The statistic in this scenario is the sample mean age of the 400 selected trees. It is calculated from the sample and provides information about a subset of the entire tree population in the park.
In this scenario, the sample mean age of the 400 selected trees is the statistic.
Specifically, the sample mean is a statistic because it is a measure calculated from a sample (subset) of the entire population of trees in the park. In contrast, if we were referring to the average age of all trees in the park (the entire population), it would be a parameter.
Parameters describe characteristics of the entire population, while statistics provide information about samples drawn from that population. In this case, the sample mean serves as a statistic representing the average age of the selected trees.