Figure 2C: The annual mortality rate by height as defined by a cubic smoothing spline. Average annual mortality was estimated from the frequency of deaths observed over the 6-yr interval (1964-1970) summarized by 0.5-m height classes.
(from the body of the paper:) "Mortality was highest among very young (<1 m) and very old (>8 m) individuals, consistent with previous observations of saguaro survivorship (Steenbergh and Lowe 1983). Because we located very few saguaros in the smallest size class (<10 cm), our model does not effectively capture the extremely high mortality rates of very young plants observed in seedling studies (Steenbergh and Lowe 1983)."