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Two Decades of Forest Productivity Enhancement

Paper Reviewed
Fernández-Martínez, M., Vicca, S., Janssens, I.A., Ciais, P., Obersteiner, M., Bartrons, M., Sardans, J., Verger, A., Canadell, J.G., Chevallier, F., Wang, X., Bernhofer, C., Curtis, P.S., Gianelle, D., Grünwald, T., Heinesch, B., Ibrom, A., Knohl, A., Laurila, T., Law, B.E., Limousin, J.M., Longdoz, B., Loustau, D., Mammarella, I., Matteucci, G., Monson, R.K., Montagnani, L., Moors, E.J., Munger, J.W., Papale, D., Piao, S.L. and Peñuelas, J. 2017. Atmospheric deposition, CO2, and change in the lank carbon sink. Scientific Reports 9: 9632, DOI:10.1038/s41598-017-08755-8.

Over three decades ago, principals of our organization predicted that in response to the well-known plant productivity gains that result from the aerial fertilization effect of the ongoing rise in atmospheric CO2, plus its transpiration-reducing effect that boosts plant water use efficiency, along with its stress-alleviating effect that lessens the negative growth impacts of resource limitations and environmental constraints, the terrestrial biosphere of our planet should be reaping tremendous benefits in the form of enhanced biospheric productivity, notwithstanding the many assaults on both natural and managed ecosystems inflicted by man and nature alike.

In the past few years an ever-growing number of scientific studies have proven this prediction to be correct. Observations from all across the globe demonstrate that rising atmospheric CO2 concentrations are stimulating the productivity of terrestrial ecosystems (see, for example, the many reviews we have posted in our Subject Index under the topic of Biospheric Productivity on this page]). The latest study to confirm as much comes from the 32-member research team of Fernández-Martínez et al. (2017).

Publishing their work in the journal Scientific Reports, the scientists performed a series of analyses on a suite of remotely-sensed, climatic, weather, carbon flux, atmospheric pollutant, and forestry-related data sets to calculate their influence on gross primary production (GPP), ecosystem respiration (Re) and net ecosystem production (NEP) for 23 temperate and boreal forest locations distributed across Europe and the USA. As illustrated in Figure 1 below, Fernández-Martínez et al. calculated that annual NEP and GPP increased by 8.4 ± 1.8 and 11.2 ± 2.5 g C m-2 yr-1, respectively over the period 1992-2013, which increase, they say, "corresponds to an annual increase of 1.1% in both C fluxes, consistent in magnitude with growth rates reported in previous studies." Utilizing these numbers with the data presented in Figure 1, it can therefore be calculated that, over the 21-year period of study, GPP increased by a respectable 19% and NEP by a whopping 31%!

Figure 1. Long-term (1992-2013) NEP (Panel A) and GPP (Panel B) trends for 23 temperate and boreal forests across Europe and the USA. Red and blue lines indicate forests with increasing and decreasing trends, respectively, and black lines indicate the average trends. The shaded area indicates the standard error of the average trend. Grey dots indicate forest-year observations; and all values were adjusted to the same mean to remove forest-specific variability. The inset shows the modelled distribution of the trends using kernel-density estimation, indicating the percentage of forests with increasing and decreasing trends. Source: Fernández-Martínez et al. (2017).

With respect to the cause of the observed increases in both NEP and GPP, Fernández-Martínez et al. note that "temperatures and drought did not significantly change [over the period of study] and, therefore, could not be responsible for the observed trends." Indeed, as shown in Figure 2, generalized linear mixed models and model averaging statistical analyses revealed negligent contributions from these two parameters on the observed change in NEP and GPP. However, in line with our hypothesis of over 30 years ago, the authors report that "increasing CO2 is the only predictor systematically associated with the observed increase in both NEP and GPP over time," adding that "for each ppm increase in atmospheric CO2 concentration, NEP and GPP increased by 4.81 ± 0.52 and 4.49 ± 0.75 g C m-2 yr-1, respectively."

In light of the above findings, Fernández-Martínez et al. say their results "provide consistent empirical evidence that support the dominant role of the CO2-fertilization effect in explaining the current positive NEP trends at local and possibly regional scales." We agree, and continue to be awed and inspired by the magnitude and scale of ecosystem enhancement that is provided courtesy of the aerial fertilization effect of atmospheric CO2.

Figure 2. Temporal contribution of the predictor variables on NEP and GPP for the period 1995-2011. ΔSPEI is considered a measure of drought. Error bars indicate standard errors. Significance levels: (*) P < 0.1; * P < 0.05; ** P < 0.01; *** P < 0.001. Source: Fernández-Martínez et al. (2017).

Posted 17 January 2018