Forest Soil Calcium Dynamics and Water Quality: Implications for Forest Management Planning.

Forest management planning is increasingly focused at the landscape scale. The resulting increase in planning unit size has fueled debate about forest sustainability, particularly at local scales. In Ontario, one concern is calcium depletion in forest soil and aquatic ecosystems. Planning decisions related to sustaining forest watershed calcium require identifying sites sensitive to calcium loss and implementing management practices that maintain background calcium pools and fluxes. We are conducting a risk assessment to identify linkages among the effects of historically high acid deposition exposure, climate change, and forest practices on watershed calcium depletion. A literature synthesis on forest soil calcium cycling and export, long-term (>10 years) trends in soil exchangeable calcium pools, changes in surface water quality, and their responses to forest practices indicated (P≤0.05) that forest floor in mature hardwood stands lost more calcium than that of conifer/mixedwood soils, which was consistent with net stream and lake calcium losses. However, surface water acid neutralizing capacity increased (P≤0.05), possibly because sulfate decreased relative to calcium. On average, based on soil concentrations and contents, forestry practices did not (P ≥0.05) deplete calcium in either cover type.

We are applying these observations with assembled regional data sets of climate, acid deposition, forest characteristics, hydrology and water quality, and soil/surficial geological properties to identify watersheds susceptible to calcium depletion. The findings will help inform our final step, which is to develop thresholds of calcium storage and export employing experimentally-manipulated watersheds that (1) estimate calcium temporal trajectories (i.e., chronosequence study substituting space for time) following forest harvesting operations and (2) identify silvicultural practices (i.e., harvesting and regeneration configurations) that minimize calcium losses. Combined our approach provides a scientifically-defendable risk assessment of calcium losses to develop mitigation options for forest management planning in regions of Ontario exposed to historically high acid deposition rates.