Assessing the Threat that Anthropogenic Calcium Depletion Poses to Forest Health and Productivity
Growing evidence from around the globe indicates that anthropogenic factors including pollution-induced acidification, associated aluminum mobility, and nitrogen saturation are disrupting natural nutrient cycles and depleting base cations from forest ecosystems. Although cation depletion can have varied and interacting influences on ecosystem function, it is the loss of calcium (Ca) that may be particularly limiting to tree health and productivity. Ca plays unique roles in plant cell function, including environmental signal transduction processes that allow cells to sense and respond to stress. Considering this, Ca depletion could impair plant response systems and predispose trees to reduced growth and increased decline. Controlled experiments with red spruce and other tree species provide mechanistic support for the hypothesis that Ca deficiencies predispose trees to decline. Importantly, several examples of species declines in the field also suggest that injury is often greater when Ca depletion and stress exposure co-occur.
Connections between contemporary species declines and Ca depletion highlight the need for monitoring forests for indicators of change, including Ca loss. Direct measures of soil and plant Ca concentrations provide one traditional means of assessing that Ca status within forests. Although often valuable, substantial variation among soils and species and a lack of comparative historical data provide obstacles to the use of these measures for evaluating Ca depletion across the landscape. An alternative approach for assessing Ca depletion is to model critical loads and exceedances of pollutant additions that lead to net losses in Ca pools and likely disrupt ecosystem Ca cycles within forests. For example, spatial associations of Ca cycling and loss to broad-scale data on forest health and productivity were recently conducted for portions of the Northeastern United States. A steady-state ecosystem process model was coupled to extensive spatial databases and used to generate maps identifying forest areas likely to experience Ca depletion. Sustainable Ca supplies in forest ecosystems are functions of forest type, timber extraction intensity, prior land use, atmospheric deposition rates, and site factors including climate, hydrology, soil mineral type and weathering rates. Considering the unique vulnerability of Ca to leaching loss and its vital role in supporting tree stress response systems, the model focused on how changes in Ca pools may influence forest health conditions. Initial comparisons within New England indicated that the model-based Ca deficiency metric was a good predictor of field-based indicators of forest health and productivity. Models such as this show promise for evaluating the threat Ca depletion poses to forest health and productivity in an integrated and spatially explicit manner in North America. This approach has already proven valuable to policy makers and managers in Europe when evaluating alternative pollution reduction or mitigation options.
- Anthropogenic Cation Depletion : Cations are naturally occurring, positively charged elements that are important constituents of soils and surface waters and play unique and critical roles in biological systems.
- Ca Depletion as a Biologically Unique Threat : Paul G. Schaberg, Eric K. Miller, and Christopher Eagar
- Experimental Evidence of Tree Health Impacts : Although based on basic understandings of the distribution and physiology of Ca in plants, experimental evidence that Ca deficiencies could reduce stress tolerance in trees has only recently surfaced.
- Field Evidence of Influences on Tree Health : Controlled experiments like the ones outlined above have provided valuable insights into the biological mechanism through which Ca depletion may influence tree physiology and health.
- Limitations of Current Indicators of Ca Depletion : Evidence from laboratory experiments and a growing number of field assessments indicate that anthropogenic Ca depletion may pose a unique threat to forest health and productivity.
- Critical Loads as an Alternative Approach : Paul G. Schaberg, Eric K. Miller, and Christopher Eagar
- Application to New England : One approach for applying the concept of critical loads to assessing the sensitivity of forests to Ca depletion grew out of the work of the Conference of New England Governors and Eastern Canadian Premiers (NEG/ECP).
- Development and Relevance of Future Models : Initial results like those described for New Hampshire and Vermont indicate that models of critical loads and forest sensitivity show real promise in helping to empirically define the threat posed to forests in an integrated and spatially explicit manner.
Encyclopedia ID: p3176
