Conference of New England Governors and Eastern Canadian Premiers
Considering the unique vulnerability of Ca to leaching loss and its vital role in supporting tree stress response systems, the model focuses on how atmospherically deposited S and N act to produce changes in Ca pools that may influence forest health conditions. Review of the literature (discussed previously) indicated that the most appropriate critical limit that could be modeled as a function of S and N deposition was whether an ecosystem could maintain a sustainable supply of the nutrient base cations Ca, Mg, and K or if the system was experiencing long-term depletion of these elements. Systems in a chronic state of cation depletion will eventually exhibit the Ca deficiency-related problems discussed above. Because the goal of the assessment was to provide estimates of the forest area potentially impaired if pollution remained at current S and N deposition levels, a steady-state modeling approach was selected. It was determined that adequate estimates of the parameters required for steady-state modeling could be developed regionwide at an appropriate spatial scale, whereas the data requirements for dynamic modeling could only be met in a few locations. It is anticipated that the results of the steady-state modeling assessment will direct future data collection efforts to high-value, high-risk areas where the cost of data collection for dynamic modeling may be justified.
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 (Miller 2005, 2006; Ouimet and others 2006). 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 (NEG/ECP Forest Mapping Group 2001). The ecosystem model and several submodels simulate these processes. The crucial determinant of an ecosystem’s ability to tolerate S and N deposition without declining Ca supplies is the rate at which primary minerals, (e.g., hornblende, plagioclase, calcite) chemically decompose, liberating the nutrient cations Ca2+, Mg2+ and K+ to plant-available pools and, thus, replenishing nutrients lost via timber removals and acid-induced leaching. A geochemical model based on the work of Sverdrup and Warvfinge (1993) was used to estimate the weathering rate of primary minerals. Considerable field and modeling efforts were required to develop the spatial data layers needed to apply this model to the New England region. The annual demand for nutrients required to regrow the biomass exported via harvesting was estimated from timber extraction rates and wood- nutrient content. This information was generally available for the New England States from combinations of State and Federal sources. Atmospheric deposition of S, N, chloride (Cl), Ca, Mg, sodium (Na), and K was estimated for a 5-year period (1999-2003) in order to provide some smoothing of year-to-year variations in climate and patterns of atmospheric transport. Total deposition, including precipitation, cloud droplet interception, and dry deposition, was estimated using atmospheric chemistry data from the US NADP, CASTNet, and NOAA-AirMon deposition monitoring networks and Ecosystems Research Group, Ltd.’s High-Resolution Deposition Model (Miller 2000, Miller and others 2005, NEG/ECP Forest Mapping Group 2001).
Although only results for Vermont and New Hampshire are presented here as examples, this assessment methodology has been applied to all of New England. Critical loads of S plus N ranged widely in New Hampshire and Vermont (0 – 21 keq ha-1 y-1) as a result of the diverse geology and climate of the region (Figure 1). Areas of Ca-rich rocks and soil materials scattered throughout the region support the highest critical loads, often in excess of 3 keq ha-1 y-1. The lowest critical loads were found primarily in northern New Hampshire where soils are developed in thin and patchy tills derived from base-poor rocks. The range of S deposition was between 3.2 and 18.9 (average 5.1) kg ha-1 y-1 and N deposition (ammonium + nitrate) ranged between 3.3 and 25.2 (average 8.4) kg ha-1 y-1, producing an aggregate acidifying and nutrient-leaching potential of 0.43 to 2.7 (average 0.92) keq ha-1 y-1 (Figure 2). The highest elevation areas received the highest S + N deposition due to orographically enhanced precipitation and cloud water inputs. Deposition was also high in the southern and western parts of the region due to proximity to emission sources. Using critical load and atmospheric deposition estimates, a deposition index can be calculated to help evaluate the relative risk for ecosystem health problems resulting from Ca limitation (Figure 3). The deposition index is calculated as the ecosystem critical load minus the atmospheric deposition. In this index, positive values reflect the capacity of a forest ecosystem to tolerate additional acidic deposition, whereas negative values correspond to the reduction in S and N deposition required to eliminate present or deter the development of nutrient limitations. Atmospheric deposition of S and N during 1999-2003 exceeded the critical load in approximately 18 percent of the forested area of NH and 30 percent of the forested area in VT (Figure 4). Critical loads were frequently exceeded where deposition was moderate (Northeast) to high (South) and where critical loads are low. An additional 10 percent of the forested area in each State experienced deposition rates during 1999-2003 that were less than 0.2 keq ha-1 y-1 below the critical load. Some locations within these areas with shallower soils and more intense harvesting than the average values used in this assessment are likely to also be at risk of Ca depletion.
Forest tree species occupy different portions of the landscape as a function of climate, soil conditions, and land use history. This distribution results in some types of forests being more severely impacted than others by the nutrient cation depletion caused by S + N deposition. For example, critical loads are exceeded in 49.6 percent of New Hampshire’s central hardwood forests, but in just 2 percent of the State’s northern hardwood forests. This discrepancy exists because the central hardwood forests (6.1 percent of total forest area) tend to occur more frequently on poor sandy soils. The northern hardwood forest (18.9 percent of forest area) occurs on somewhat richer sites. Stands dominated by sugar maple (11.4 percent of forest area) occupy mid-elevation sites and have the highest Ca requirement of the northern hardwood forest variants. We estimate that the critical load is exceeded in 39.8 percent (88,167 ha) of New Hampshire’s sugar maple stands, and deposition is within 10 percent of the critical load in an additional 3.6 percent (7,826 ha).
Encyclopedia ID: p3200
