Laboratory and Field Data Indicate that Acidic Deposition-induced Calcium Depletion Disrupts the Nutrition and Physiology of Trees, Predisposing Them to Decline

Paul G. Schaberg1, Gary J. Hawley2, Joshua M. Halman2 and Paula F. Murakami1

Substantial evidence indicates that high acid loading from atmospheric deposition that promotes calcium (Ca) leaching and mobilizes aluminum in the soil disrupts biogeochemical cycles and depletes Ca from forest ecosystems in the northeastern US. Because Ca is an essential plant nutrient, Ca depletion raises important questions concerning the continued health and sustainability of forest ecosystems. Ca plays critical roles in plant cell function, including enhancing the stability of cell walls and membranes, and signal transduction processes that allow cells to sense and respond to stress. Considering the fundamental role that Ca plays in plant stress response, biological Ca depletion could suppress the ability of trees to adequately sense and respond to stress, and predispose them to decline following exposure to even “normal” levels of stress that otherwise would pose no threat.

Initial evidence that Ca depletion reduces tree stress tolerance was obtained via investigation of the mechanism through which acid deposition increases freezing injury in red spruce (Picea rubens Sarg.). Experiments demonstrated that acid deposition directly leached Ca from the plasma membranes of red spruce foliar cells and that this loss of Ca destabilized cells, depleted pools of signal Ca, and increased the susceptibility of cells to freezing injury. Increased freezing injury also resulted from soil-based Ca depletion. In 2003 (a year of extensive region-wide injury in New York and New England), freezing injury was greatest in areas where acidic inputs were elevated, and injury was significantly reduced in a watershed fertilized with Ca. New experimental evidence indicates that Ca depletion down-regulates other Ca-dependent processes in red spruce, including stomatal closure, antioxidant enzyme activity, and foliar sugar accumulation – indicating that Ca depletion has physiological impacts beyond influences on foliar winter injury. Furthermore, data indicates that other tree species (e.g., eastern hemlock (Tsuga canadensis (L.) Carr.), balsam fir (Abies balsamea (L.) Mill.), eastern white pine (Pinus strobus L.) and sugar maple (Acer saccharum Marsh)) experience mechanistic changes in Ca nutrition and physiology similar to those documented for red spruce. Indeed, decline symptoms (crown deterioration, slow growth and reduced wound closure) for sugar maple trees in the field can be alleviated via Ca fertilization. Together, these data suggest that the negative influences of Ca depletion on the health and productivity of forest trees are not limited to red spruce winter injury, but have broader relevance to a variety of species and physiological processes.

1 USDA Forest Service, Northern Research Station, 705 Spear Street, South Burlington, VT 05403; Tel: 802-951-6771; e-mail:
2 The University of Vermont, Rubenstein School of Environment and Natural Resources, 81 Carrigan Drive, Burlington, VT 05405