Acid Rain and Calcium Depletion
Acid rain and other anthropogenic factors can leach calcium (Ca) from forest ecosystems and mobilize potentially toxic aluminum (Al) in soils. Considering the unique role Ca plays in the physiological response of cells to environmental stress, we propose that depletion of biological Ca would impair basic stress recognition and response systems, and predispose trees to exaggerated injury following exposure to other environmental stresses. Because Ca competes with Al for uptake, soil Ca deficiency would also increase the likelihood of Al toxicity and associated damage.To date, Ca depletion has been implicated in the decline and mortality of at least two tree species: red spruce (Picea rubens Sarg.) and sugar maple (Acer saccharum Marsh.). Our research on the causes of red spruce decline showed that acid rain directly leaches important pools of Ca from the cell membranes of red spruce foliage. This loss of Ca reduces that stability of cells and diminishes the cold hardiness of foliage – increasing the risk of freezing injury and decline. New research on the broader significance of Ca depletion indicates that the same disruptions documented for red spruce can occur for other evergreen species (e.g., eastern hemlock (Tsuga canadensis (L.) Carr.), balsam fir (Abies balsamea (L.) Mill.), and eastern white pine (Pinus strobus L.)), and that soil-based Ca deficiency is associated with the decline of sugar maple trees in the field.
Considering the unique role Ca plays in the physiological response of cells to environmental stress, we propose that depletion of biological Ca would impair basic stress recognition and response systems, and predispose plants to exaggerated injury following exposure to other environmental stresses. Diminished stress response would be particularly problematic now because numerous human activities (e.g., pollution production, ozone depletion, climate change, the spread of exotic pests and pathogens, etc.) are simultaneously subjecting forests to an increasing level and diversity of stresses. Because Ca competes with Al for uptake, soil Ca deficiency would also increase the likelihood of Al toxicity and associated damage.
We are assessing the influence of Ca depletion and Al toxicity on the health and productivity of several tree species in the Northern Forest. For example, we have worked on two long-term studies at the Hubbard Brook Experimental Forest (HBEF) in NH to assess the tree health impacts of adding back Ca to soils to levels that existed before acid rain-induced leaching. In one study where Ca was added to an entire watershed, Ca addition significantly improved the health of red spruce trees by increasing foliar Ca and sugar levels, increasing the activity of protective antioxidant enzymes, increasing foliar cold tolerance, and reducing the winter injury of foliage by about 3-fold during a year of otherwise high injury. In a separate replicated experiment, Ca addition reversed the well-established symptoms of sugar maple decline (crown dieback and reduced woody growth), and significantly increased wound closure on tree stems. The influence of Ca nutrition on wound closure has particular relevance for sugar maple trees because they are regularly wounded during maple syrup production. We now expanding our research scope to assess 1) if Ca depletion plays a role in the decline of other tree species (e.g., paper birch: Betula papyrifera Marsh.), and 2) if Ca depletion may limit carbon (C) sequestration in the Northern Forest. Reduced C sequestration would limit forest C uptake exactly when it is needed most to offset the anthropogenic CO2 emissions that promote climate change.
Concerns about the influence of Ca depletion on forest health exist for industrialized regions around the world including Europe, eastern North America, and increasingly China. Growing experimental evidence and examples from the field indicate that the threat posed to forest ecosystems from anthropogenic Ca depletion is real and potentially widespread. Knowledge of the influence of pollution loading on the cation pools that sustain forest health and productivity provides further scientific grounding and impetus for policy makers to modify existing pollution control measures. In addition, an increased recognition of the potential consequences of Ca depletion has functional relevance to managers in the field. Especially in regions with low inherent soil fertility and/or high precipitation leaching, management options that either add Ca to systems or decrease its removal are increasingly being examined and employed.
Halman, J.M.; Schaberg, P.G.; Hawley, G.J.; Eagar, C. 2008. Calcium addition at the Hubbard Brook Experiment Forest increases sugar storage, antioxidant activity and cold tolerance in native red spruce (Picea rubens). Tree Physiology 28:855-862.
Huggett, B.A.; Schaberg, P.G.; Hawley, G.J.; Eagar, C. 2007. Long-term calcium addition increases growth release, wound closure, and health of sugar maple (Acer saccharum) trees at the Hubbard Brook Experimental Forest. Canadian Journal of Forest Research 37:1692-1700.
Schaberg, P.G.; Tilley, J.W.; Hawley, G.J.; DeHayes, D.H.; Bailey, S.W. 2006. Associations of calcium and aluminum with the growth and health of sugar maple trees in Vermont. Forest Ecology and Management 223: 159-169.
- Paul G. Schaberg, US Forest Service - Northern Research Station - Research Plant Physiologist
- Gary J. Hawley, The University of Vermont - Senior Researcher
- Joshua M. Halman, The University of Vermont - Research Field Technician
- Brett Huggett, Harvard University - Ph.D. Student
- Chris Eagar, US Forest Service - Northern Research Station - Research Ecologist
Last Modified: 01/27/2010