Effects of changing atmospheric deposition on the structure and function of the Northern Forest: Long-term measurements, experiments and future model projections from the Hubbard Brook Experimental Forest, New Hampshire, USA
Results from long-term measurements, a watershed-scale experiment and model projections provide insight on the effects of changing acid deposition on the structure and function of the Northern Forest at the Hubbard Brook Experimental Forest (HBR) in New Hampshire, USA. HBR is a U.S. Forest Service intensive research site and a member of the National Science Foundation Long-Term Ecological Research (LTER) network. We used long-term measurements of precipitation, soil, vegetation and streamwater; a watershed-level addition of calcium silicate; and model calculations using PnET-BGC to assess past impacts and to project the future recovery from acid deposition. The 3,037-ha HBR valley is located in the White Mountain National Forest (43°56’N, 71°45’W). The climate is humid continental. Vegetation is dominated by northern hardwoods, i.e., American beech (Fagus grandifolia), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) that grade into red spruce (Picea rubens) and balsam fir (Abies balsamea) at higher elevations. Soils are typically 25-75 cm deep, acidic (pH ~4.0 in the surface organic horizons) typic and aquic Haplorthods developed from unsorted basal tills derived mainly from silicate crystalline rocks.
As a result of acid-sensitive soils and surficial materials at HBR, elevated inputs of acidic deposition in the second half of the 20th century acidified soils by leaching calcium from available soil pools, mobilizing dissolved inorganic aluminum, and impacting streamwater. These conditions contributed to declines in the health of the sensitive tree species, particularly red spruce and sugar maple. To elucidate these effects a watershed-level calcium silicate addition experiment was initiated in 1999. This manipulation demonstrated that experimental replacement of calcium previously lost from decades of acid deposition could recover the base status of soil and improve the health of sugar maple. On-going, long-term measurements of bulk and wet-only precipitation and streamwater from the HBR show that U.S. air quality management programs have resulted in decreases acid deposition and are driving recovery of soil and surface waters. However, future projections of the response of experimental watersheds (using the biogeochemical model PnET-BGC) show that recovery will occur over many decades, and that the recovery of the base status of soils will lag behind the recovery of streamwaters.