Unlike spruces or firs, most sugar maples are native to areas that receive a lot of acid rain. Spruces and firs, unlike sugar maples, can extract calcium from a mineral compound that is common in soil and is not affected by acid rain. Sugar maples require more calcium in the spring and summer than they do in the fall and winter. Sugar maples that do not receive enough calcium deteriorate less rapidly than spruces or firs that do not receive enough calcium.
When a forest is subject to acid rain, the calcium level in the soil declines. Spruce, fir, and sugar maple trees all need calcium to survive. However, sugar maples in forests that receive significant acid rain are much more likely to show signs of decline consistent with calcium deficiency than are spruces or firs in such forests.
Which one of the following, if true, most helps to explain the greater decline among sugar maples?