Acidic Deposition and Climate Warming as Drivers of Tree Growth in High-Elevation Spruce-Fir Forests of the Northeastern US

Jay W Wason,Martin Dovciak,John J Battles,Colin M Beier

doi:10.3389/ffgc.2019.00063

Abstract

Acid rain in eastern North America contributed to the widespread decline of red spruce in high-elevation spruce-fir forests. With recent reductions in acid deposition and a warming climate, resurgence of red spruce growth has been reported in some mountain areas. Based on an extensive tree-ring sampling network established across elevations (600 to 1200 m above sea level) in spruce-fir forests on 10 mountains in the northeastern US (New York, Vermont, New Hampshire, and Maine), we investigated whether this resurgence was specific only to red spruce or if it occurred also in the codominant balsam fir. Furthermore, we tested if tree growth changes for both species were related to recent trends in acidic deposition and climate. Sharply increasing growth rates of red spruce were evident at all elevations and most closely correlated with increasing rainwater pH. Although climate of the previous year (cool July and warm November) explained higher spruce growth in certain years, recent trends in climate did not drive observed increases in spruce growth. In contrast, balsam fir exhibited no regional growth surge during the period of spruce recovery. Thus, spruce growth resurgence appears to be mediated primarily by declining acid deposition and not climatic changes or stand dynamics that would also impact fir growth. Although high-elevation forests may ultimately be at risk for future warming-related heat and drought stress, the observed recent dramatic resurgence of spruce growth illustrates the benefits of policy-driven reductions in acidic deposition for the health and productivity of northeastern US forests.

Highlights

Acidic deposition has severely impacted major forested areas of Europe, Asia, and North America (Vet et al, 2014)
Recent empirical studies generally support the negative impact of air pollution on tree growth but differences in research design, geographic scope, and analytical framework have led to contrasting conclusions about the relative contributions of how increasing air temperatures and decreasing acidic deposition contribute to enhanced red spruce growth (Kosiba et al, 2018; Mathias and Thomas, 2018)
The overall 62% average increase in red spruce basal area increment (BAI) since the 1960s was primarily related to concomitant increases in rainfall pH, i.e., a strong declining trend in bulk acidic deposition (Figure 3A; Table 2; Figure S4A)

Summary

Introduction

Acidic deposition has severely impacted major forested areas of Europe, Asia, and North America (Vet et al, 2014). Across the northeastern US, acidic deposition contributed to the widespread morbidity and mortality of high-elevation red spruce (Picea rubens) in the 1980’s (Adams et al, 2012) This decline was specific to red spruce and compelling evidence revealed that acidic deposition leached membrane-bound calcium from foliage and decreased the freezing tolerance. Recent empirical studies generally support the negative impact of air pollution on tree growth but differences in research design, geographic scope, and analytical framework have led to contrasting conclusions about the relative contributions of how increasing air temperatures and decreasing acidic deposition contribute to enhanced red spruce growth (Kosiba et al, 2018; Mathias and Thomas, 2018)

Methods

Results

Conclusion