State the problem. You would then go outside and look at that area. Compare as many likely factors that you can think of. The sparse area is surrounded by several evergreen trees, which drop needles and block much of the sunlight. The soil appears just as rich as the soil in other areas, but the pH is lower.
All areas seem to be getting similar amounts of water.
The temperature in the shaded area is lower than the areas that are not shaded. Based on the information that you gathered, and your knowledge of Biology, you are ready to form a hypothesis. Remember, a hypothesis is an educated guess. It is only your background knowledge in this subject that separates a true hypothesis from what would merely be a guess. Now, considering the observations you made, you might decide that pH of the soil in the sparse area is the problem.