Passage Summary: Acid rain takes calcium out of the dirt. Even though three types of trees need that calcium, the sugar maples are struggling way more than the others.

Reasoning: Acid rain depletes soil calcium, a nutrient required by spruce, fir, and sugar maple trees, yet sugar maples exhibit significantly higher rates of calcium-deficiency symptoms than the other two species.

Analysis: The paradox here is a 'difference in effect' despite a 'similarity in needs.' We are looking for an answer that explains why sugar maples are uniquely sensitive to the loss of calcium compared to spruces and firs. Perhaps maples have a higher biological demand for calcium, or perhaps the other trees have deeper root systems that access calcium stores unaffected by acid rain.