Assessing the Threat that Anthropogenic Calcium Depletion Poses to Forest Health and Productivity

Paul G. Schaberg, Eric K. Miller, and Cristopher Eagar

USDA Forest Service Northeastern Research Station (1,3) and Ecosystems Research Group(2)

Considerable evidence now indicates that hydrogen ion (H+), nitrogen (N) and sulfur (S) additions from anthropogenic pollutant sources contribute to the leaching and depletion base cations such as calcium (Ca), magnesium (Mg) and potassium (K) from forest soils and ecosystems.  Although the depletion of base cations can have varied and interacting influences on ecosystem function, it is the loss of Ca that may be particularly limiting to tree health and productivity.  In contrast to other cations, Ca is not mobile in the phloem and is often immobilized in plant tissues in insoluble forms - processes that limit its biological availability and redistribution.  In addition, Ca is often concentrated outside of cell membranes, uniquely increasing its vulnerability to direct leaching loss.  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 these roles, Ca deficiency is expected to reduce tree growth and increase forest decline following exposure to even “normal” levels of stress that otherwise would pose no threat. 

Controlled experiments with red spruce, sugar maple, and other species provide mechanistic support for theoretical expectations regarding the impacts of Ca depletion on tree health and productivity.  For example, both H+ and N additions have been shown to reduce available Ca in red spruce foliage, simultaneously reducing foliar cold tolerance and increasing winter injury and crown degradation.  New experimental evidence indicates that Ca depletion down-regulates another Ca-dependent process (stomatal closure), predisposing red spruce to drought damage.  Data show that other tree species (eastern hemlock, balsam fir, and eastern white pine) experience the same mechanistic changes in Ca nutrition and physiology documented for red spruce.  Importantly, many real-world examples for a variety of tree species (sugar maple decline, dogwood susceptibility to anthracnose, and hemlock susceptibility to the hemlock wooly adelgid) show that injury is often greater when Ca depletion and stress exposure co-occur.

Concerns about the influence of H+ and N deposition on Ca nutrition and forest health exist for industrialized regions around the world including eastern North America, Europe, and increasingly China.  Indeed, 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 being examined and sometimes employed.  However, because not all tree species access, sequester or require Ca in equal levels, uniform standards for assessing thresholds in Ca depletion that require managerial actions remain elusive. 

An alternative approach to defining plant-based thresholds for Ca deficiency is to model critical loads and exceedances in pollutant additions that likely disrupt ecosystem Ca cycles and lead to net losses in Ca pools 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, and soil mineral weathering rates.  Considering the unique vulnerability of Ca to leaching loss and its vital role in supporting tree stress response systems, the model focuses on how changes in Ca pools may influence forest health conditions.  The model-based nutrient deficiency metric is a good predictor of independent “on-the-ground” indicators of current forest health and productivity.  For oak and pine forests in Massachusetts, tree height and canopy transparency were significantly related to foliar Ca levels.  A separate evaluation also showed promising results: a comparison of model results with multiple-year aerial surveys of forest damage in Vermont indicated that both the frequency of damage and size of damaged areas were related to modeled Ca deficiency.  This model-based threat assessment identified 18-30% of NH and VT forests to be at risk of anthropogenic Ca depletion under current atmospheric deposition and harvesting rates.

Land Session - Wednesday Afternoon

corresponding author:

Paul Schaberg
USDA Forest Service
Northeastern Research Station
705 Spear Street
Box 968
Burlington, VT 05402-0968
802-951-6771 x1120
pschaberg@fs.fed.us