Disentangling the effects of acidic air pollution, atmospheric CO₂, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

Abstract

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75-year tree ring chronologies of basal tree growth, carbon isotope discrimination (∆ C, a proxy for leaf gas exchange), and δ N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO , while reductions in pollutant emissions of NO and warmer springs played smaller, but significant roles. Tree ring ∆ C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75-year chronology, and was driven by changes in atmospheric CO and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.

Temporal analysis of threats causing species endangerment in the United States

“…For pollution, we hypothesize that passage of certain environmental regulations, such as the Clean Air Act and Clean Water Act, have improved conditions for biodiversity. For example, native fish populations steadily increased while those for exotic species decreased in Illinois Rivers as a result of the Clean Water Act (Gibson-Reinemer et al, 2017), and the red spruce forest ecosystem of the Central Appalachian Mountains recovered in response to reduced acidic air pollution since passage of the Clean Air Act (Mathias & Thomas, 2018). A perplexing result of our research is the quadratic association between demographic stochasticity occurrence and year of listing ( Figure 2).…”

“…These efforts include developing models that predict where expansions in problem species geographical ranges will occur based on changing climate conditions, as well as provide the required resources to control problem species. For example, native fish populations steadily increased while those for exotic species decreased in Illinois Rivers as a result of the Clean Water Act (Gibson-Reinemer et al, 2017), and the red spruce forest ecosystem of the Central Appalachian Mountains recovered in response to reduced acidic air pollution since passage of the Clean Air Act (Mathias & Thomas, 2018). For example, we found three threats (overutilization, pollution, and demographic stochasticity) that have decreased in occurrence in the U.S. Overutilization occurrence decreased linearly between 1975 and 2017, while pollution occurrence increased up to the 2000s but then decreased (Figure 2).…”

Growing season moisture drives interannual variation in woody productivity of a temperate deciduous forest

“…By contrast with the negative impact of temperature during the growing season, tree growth and ANPP stem responded positively to high April temperatures, indicative of thermal limitations to the start of the growing season and consistent with other tree‐ring observations in the central Appalachians (Mathias & Thomas, ) and with findings that GPP and NEE (White et al ., ; Baldocchi et al ., ) are sensitive to growing season length. However, the influence of April temperatures on ANPP stem was quite modest relative to conditions in May–August and even the previous early growing season (Fig.…”

Higher CO2 Concentrations and Lower Acidic Deposition Have Not Changed Drought Response in Tree Growth But Do Influence iWUE in Hardwood Trees in the Midwestern United States

“…Pollution such as acidic deposition can negatively influence tree growth and the ability of trees to tolerate climatic stress (Cook et al, ; Johnson et al, ; LeBlanc et al, ). Acidic deposition has decreased in recent decades in the Midwest and this could have resulted in an increase in tree growth (Engel et al, ; Kosiba et al, ; Mathias & Thomas, ; Thomas et al, ), although other studies have shown little to no impact on growth (Bishop et al, ; Schaberg et al, ). Further, nitrogen deposition can act as a fertilizer to increase carbon storage of trees (Horn et al, ; Thomas et al, ) .…”