The average age of pion pine trees in the coast ranges of California was investigated by placing

The average age of piñon pine trees in the coast ranges of California was investigated by placing 500 10-hectare plots randomly on a distribution map of the species using a computer. Researchers then found the location of each random plot in the field, and they measured the age of every piñon pine tree within each of the 10-hectare plots. The average age within the  plot was used as the unit measurement. These unit measurements were then used to estimate the average age of California piñon pines.

a. What is the population of interest in this study?

b. Why did the researchers take an average of the ages of trees within each plot as their unit measurement, rather than combine into a single sample the ages of all the trees from all the plots?