The independent variable in an experiment is the variable that the scientist/experimenter manipulates or changes in order to bring about a response in the dependent variable.
In this case, the independent variable is the AMOUNT OF RAINFALL that the Palo verde trees were exposed to. This is because the amount of rainfall received is what is changed in this experiment while the growth of the Palo verde trees is the dependent variable, which is dependent on the amount of rainfall received (independent).
