A new study conducted by a US research team revealed that acid rain damage to
America's forests may be much more widespread than previously believed. It may actually
create conditions in trees similar to lessened immune systems in humans, making them
vulnerable with serious potential implications. Thus, an "immune response" hypothesis was
proposed by the research team to discover how plants have been threatened by acid rain.
Plants may appear and function as if they were healthy. However, if they are exposed to
even a routine stress or disease, the resulting decline can be far worse than expected.
Up to now, acid rain has been associated with the decline of forests in certain
specific locations. The mechanism through which acid rain depletes calcium and weakens
red spruce trees, making them more vulnerable to winter freezing injury, are documented.
The new work shows that this mechanism is also applicable to other tree species,
including balsam fir, white pine, and eastern hemlock. Because calcium is a critical
ingredient in the plant's stress response system, acid rain's depletion of cellular calcium may
suppress the capacity of trees to survive environmental stresses.
This connection between calcium deficiency and environmental stress exposure are
common components in the declines of several tree species, including red spruce, sugar
maple, and flowering dogwood. The research team’s "immune response" hypothesis
provides an overarching explanation of how acid rain ultimately threatens forests. The
findings are especially relevant now because a growing assortment of human influences--
climate change, pollutants, and new pests and diseases, are burdening our forests.
If extensive, the decline of individual species would radiate through plant
communities. It would alter the competition and survival of populations, perhaps even
species, including animals at higher levels of the forest food chains. Those calcium
deficiencies in plants are passed onto herbivores, altering their nutrition. For instance, birds
eating calcium-deficient plant material might have less calcium for egg production. Insects
could experience weaker exoskeletons. Mammals could have weaker bones or change in the
quantity or quality of milk production. The problems continue through the ecosystem and
into economic system.