Acid rain leaches calcium and other nutrients from forest ecosystems. In places where soil calcium levels are naturally low, where acid rain inputs are high, or especially where both these conditions exist, tree calcium deficiencies may arise. Because calcium is an essential element that supports tree health and stress response, calcium deficiencies can take a toll on forest health. Until recently, the connection between calcium depletion and the increased susceptibility of red spruce tree needles to freezing injury and death was the best-documented example of acid rain’s influence on tree health in the mountains of the Northeast. However, recent research has shown that sugar maples may also decline because of calcium depletion.
Sugar maple decline is characterized by a gradual dieback of tree crowns followed by a reduction in stem growth, sometimes leading to tree death. Past work has shown that these symptoms are most likely on sites with calcium-poor soils, especially when some other stress agent, such as insect defoliation, is present.
Recent research at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire has provided evidence that calcium depletion by acid rain may help drive maple decline. Our collaborative research team, scientists from the U. S. Forest Service and researchers from the University of Vermont, compared some key features (nutrition, crown vigor, growth, and wound response of sugar maple trees) on a series of control plots known to have lost calcium due to acid rain with plots that had been fertilized with calcium.
We found that trees growing on control plots exhibited the type of crown die-back and poor stem growth typical of maple decline. In contrast, trees on the calcium-enriched plots had healthier crowns and greater stem growth.
One notable difference between the two sets of plots was how trees responded following the 1998 ice storm, when ice damage thinned the crowns of large, dominant trees and provided smaller trees with more light and other resources for growth. Crown damage from this unusual event reduced overstory competition equally on both the control and the calcium-enriched plots. The corresponding growth release was not equal, however: growth was much greater among trees on the enriched plots compared with trees on the unfertilized plots, and the effect remained measurable for years after the ice storm.
Calcium depletion, in addition to affecting growth, appears to influence the closure of stem wounds as well. We made roughly ½-inch bark wounds (down to the wood) at approximately breast height on the stem of each tree and measured the amount of wound closure after one growing season. Trees on the calcium-depleted control plots showed no measurable wound closure, whereas wounds on trees in calcium-enriched plots were almost halfway closed. Adequate wound response is particularly important for sugar maples because many of them are wounded each spring to allow for syrup production.
Results from this study highlight the influence of calcium depletion on the classic symptoms of maple decline but also show that calcium depletion can reduce the growth of maple trees following release from competition and inhibit stem-wound closure. Our research group is now examining the influence of calcium depletion on other aspects of maple physiology and health, as well as potential effects on other tree species.