(Answers are available in Sect. A.10)

Researchers in New Zealand were interested in:

…the behaviour of heather (Calluna vulgaris) in Tongariro National Park…: the potential for herbicidal control; likely changes in secondary vegetation if biological control is effective; some aspects of its demography; and, finally, whether heather acts to facilitate or impede rates of native shrub invasion of red tussock grassland.

Rogers (1996)

The graph in Fig. 10.8 comes from this study examining heather in the central North Island of New Zealand.

1. Identify a possible research question being answered by the graph.
2. Critique the graph
3. Compute the value of $$r$$, and interpret.
4. For a stem age of 20 years, what does the regression model predict as the mean stem diameter?
5. Interpret the meaning of the slope and intercept in the regression equation. Comment on these interpretations.

### References

Rogers, G. M. 1996. “Control, Demography, and Post-Control Response of Heather in the Central North Island: Part 2.” 29. New Zealand Department of Conservation.