succes- Aspen Mixed aspen-firFir Successional stage

Answer 1

3.

• Fir trees when they are present very close to aspen ( < 0.5m), it inhibits the growth of aspen trees.
• Initially , apen trees aid in the growth of fir trees . But later some time during climax stage of secondary succession , fir trees will be abundant and aspen trees shows a decline in the abundancy .
• Slowly, the conifers dominate the area and aspen trees are reduced. This condition is prevalent until next forest disturbance.
• Aspen trees facilitate the growwth of fir trees but fir trees doesnot.
• Due to presence of fir trees aspen trees cannot survive , similar to pathogenicity.
• Thus , it can be stated that mortality of aspen trees increases as abundancy of fir trees increases.
• No, the data doesnt support previous explanation.