You wanted to track the spread of the invasive polyphagous shot-hole borer, so each year for the last 3 years you counted the number of trees in the Arroyo that were infested with them. This is an example of an:

Observational study

Explanation:

The illustration is an example of an observational study.

Observational studies generally involve observing and recording data from a population of study without trying to manipulate any variable within the population.

Observational study is as opposed to experimental study with the later involving assigning the members of the population of study into different groups and subjecting them to different treatments in order to see the effects of variable manipulation.

The example shown in the illustration involved observing the population of study (the trees) in their natural habitat then counting the number of trees infested by the shot-hole borer. It does not involve assigning the members of the population into different groups.

This is an example of an observational study.

sodium i think the chemical element of atomic number 11, a soft silver-white reactive metal of the alkali metal group.