In the scenario the forester wonders why one forest had mature hemlock and very few young hemlock which part of a scientific method dose this best illustrate

The forester made an observation that the forest had mostly mature hemlock and very few young hemlock. This is common for outdoors scientists to make observations on the  part of the natural world that is their field of study, and then draw conclusions from these observations. For example, the observation that the western edge of North and South America has a string of volcanoes or the ring of fire, plus the concordance between the east coast of South America and the western coast of Africa let first to the theory of continental drift and finally to plate tectonics. Similarly, the paucity of young hemlock and the existence of mature hemlock could also spark a conclusion, such as maybe the mature hemlock were diseased so couldn't create new offspring or perhaps a fire had wiped out the young ones, as two possible explanations which would depend on the actual circumstances observed by the forester.