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A CO2-induced Stimulation of Red Spruce Forest Growth Since 1989

Paper Reviewed
Mathias, J.M. and Thomas, R.B. 2018. Disentangling the effects of acidic air pollution, atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains. Global Change Biology 24: 3938-3953.

Writing as background for their study, Mathias and Thomas (2018) say that "tree ring growth and isotope chronologies have become important proxies used to examine historical changes in productivity, plant physiology, and ecosystem processes in a rapidly changing environment."

In producing three such proxies of their own, the two researchers analyzed red spruce (Picea rubens) tree rings from three forest locations in the Central Appalachian Mountains of West Virginia, creating chronologies of basal tree growth, leaf gas exchange (Δ13C) and ecosystem N status (δ15N) for this region covering the past 75 years. Then, using statistical analyses (Kendall's rank correlation and generalized linear mixed models), Mathias and Thomas investigated "the multiple environmental effects and their sensitivities on red spruce tree growth into discreet components."

The results of their study revealed that red spruce basal area increment (BAI) declined by nearly 50% between 1940 and 1989, but increased 105.8% from 1989 to 2014. Further analyses revealed that "increasing atmospheric CO2 was responsible for the largest change in basal area of the red spruce trees," followed by "reductions in national SO2 emissions," which was the second most important factor. Reductions in national emissions of NOx and increases in mean April temperatures also contributed to the BAI trend, but to a much lesser degree (see Figure 1). Tree ring Δ13C also increased significantly since 1989, concurrent with significant declines in tree ring δ15N signatures, all of which findings led the two researchers to write that "these isotope chronologies provide strong evidence that simultaneous changes in carbon and nitrogen cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem nitrogen retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution."

In commenting on their findings, Mathias and Thomas say that "isolating the effects of increasing CO2 on tree growth in forest ecosystems has been difficult to confirm [in the field] due to the small annual step changes in CO2 and the myriad of interacting environmental factors known to affect whole-tree growth over multi-decadal time spans." However, they add that "this study not only provides evidence for increasing CO2 having a positive effect on tree growth but also indicates a broad range of complexity as red spruce forest ecosystems recover from decades of acidic air pollution and the realization that none of the environmental factors that are examined here act in isolation." Yet, as more and more time goes by, and as the aerial fertilization effect of Earth's rising atmospheric CO2 concentration increases in magnitude, it will likely become easier to recognize the growth benefits of this important trace gas.

Figure 1. Panel a: Contribution of atmospheric CO2, NOx emissions, SO2 emissions, and mean April temperatures (April Tmean) to the change in red spruce basal area increment (BAI) each year predicted by the GLMM average model for 1989-2014. Panel b: the contribution of CO2, NOx emissions, SO2 emissions, and April Tmean to the total change in BAI over 1989-2014. Numbers in parentheses indicate the direction and magnitude of changes in environmental parameters. Numbers in brackets in Panel (a) represent the trend in each respective environmental parameter over 1989-2014, while numbers in brackets in Panel (b) represent the total change in each respective environmental parameter over 1989-2014. Units for CO2 and April Tmean are ppm and °C, respectively, while units for NOx and SO2 are 106 metric tons. Unknown contributions in Panel (a) were calculated as the difference between the observed change and all known contributions. Asterisks (*) indicate significance values (P < 0.05). Source: Mathias and Thomas (2018).

Posted 20 December 2018