Assessing relationships between red spruce radial growth and pollution critical load exceedance values

Acidic sulfur (S) and nitrogen (N) deposition depletes cations such as calcium (Ca) from forest soils and has been linked to increases in foliar winter injury that led to the decline of red spruce (Picea rubens Sarg.) in the northeastern United States. We used results from a 30 m resolution steady-state S and N critical load exceedance model for New England to better understand the spatial connections between Ca depletion and red spruce productivity. Atmospheric deposition and other inputs were estimated for a 5-year period (1984-1988) in order to smooth year-to-year variations in climate and patterns of atmospheric transport. Deposition levels prior to the reductions that followed the 1990 Amendments to the Clean Air Act were used because tree health and productivity declines were expected to be most responsive to high acid loading. We examined how radial growth (basal area increment) of 441 dominant and co-dominant red spruce trees from 37 sites across Vermont and New Hampshire was related to modeled estimates of S and N critical load exceedance. We assessed growth using statistical models with exceedance as a source of variation, but which also included “year” and “elevation class” (to help account for climatic variability) and interactions among factors. As expected, yearly climate-related sources of variation accounted for most of the differences in growth. However, exceedance was significantly and negatively associated with mean growth for the study period (1951-2010) overall, and particularly for the 1980s and 2000s - periods of numerous and/or severe foliar winter injury events. Because high winter injury reflects the convergence of predisposing (cation depletion) and inciting (weather) factors, exceedance alone appears insufficient to define associated patterns of growth reduction. Significant interactions indicated that exceedance had little influence on growth at low elevations (where intrinsic conditions for growth were generally good) or high elevations (where growth was uniformly poor), whereas exceedance was significantly associated with reduced growth at mid elevations over long periods of time. Exceedance was also linked to reduced growth rebounds following a region-wide foliar winter injury event in 2003. Overall, our analyses suggest that modeled S and N critical load exceedance can help account for red spruce growth and rebound from injury in the field. Interestingly, recent growth for red spruce is above average for the 20th to 21st century dendrochronological record - indicating that the factors shaping growth may be changing. The influence of reduced pollution inputs on this recent growth surge is under investigation.